[RFC,PATCH 14/14] utrace core

[Oleg Nesterov]

From: Roland McGrath <roland@xxxxxxxxxx>

This adds the utrace facility, a new modular interface in the kernel
for implementing user thread tracing and debugging.  This fits on top
of the tracehook_* layer, so the new code is well-isolated.

The new interface is in <linux/utrace.h> and the DocBook utrace book
describes it.  It allows for multiple separate tracing engines to work
in parallel without interfering with each other.  Higher-level tracing
facilities can be implemented as loadable kernel modules using this layer.

The new facility is made optional under CONFIG_UTRACE. It can only be
enabled on machines that have all the prerequisites and select
CONFIG_HAVE_ARCH_TRACEHOOK.

If CONFIG_UTRACE is set ptrace uses the utrace facilities, it can play
nicely with other utrace-based things tracing the same threads.

Signed-off-by: Roland McGrath <roland@xxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---

 Documentation/DocBook/Makefile    |    2 
 Documentation/DocBook/utrace.tmpl |  590 +++++++++
 fs/proc/array.c                   |    3 
 include/linux/sched.h             |    5 
 include/linux/tracehook.h         |   91 +
 include/linux/utrace.h            |  729 +++++++++++
 init/Kconfig                      |    9 
 kernel/fork.c                     |    3 
 kernel/Makefile                   |    1 
 kernel/utrace.c                   | 2430 ++++++++++++++++++++++++++++++++++++++
 10 files changed, 3861 insertions(+), 2 deletions(-)

--- V1/Documentation/DocBook/Makefile~14_UTRACE	2009-11-24 20:27:22.000000000 +0100
+++ V1/Documentation/DocBook/Makefile	2009-11-24 20:30:17.000000000 +0100
@@ -9,7 +9,7 @@
 DOCBOOKS := z8530book.xml mcabook.xml device-drivers.xml \
 	    kernel-hacking.xml kernel-locking.xml deviceiobook.xml \
 	    procfs-guide.xml writing_usb_driver.xml networking.xml \
-	    kernel-api.xml filesystems.xml lsm.xml usb.xml kgdb.xml \
+	    kernel-api.xml filesystems.xml lsm.xml usb.xml kgdb.xml utrace.xml \
 	    gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml \
 	    genericirq.xml s390-drivers.xml uio-howto.xml scsi.xml \
 	    mac80211.xml debugobjects.xml sh.xml regulator.xml \
--- /dev/null	2009-11-24 16:51:11.614043008 +0100
+++ V1/Documentation/DocBook/utrace.tmpl	2009-11-24 20:30:17.000000000 +0100
@@ -0,0 +1,590 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"; []>
+
+<book id="utrace">
+  <bookinfo>
+    <title>The utrace User Debugging Infrastructure</title>
+  </bookinfo>
+
+  <toc></toc>
+
+  <chapter id="concepts"><title>utrace concepts</title>
+
+  <sect1 id="intro"><title>Introduction</title>
+
+  <para>
+    <application>utrace</application> is infrastructure code for tracing
+    and controlling user threads.  This is the foundation for writing
+    tracing engines, which can be loadable kernel modules.
+  </para>
+
+  <para>
+    The basic actors in <application>utrace</application> are the thread
+    and the tracing engine.  A tracing engine is some body of code that
+    calls into the <filename>&lt;linux/utrace.h&gt;</filename>
+    interfaces, represented by a <structname>struct
+    utrace_engine_ops</structname>.  (Usually it's a kernel module,
+    though the legacy <function>ptrace</function> support is a tracing
+    engine that is not in a kernel module.)  The interface operates on
+    individual threads (<structname>struct task_struct</structname>).
+    If an engine wants to treat several threads as a group, that is up
+    to its higher-level code.
+  </para>
+
+  <para>
+    Tracing begins by attaching an engine to a thread, using
+    <function>utrace_attach_task</function> or
+    <function>utrace_attach_pid</function>.  If successful, it returns a
+    pointer that is the handle used in all other calls.
+  </para>
+
+  </sect1>
+
+  <sect1 id="callbacks"><title>Events and Callbacks</title>
+
+  <para>
+    An attached engine does nothing by default.  An engine makes something
+    happen by requesting callbacks via <function>utrace_set_events</function>
+    and poking the thread with <function>utrace_control</function>.
+    The synchronization issues related to these two calls
+    are discussed further below in <xref linkend="teardown"/>.
+  </para>
+
+  <para>
+    Events are specified using the macro
+    <constant>UTRACE_EVENT(<replaceable>type</replaceable>)</constant>.
+    Each event type is associated with a callback in <structname>struct
+    utrace_engine_ops</structname>.  A tracing engine can leave unused
+    callbacks <constant>NULL</constant>.  The only callbacks required
+    are those used by the event flags it sets.
+  </para>
+
+  <para>
+    Many engines can be attached to each thread.  When a thread has an
+    event, each engine gets a callback if it has set the event flag for
+    that event type.  For most events, engines are called in the order they
+    attached.  Engines that attach after the event has occurred do not get
+    callbacks for that event.  This includes any new engines just attached
+    by an existing engine's callback function.  Once the sequence of
+    callbacks for that one event has completed, such new engines are then
+    eligible in the next sequence that starts when there is another event.
+  </para>
+
+  <para>
+    Event reporting callbacks have details particular to the event type,
+    but are all called in similar environments and have the same
+    constraints.  Callbacks are made from safe points, where no locks
+    are held, no special resources are pinned (usually), and the
+    user-mode state of the thread is accessible.  So, callback code has
+    a pretty free hand.  But to be a good citizen, callback code should
+    never block for long periods.  It is fine to block in
+    <function>kmalloc</function> and the like, but never wait for i/o or
+    for user mode to do something.  If you need the thread to wait, use
+    <constant>UTRACE_STOP</constant> and return from the callback
+    quickly.  When your i/o finishes or whatever, you can use
+    <function>utrace_control</function> to resume the thread.
+  </para>
+
+  <para>
+    The <constant>UTRACE_EVENT(SYSCALL_ENTRY)</constant> event is a special
+    case.  While other events happen in the kernel when it will return to
+    user mode soon, this event happens when entering the kernel before it
+    will proceed with the work requested from user mode.  Because of this
+    difference, the <function>report_syscall_entry</function> callback is
+    special in two ways.  For this event, engines are called in reverse of
+    the normal order (this includes the <function>report_quiesce</function>
+    call that precedes a <function>report_syscall_entry</function> call).
+    This preserves the semantics that the last engine to attach is called
+    "closest to user mode"--the engine that is first to see a thread's user
+    state when it enters the kernel is also the last to see that state when
+    the thread returns to user mode.  For the same reason, if these
+    callbacks use <constant>UTRACE_STOP</constant> (see the next section),
+    the thread stops immediately after callbacks rather than only when it's
+    ready to return to user mode; when allowed to resume, it will actually
+    attempt the system call indicated by the register values at that time.
+  </para>
+
+  </sect1>
+
+  <sect1 id="safely"><title>Stopping Safely</title>
+
+  <sect2 id="well-behaved"><title>Writing well-behaved callbacks</title>
+
+  <para>
+    Well-behaved callbacks are important to maintain two essential
+    properties of the interface.  The first of these is that unrelated
+    tracing engines should not interfere with each other.  If your engine's
+    event callback does not return quickly, then another engine won't get
+    the event notification in a timely manner.  The second important
+    property is that tracing should be as noninvasive as possible to the
+    normal operation of the system overall and of the traced thread in
+    particular.  That is, attached tracing engines should not perturb a
+    thread's behavior, except to the extent that changing its user-visible
+    state is explicitly what you want to do.  (Obviously some perturbation
+    is unavoidable, primarily timing changes, ranging from small delays due
+    to the overhead of tracing, to arbitrary pauses in user code execution
+    when a user stops a thread with a debugger for examination.)  Even when
+    you explicitly want the perturbation of making the traced thread block,
+    just blocking directly in your callback has more unwanted effects.  For
+    example, the <constant>CLONE</constant> event callbacks are called when
+    the new child thread has been created but not yet started running; the
+    child can never be scheduled until the <constant>CLONE</constant>
+    tracing callbacks return.  (This allows engines tracing the parent to
+    attach to the child.)  If a <constant>CLONE</constant> event callback
+    blocks the parent thread, it also prevents the child thread from
+    running (even to process a <constant>SIGKILL</constant>).  If what you
+    want is to make both the parent and child block, then use
+    <function>utrace_attach_task</function> on the child and then use
+    <constant>UTRACE_STOP</constant> on both threads.  A more crucial
+    problem with blocking in callbacks is that it can prevent
+    <constant>SIGKILL</constant> from working.  A thread that is blocking
+    due to <constant>UTRACE_STOP</constant> will still wake up and die
+    immediately when sent a <constant>SIGKILL</constant>, as all threads
+    should.  Relying on the <application>utrace</application>
+    infrastructure rather than on private synchronization calls in event
+    callbacks is an important way to help keep tracing robustly
+    noninvasive.
+  </para>
+
+  </sect2>
+
+  <sect2 id="UTRACE_STOP"><title>Using <constant>UTRACE_STOP</constant></title>
+
+  <para>
+    To control another thread and access its state, it must be stopped
+    with <constant>UTRACE_STOP</constant>.  This means that it is
+    stopped and won't start running again while we access it.  When a
+    thread is not already stopped, <function>utrace_control</function>
+    returns <constant>-EINPROGRESS</constant> and an engine must wait
+    for an event callback when the thread is ready to stop.  The thread
+    may be running on another CPU or may be blocked.  When it is ready
+    to be examined, it will make callbacks to engines that set the
+    <constant>UTRACE_EVENT(QUIESCE)</constant> event bit.  To wake up an
+    interruptible wait, use <constant>UTRACE_INTERRUPT</constant>.
+  </para>
+
+  <para>
+    As long as some engine has used <constant>UTRACE_STOP</constant> and
+    not called <function>utrace_control</function> to resume the thread,
+    then the thread will remain stopped.  <constant>SIGKILL</constant>
+    will wake it up, but it will not run user code.  When the stop is
+    cleared with <function>utrace_control</function> or a callback
+    return value, the thread starts running again.
+    (See also <xref linkend="teardown"/>.)
+  </para>
+
+  </sect2>
+
+  </sect1>
+
+  <sect1 id="teardown"><title>Tear-down Races</title>
+
+  <sect2 id="SIGKILL"><title>Primacy of <constant>SIGKILL</constant></title>
+  <para>
+    Ordinarily synchronization issues for tracing engines are kept fairly
+    straightforward by using <constant>UTRACE_STOP</constant>.  You ask a
+    thread to stop, and then once it makes the
+    <function>report_quiesce</function> callback it cannot do anything else
+    that would result in another callback, until you let it with a
+    <function>utrace_control</function> call.  This simple arrangement
+    avoids complex and error-prone code in each one of a tracing engine's
+    event callbacks to keep them serialized with the engine's other
+    operations done on that thread from another thread of control.
+    However, giving tracing engines complete power to keep a traced thread
+    stuck in place runs afoul of a more important kind of simplicity that
+    the kernel overall guarantees: nothing can prevent or delay
+    <constant>SIGKILL</constant> from making a thread die and release its
+    resources.  To preserve this important property of
+    <constant>SIGKILL</constant>, it as a special case can break
+    <constant>UTRACE_STOP</constant> like nothing else normally can.  This
+    includes both explicit <constant>SIGKILL</constant> signals and the
+    implicit <constant>SIGKILL</constant> sent to each other thread in the
+    same thread group by a thread doing an exec, or processing a fatal
+    signal, or making an <function>exit_group</function> system call.  A
+    tracing engine can prevent a thread from beginning the exit or exec or
+    dying by signal (other than <constant>SIGKILL</constant>) if it is
+    attached to that thread, but once the operation begins, no tracing
+    engine can prevent or delay all other threads in the same thread group
+    dying.
+  </para>
+  </sect2>
+
+  <sect2 id="reap"><title>Final callbacks</title>
+  <para>
+    The <function>report_reap</function> callback is always the final event
+    in the life cycle of a traced thread.  Tracing engines can use this as
+    the trigger to clean up their own data structures.  The
+    <function>report_death</function> callback is always the penultimate
+    event a tracing engine might see; it's seen unless the thread was
+    already in the midst of dying when the engine attached.  Many tracing
+    engines will have no interest in when a parent reaps a dead process,
+    and nothing they want to do with a zombie thread once it dies; for
+    them, the <function>report_death</function> callback is the natural
+    place to clean up data structures and detach.  To facilitate writing
+    such engines robustly, given the asynchrony of
+    <constant>SIGKILL</constant>, and without error-prone manual
+    implementation of synchronization schemes, the
+    <application>utrace</application> infrastructure provides some special
+    guarantees about the <function>report_death</function> and
+    <function>report_reap</function> callbacks.  It still takes some care
+    to be sure your tracing engine is robust to tear-down races, but these
+    rules make it reasonably straightforward and concise to handle a lot of
+    corner cases correctly.
+  </para>
+  </sect2>
+
+  <sect2 id="refcount"><title>Engine and task pointers</title>
+  <para>
+    The first sort of guarantee concerns the core data structures
+    themselves.  <structname>struct utrace_engine</structname> is
+    a reference-counted data structure.  While you hold a reference, an
+    engine pointer will always stay valid so that you can safely pass it to
+    any <application>utrace</application> call.  Each call to
+    <function>utrace_attach_task</function> or
+    <function>utrace_attach_pid</function> returns an engine pointer with a
+    reference belonging to the caller.  You own that reference until you
+    drop it using <function>utrace_engine_put</function>.  There is an
+    implicit reference on the engine while it is attached.  So if you drop
+    your only reference, and then use
+    <function>utrace_attach_task</function> without
+    <constant>UTRACE_ATTACH_CREATE</constant> to look up that same engine,
+    you will get the same pointer with a new reference to replace the one
+    you dropped, just like calling <function>utrace_engine_get</function>.
+    When an engine has been detached, either explicitly with
+    <constant>UTRACE_DETACH</constant> or implicitly after
+    <function>report_reap</function>, then any references you hold are all
+    that keep the old engine pointer alive.
+  </para>
+
+  <para>
+    There is nothing a kernel module can do to keep a <structname>struct
+    task_struct</structname> alive outside of
+    <function>rcu_read_lock</function>.  When the task dies and is reaped
+    by its parent (or itself), that structure can be freed so that any
+    dangling pointers you have stored become invalid.
+    <application>utrace</application> will not prevent this, but it can
+    help you detect it safely.  By definition, a task that has been reaped
+    has had all its engines detached.  All
+    <application>utrace</application> calls can be safely called on a
+    detached engine if the caller holds a reference on that engine pointer,
+    even if the task pointer passed in the call is invalid.  All calls
+    return <constant>-ESRCH</constant> for a detached engine, which tells
+    you that the task pointer you passed could be invalid now.  Since
+    <function>utrace_control</function> and
+    <function>utrace_set_events</function> do not block, you can call those
+    inside a <function>rcu_read_lock</function> section and be sure after
+    they don't return <constant>-ESRCH</constant> that the task pointer is
+    still valid until <function>rcu_read_unlock</function>.  The
+    infrastructure never holds task references of its own.  Though neither
+    <function>rcu_read_lock</function> nor any other lock is held while
+    making a callback, it's always guaranteed that the <structname>struct
+    task_struct</structname> and the <structname>struct
+    utrace_engine</structname> passed as arguments remain valid
+    until the callback function returns.
+  </para>
+
+  <para>
+    The common means for safely holding task pointers that is available to
+    kernel modules is to use <structname>struct pid</structname>, which
+    permits <function>put_pid</function> from kernel modules.  When using
+    that, the calls <function>utrace_attach_pid</function>,
+    <function>utrace_control_pid</function>,
+    <function>utrace_set_events_pid</function>, and
+    <function>utrace_barrier_pid</function> are available.
+  </para>
+  </sect2>
+
+  <sect2 id="reap-after-death">
+    <title>
+      Serialization of <constant>DEATH</constant> and <constant>REAP</constant>
+    </title>
+    <para>
+      The second guarantee is the serialization of
+      <constant>DEATH</constant> and <constant>REAP</constant> event
+      callbacks for a given thread.  The actual reaping by the parent
+      (<function>release_task</function> call) can occur simultaneously
+      while the thread is still doing the final steps of dying, including
+      the <function>report_death</function> callback.  If a tracing engine
+      has requested both <constant>DEATH</constant> and
+      <constant>REAP</constant> event reports, it's guaranteed that the
+      <function>report_reap</function> callback will not be made until
+      after the <function>report_death</function> callback has returned.
+      If the <function>report_death</function> callback itself detaches
+      from the thread, then the <function>report_reap</function> callback
+      will never be made.  Thus it is safe for a
+      <function>report_death</function> callback to clean up data
+      structures and detach.
+    </para>
+  </sect2>
+
+  <sect2 id="interlock"><title>Interlock with final callbacks</title>
+  <para>
+    The final sort of guarantee is that a tracing engine will know for sure
+    whether or not the <function>report_death</function> and/or
+    <function>report_reap</function> callbacks will be made for a certain
+    thread.  These tear-down races are disambiguated by the error return
+    values of <function>utrace_set_events</function> and
+    <function>utrace_control</function>.  Normally
+    <function>utrace_control</function> called with
+    <constant>UTRACE_DETACH</constant> returns zero, and this means that no
+    more callbacks will be made.  If the thread is in the midst of dying,
+    it returns <constant>-EALREADY</constant> to indicate that the
+    <constant>report_death</constant> callback may already be in progress;
+    when you get this error, you know that any cleanup your
+    <function>report_death</function> callback does is about to happen or
+    has just happened--note that if the <function>report_death</function>
+    callback does not detach, the engine remains attached until the thread
+    gets reaped.  If the thread is in the midst of being reaped,
+    <function>utrace_control</function> returns <constant>-ESRCH</constant>
+    to indicate that the <function>report_reap</function> callback may
+    already be in progress; this means the engine is implicitly detached
+    when the callback completes.  This makes it possible for a tracing
+    engine that has decided asynchronously to detach from a thread to
+    safely clean up its data structures, knowing that no
+    <function>report_death</function> or <function>report_reap</function>
+    callback will try to do the same.  <constant>utrace_detach</constant>
+    returns <constant>-ESRCH</constant> when the <structname>struct
+    utrace_engine</structname> has already been detached, but is
+    still a valid pointer because of its reference count.  A tracing engine
+    can use this to safely synchronize its own independent multiple threads
+    of control with each other and with its event callbacks that detach.
+  </para>
+
+  <para>
+    In the same vein, <function>utrace_set_events</function> normally
+    returns zero; if the target thread was stopped before the call, then
+    after a successful call, no event callbacks not requested in the new
+    flags will be made.  It fails with <constant>-EALREADY</constant> if
+    you try to clear <constant>UTRACE_EVENT(DEATH)</constant> when the
+    <function>report_death</function> callback may already have begun, if
+    you try to clear <constant>UTRACE_EVENT(REAP)</constant> when the
+    <function>report_reap</function> callback may already have begun, or if
+    you try to newly set <constant>UTRACE_EVENT(DEATH)</constant> or
+    <constant>UTRACE_EVENT(QUIESCE)</constant> when the target is already
+    dead or dying.  Like <function>utrace_control</function>, it returns
+    <constant>-ESRCH</constant> when the thread has already been detached
+    (including forcible detach on reaping).  This lets the tracing engine
+    know for sure which event callbacks it will or won't see after
+    <function>utrace_set_events</function> has returned.  By checking for
+    errors, it can know whether to clean up its data structures immediately
+    or to let its callbacks do the work.
+  </para>
+  </sect2>
+
+  <sect2 id="barrier"><title>Using <function>utrace_barrier</function></title>
+  <para>
+    When a thread is safely stopped, calling
+    <function>utrace_control</function> with <constant>UTRACE_DETACH</constant>
+    or calling <function>utrace_set_events</function> to disable some events
+    ensures synchronously that your engine won't get any more of the callbacks
+    that have been disabled (none at all when detaching).  But these can also
+    be used while the thread is not stopped, when it might be simultaneously
+    making a callback to your engine.  For this situation, these calls return
+    <constant>-EINPROGRESS</constant> when it's possible a callback is in
+    progress.  If you are not prepared to have your old callbacks still run,
+    then you can synchronize to be sure all the old callbacks are finished,
+    using <function>utrace_barrier</function>.  This is necessary if the
+    kernel module containing your callback code is going to be unloaded.
+  </para>
+  <para>
+    After using <constant>UTRACE_DETACH</constant> once, further calls to
+    <function>utrace_control</function> with the same engine pointer will
+    return <constant>-ESRCH</constant>.  In contrast, after getting
+    <constant>-EINPROGRESS</constant> from
+    <function>utrace_set_events</function>, you can call
+    <function>utrace_set_events</function> again later and if it returns zero
+    then know the old callbacks have finished.
+  </para>
+  <para>
+    Unlike all other calls, <function>utrace_barrier</function> (and
+    <function>utrace_barrier_pid</function>) will accept any engine pointer you
+    hold a reference on, even if <constant>UTRACE_DETACH</constant> has already
+    been used.  After any <function>utrace_control</function> or
+    <function>utrace_set_events</function> call (these do not block), you can
+    call <function>utrace_barrier</function> to block until callbacks have
+    finished.  This returns <constant>-ESRCH</constant> only if the engine is
+    completely detached (finished all callbacks).  Otherwise it waits
+    until the thread is definitely not in the midst of a callback to this
+    engine and then returns zero, but can return
+    <constant>-ERESTARTSYS</constant> if its wait is interrupted.
+  </para>
+  </sect2>
+
+</sect1>
+
+</chapter>
+
+<chapter id="core"><title>utrace core API</title>
+
+<para>
+  The utrace API is declared in <filename>&lt;linux/utrace.h&gt;</filename>.
+</para>
+
+!Iinclude/linux/utrace.h
+!Ekernel/utrace.c
+
+</chapter>
+
+<chapter id="machine"><title>Machine State</title>
+
+<para>
+  The <function>task_current_syscall</function> function can be used on any
+  valid <structname>struct task_struct</structname> at any time, and does
+  not even require that <function>utrace_attach_task</function> was used at all.
+</para>
+
+<para>
+  The other ways to access the registers and other machine-dependent state of
+  a task can only be used on a task that is at a known safe point.  The safe
+  points are all the places where <function>utrace_set_events</function> can
+  request callbacks (except for the <constant>DEATH</constant> and
+  <constant>REAP</constant> events).  So at any event callback, it is safe to
+  examine <varname>current</varname>.
+</para>
+
+<para>
+  One task can examine another only after a callback in the target task that
+  returns <constant>UTRACE_STOP</constant> so that task will not return to user
+  mode after the safe point.  This guarantees that the task will not resume
+  until the same engine uses <function>utrace_control</function>, unless the
+  task dies suddenly.  To examine safely, one must use a pair of calls to
+  <function>utrace_prepare_examine</function> and
+  <function>utrace_finish_examine</function> surrounding the calls to
+  <structname>struct user_regset</structname> functions or direct examination
+  of task data structures.  <function>utrace_prepare_examine</function> returns
+  an error if the task is not properly stopped and not dead.  After a
+  successful examination, the paired <function>utrace_finish_examine</function>
+  call returns an error if the task ever woke up during the examination.  If
+  so, any data gathered may be scrambled and should be discarded.  This means
+  there was a spurious wake-up (which should not happen), or a sudden death.
+</para>
+
+<sect1 id="regset"><title><structname>struct user_regset</structname></title>
+
+<para>
+  The <structname>struct user_regset</structname> API
+  is declared in <filename>&lt;linux/regset.h&gt;</filename>.
+</para>
+
+!Finclude/linux/regset.h
+
+</sect1>
+
+<sect1 id="task_current_syscall">
+  <title><filename>System Call Information</filename></title>
+
+<para>
+  This function is declared in <filename>&lt;linux/ptrace.h&gt;</filename>.
+</para>
+
+!Elib/syscall.c
+
+</sect1>
+
+<sect1 id="syscall"><title><filename>System Call Tracing</filename></title>
+
+<para>
+  The arch API for system call information is declared in
+  <filename>&lt;asm/syscall.h&gt;</filename>.
+  Each of these calls can be used only at system call entry tracing,
+  or can be used only at system call exit and the subsequent safe points
+  before returning to user mode.
+  At system call entry tracing means either during a
+  <structfield>report_syscall_entry</structfield> callback,
+  or any time after that callback has returned <constant>UTRACE_STOP</constant>.
+</para>
+
+!Finclude/asm-generic/syscall.h
+
+</sect1>
+
+</chapter>
+
+<chapter id="internals"><title>Kernel Internals</title>
+
+<para>
+  This chapter covers the interface to the tracing infrastructure
+  from the core of the kernel and the architecture-specific code.
+  This is for maintainers of the kernel and arch code, and not relevant
+  to using the tracing facilities described in preceding chapters.
+</para>
+
+<sect1 id="tracehook"><title>Core Calls In</title>
+
+<para>
+  These calls are declared in <filename>&lt;linux/tracehook.h&gt;</filename>.
+  The core kernel calls these functions at various important places.
+</para>
+
+!Finclude/linux/tracehook.h
+
+</sect1>
+
+<sect1 id="arch"><title>Architecture Calls Out</title>
+
+<para>
+  An arch that has done all these things sets
+  <constant>CONFIG_HAVE_ARCH_TRACEHOOK</constant>.
+  This is required to enable the <application>utrace</application> code.
+</para>
+
+<sect2 id="arch-ptrace"><title><filename>&lt;asm/ptrace.h&gt;</filename></title>
+
+<para>
+  An arch defines these in <filename>&lt;asm/ptrace.h&gt;</filename>
+  if it supports hardware single-step or block-step features.
+</para>
+
+!Finclude/linux/ptrace.h arch_has_single_step arch_has_block_step
+!Finclude/linux/ptrace.h user_enable_single_step user_enable_block_step
+!Finclude/linux/ptrace.h user_disable_single_step
+
+</sect2>
+
+<sect2 id="arch-syscall">
+  <title><filename>&lt;asm/syscall.h&gt;</filename></title>
+
+  <para>
+    An arch provides <filename>&lt;asm/syscall.h&gt;</filename> that
+    defines these as inlines, or declares them as exported functions.
+    These interfaces are described in <xref linkend="syscall"/>.
+  </para>
+
+</sect2>
+
+<sect2 id="arch-tracehook">
+  <title><filename>&lt;linux/tracehook.h&gt;</filename></title>
+
+  <para>
+    An arch must define <constant>TIF_NOTIFY_RESUME</constant>
+    and <constant>TIF_SYSCALL_TRACE</constant>
+    in its <filename>&lt;asm/thread_info.h&gt;</filename>.
+    The arch code must call the following functions, all declared
+    in <filename>&lt;linux/tracehook.h&gt;</filename> and
+    described in <xref linkend="tracehook"/>:
+
+    <itemizedlist>
+      <listitem>
+	<para><function>tracehook_notify_resume</function></para>
+      </listitem>
+      <listitem>
+	<para><function>tracehook_report_syscall_entry</function></para>
+      </listitem>
+      <listitem>
+	<para><function>tracehook_report_syscall_exit</function></para>
+      </listitem>
+      <listitem>
+	<para><function>tracehook_signal_handler</function></para>
+      </listitem>
+    </itemizedlist>
+
+  </para>
+
+</sect2>
+
+</sect1>
+
+</chapter>
+
+</book>
--- V1/fs/proc/array.c~14_UTRACE	2009-11-24 20:27:22.000000000 +0100
+++ V1/fs/proc/array.c	2009-11-24 20:30:17.000000000 +0100
@@ -82,6 +82,7 @@
 #include <linux/pid_namespace.h>
 #include <linux/ptrace.h>
 #include <linux/tracehook.h>
+#include <linux/utrace.h>
 #include <linux/swapops.h>
 
 #include <asm/pgtable.h>
@@ -189,6 +190,8 @@ static inline void task_state(struct seq
 		cred->uid, cred->euid, cred->suid, cred->fsuid,
 		cred->gid, cred->egid, cred->sgid, cred->fsgid);
 
+	task_utrace_proc_status(m, p);
+
 	task_lock(p);
 	if (p->files)
 		fdt = files_fdtable(p->files);
--- V1/include/linux/sched.h~14_UTRACE	2009-11-24 20:30:16.000000000 +0100
+++ V1/include/linux/sched.h	2009-11-24 20:30:17.000000000 +0100
@@ -1393,6 +1393,11 @@ struct task_struct {
 #endif
 	seccomp_t seccomp;
 
+#ifdef CONFIG_UTRACE
+	struct utrace *utrace;
+	unsigned long utrace_flags;
+#endif
+
 /* Thread group tracking */
    	u32 parent_exec_id;
    	u32 self_exec_id;
--- V1/include/linux/tracehook.h~14_UTRACE	2009-11-24 20:30:15.000000000 +0100
+++ V1/include/linux/tracehook.h	2009-11-24 20:30:17.000000000 +0100
@@ -49,6 +49,7 @@
 #include <linux/sched.h>
 #include <linux/ptrace.h>
 #include <linux/security.h>
+#include <linux/utrace.h>
 struct linux_binprm;
 
 /**
@@ -63,6 +64,8 @@ struct linux_binprm;
  */
 static inline int tracehook_expect_breakpoints(struct task_struct *task)
 {
+	if (unlikely(task_utrace_flags(task) & UTRACE_EVENT(SIGNAL_CORE)))
+		return 1;
 	return (task_ptrace(task) & PT_PTRACED) != 0;
 }
 
@@ -111,6 +114,9 @@ static inline void ptrace_report_syscall
 static inline __must_check int tracehook_report_syscall_entry(
 	struct pt_regs *regs)
 {
+	if ((task_utrace_flags(current) & UTRACE_EVENT(SYSCALL_ENTRY)) &&
+	    utrace_report_syscall_entry(regs))
+		return 1;
 	ptrace_report_syscall(regs);
 	return 0;
 }
@@ -134,6 +140,9 @@ static inline __must_check int tracehook
  */
 static inline void tracehook_report_syscall_exit(struct pt_regs *regs, int step)
 {
+	if (task_utrace_flags(current) & UTRACE_EVENT(SYSCALL_EXIT))
+		utrace_report_syscall_exit(regs);
+
 	if (step && (task_ptrace(current) & PT_PTRACED)) {
 		siginfo_t info;
 		user_single_step_siginfo(current, regs, &info);
@@ -201,6 +210,8 @@ static inline void tracehook_report_exec
 					 struct linux_binprm *bprm,
 					 struct pt_regs *regs)
 {
+	if (unlikely(task_utrace_flags(current) & UTRACE_EVENT(EXEC)))
+		utrace_report_exec(fmt, bprm, regs);
 	if (!ptrace_event(PT_TRACE_EXEC, PTRACE_EVENT_EXEC, 0) &&
 	    unlikely(task_ptrace(current) & PT_PTRACED))
 		send_sig(SIGTRAP, current, 0);
@@ -218,10 +229,37 @@ static inline void tracehook_report_exec
  */
 static inline void tracehook_report_exit(long *exit_code)
 {
+	if (unlikely(task_utrace_flags(current) & UTRACE_EVENT(EXIT)))
+		utrace_report_exit(exit_code);
 	ptrace_event(PT_TRACE_EXIT, PTRACE_EVENT_EXIT, *exit_code);
 }
 
 /**
+ * tracehook_init_task - task_struct has just been copied
+ * @task:		new &struct task_struct just copied from parent
+ *
+ * Called from do_fork() when @task has just been duplicated.
+ * After this, @task will be passed to tracehook_free_task()
+ * even if the rest of its setup fails before it is fully created.
+ */
+static inline void tracehook_init_task(struct task_struct *task)
+{
+	utrace_init_task(task);
+}
+
+/**
+ * tracehook_free_task - task_struct is being freed
+ * @task:		dead &struct task_struct being freed
+ *
+ * Called from free_task() when @task is no longer in use.
+ */
+static inline void tracehook_free_task(struct task_struct *task)
+{
+	if (task_utrace_struct(task))
+		utrace_free_task(task);
+}
+
+/**
  * tracehook_prepare_clone - prepare for new child to be cloned
  * @clone_flags:	%CLONE_* flags from clone/fork/vfork system call
  *
@@ -285,6 +323,8 @@ static inline void tracehook_report_clon
 					  unsigned long clone_flags,
 					  pid_t pid, struct task_struct *child)
 {
+	if (unlikely(task_utrace_flags(current) & UTRACE_EVENT(CLONE)))
+		utrace_report_clone(clone_flags, child);
 	if (unlikely(task_ptrace(child))) {
 		/*
 		 * It doesn't matter who attached/attaching to this
@@ -317,6 +357,9 @@ static inline void tracehook_report_clon
 						   pid_t pid,
 						   struct task_struct *child)
 {
+	if (unlikely(task_utrace_flags(current) & UTRACE_EVENT(CLONE)) &&
+	    (clone_flags & CLONE_VFORK))
+		utrace_finish_vfork(current);
 	if (unlikely(trace))
 		ptrace_event(0, trace, pid);
 }
@@ -351,6 +394,10 @@ static inline void tracehook_report_vfor
  */
 static inline void tracehook_prepare_release_task(struct task_struct *task)
 {
+	/* see utrace_add_engine() about this barrier */
+	smp_mb();
+	if (task_utrace_flags(task))
+		utrace_release_task(task);
 }
 
 /**
@@ -365,6 +412,7 @@ static inline void tracehook_prepare_rel
 static inline void tracehook_finish_release_task(struct task_struct *task)
 {
 	ptrace_release_task(task);
+	BUG_ON(task->exit_state != EXIT_DEAD);
 }
 
 /**
@@ -386,6 +434,8 @@ static inline void tracehook_signal_hand
 					    const struct k_sigaction *ka,
 					    struct pt_regs *regs, int stepping)
 {
+	if (task_utrace_flags(current))
+		utrace_signal_handler(current, stepping);
 	if (stepping && (task_ptrace(current) & PT_PTRACED))
 		ptrace_notify(SIGTRAP);
 }
@@ -403,6 +453,8 @@ static inline void tracehook_signal_hand
 static inline int tracehook_consider_ignored_signal(struct task_struct *task,
 						    int sig)
 {
+	if (unlikely(task_utrace_flags(task) & UTRACE_EVENT(SIGNAL_IGN)))
+		return 1;
 	return (task_ptrace(task) & PT_PTRACED) != 0;
 }
 
@@ -422,6 +474,9 @@ static inline int tracehook_consider_ign
 static inline int tracehook_consider_fatal_signal(struct task_struct *task,
 						  int sig)
 {
+	if (unlikely(task_utrace_flags(task) & (UTRACE_EVENT(SIGNAL_TERM) |
+						UTRACE_EVENT(SIGNAL_CORE))))
+		return 1;
 	return (task_ptrace(task) & PT_PTRACED) != 0;
 }
 
@@ -436,6 +491,8 @@ static inline int tracehook_consider_fat
  */
 static inline int tracehook_force_sigpending(void)
 {
+	if (unlikely(task_utrace_flags(current)))
+		return utrace_interrupt_pending();
 	return 0;
 }
 
@@ -465,6 +522,8 @@ static inline int tracehook_get_signal(s
 				       siginfo_t *info,
 				       struct k_sigaction *return_ka)
 {
+	if (unlikely(task_utrace_flags(task)))
+		return utrace_get_signal(task, regs, info, return_ka);
 	return 0;
 }
 
@@ -492,6 +551,8 @@ static inline int tracehook_get_signal(s
  */
 static inline int tracehook_notify_jctl(int notify, int why)
 {
+	if (task_utrace_flags(current) & UTRACE_EVENT(JCTL))
+		utrace_report_jctl(notify, why);
 	return notify ?: task_ptrace(current) ? why : 0;
 }
 
@@ -502,6 +563,8 @@ static inline int tracehook_notify_jctl(
  */
 static inline void tracehook_finish_jctl(void)
 {
+	if (task_utrace_flags(current))
+		utrace_finish_stop();
 }
 
 #define DEATH_REAP			-1
@@ -524,6 +587,8 @@ static inline void tracehook_finish_jctl
 static inline int tracehook_notify_death(struct task_struct *task,
 					 void **death_cookie, int group_dead)
 {
+	*death_cookie = task_utrace_struct(task);
+
 	if (task_detached(task))
 		return task->ptrace ? SIGCHLD : DEATH_REAP;
 
@@ -560,6 +625,20 @@ static inline void tracehook_report_deat
 					  int signal, void *death_cookie,
 					  int group_dead)
 {
+	/*
+	 * This barrier ensures that our caller's setting of
+	 * @task->exit_state precedes checking @task->utrace_flags here.
+	 * If utrace_set_events() was just called to enable
+	 * UTRACE_EVENT(DEATH), then we are obliged to call
+	 * utrace_report_death() and not miss it.  utrace_set_events()
+	 * uses tasklist_lock to synchronize enabling the bit with the
+	 * actual change to @task->exit_state, but we need this barrier
+	 * to be sure we see a flags change made just before our caller
+	 * took the tasklist_lock.
+	 */
+	smp_mb();
+	if (task_utrace_flags(task) & _UTRACE_DEATH_EVENTS)
+		utrace_report_death(task, death_cookie, group_dead, signal);
 }
 
 #ifdef TIF_NOTIFY_RESUME
@@ -589,10 +668,20 @@ static inline void set_notify_resume(str
  * asynchronously, this will be called again before we return to
  * user mode.
  *
- * Called without locks.
+ * Called without locks.  However, on some machines this may be
+ * called with interrupts disabled.
  */
 static inline void tracehook_notify_resume(struct pt_regs *regs)
 {
+	struct task_struct *task = current;
+	/*
+	 * This pairs with the barrier implicit in set_notify_resume().
+	 * It ensures that we read the nonzero utrace_flags set before
+	 * set_notify_resume() was called by utrace setup.
+	 */
+	smp_rmb();
+	if (task_utrace_flags(task))
+		utrace_resume(task, regs);
 }
 #endif	/* TIF_NOTIFY_RESUME */
 
--- /dev/null	2009-11-24 16:51:11.614043008 +0100
+++ V1/include/linux/utrace.h	2009-11-24 20:30:17.000000000 +0100
@@ -0,0 +1,729 @@
+/*
+ * utrace infrastructure interface for debugging user processes
+ *
+ * Copyright (C) 2006-2009 Red Hat, Inc.  All rights reserved.
+ *
+ * This copyrighted material is made available to anyone wishing to use,
+ * modify, copy, or redistribute it subject to the terms and conditions
+ * of the GNU General Public License v.2.
+ *
+ * Red Hat Author: Roland McGrath.
+ *
+ * This interface allows for notification of interesting events in a
+ * thread.  It also mediates access to thread state such as registers.
+ * Multiple unrelated users can be associated with a single thread.
+ * We call each of these a tracing engine.
+ *
+ * A tracing engine starts by calling utrace_attach_task() or
+ * utrace_attach_pid() on the chosen thread, passing in a set of hooks
+ * (&struct utrace_engine_ops), and some associated data.  This produces a
+ * &struct utrace_engine, which is the handle used for all other
+ * operations.  An attached engine has its ops vector, its data, and an
+ * event mask controlled by utrace_set_events().
+ *
+ * For each event bit that is set, that engine will get the
+ * appropriate ops->report_*() callback when the event occurs.  The
+ * &struct utrace_engine_ops need not provide callbacks for an event
+ * unless the engine sets one of the associated event bits.
+ */
+
+#ifndef _LINUX_UTRACE_H
+#define _LINUX_UTRACE_H	1
+
+#include <linux/list.h>
+#include <linux/kref.h>
+#include <linux/signal.h>
+#include <linux/sched.h>
+
+struct linux_binprm;
+struct pt_regs;
+struct utrace;
+struct user_regset;
+struct user_regset_view;
+
+/*
+ * Event bits passed to utrace_set_events().
+ * These appear in &struct task_struct.@utrace_flags
+ * and &struct utrace_engine.@flags.
+ */
+enum utrace_events {
+	_UTRACE_EVENT_QUIESCE,	/* Thread is available for examination.  */
+	_UTRACE_EVENT_REAP,  	/* Zombie reaped, no more tracing possible.  */
+	_UTRACE_EVENT_CLONE,	/* Successful clone/fork/vfork just done.  */
+	_UTRACE_EVENT_EXEC,	/* Successful execve just completed.  */
+	_UTRACE_EVENT_EXIT,	/* Thread exit in progress.  */
+	_UTRACE_EVENT_DEATH,	/* Thread has died.  */
+	_UTRACE_EVENT_SYSCALL_ENTRY, /* User entered kernel for system call. */
+	_UTRACE_EVENT_SYSCALL_EXIT, /* Returning to user after system call.  */
+	_UTRACE_EVENT_SIGNAL,	/* Signal delivery will run a user handler.  */
+	_UTRACE_EVENT_SIGNAL_IGN, /* No-op signal to be delivered.  */
+	_UTRACE_EVENT_SIGNAL_STOP, /* Signal delivery will suspend.  */
+	_UTRACE_EVENT_SIGNAL_TERM, /* Signal delivery will terminate.  */
+	_UTRACE_EVENT_SIGNAL_CORE, /* Signal delivery will dump core.  */
+	_UTRACE_EVENT_JCTL,	/* Job control stop or continue completed.  */
+	_UTRACE_NEVENTS
+};
+#define UTRACE_EVENT(type)	(1UL << _UTRACE_EVENT_##type)
+
+/*
+ * All the kinds of signal events.
+ * These all use the @report_signal() callback.
+ */
+#define UTRACE_EVENT_SIGNAL_ALL	(UTRACE_EVENT(SIGNAL) \
+				 | UTRACE_EVENT(SIGNAL_IGN) \
+				 | UTRACE_EVENT(SIGNAL_STOP) \
+				 | UTRACE_EVENT(SIGNAL_TERM) \
+				 | UTRACE_EVENT(SIGNAL_CORE))
+/*
+ * Both kinds of syscall events; these call the @report_syscall_entry()
+ * and @report_syscall_exit() callbacks, respectively.
+ */
+#define UTRACE_EVENT_SYSCALL	\
+	(UTRACE_EVENT(SYSCALL_ENTRY) | UTRACE_EVENT(SYSCALL_EXIT))
+
+/*
+ * The event reports triggered synchronously by task death.
+ */
+#define _UTRACE_DEATH_EVENTS (UTRACE_EVENT(DEATH) | UTRACE_EVENT(QUIESCE))
+
+/*
+ * Hooks in <linux/tracehook.h> call these entry points to the
+ * utrace dispatch.  They are weak references here only so
+ * tracehook.h doesn't need to #ifndef CONFIG_UTRACE them to
+ * avoid external references in case of unoptimized compilation.
+ */
+void utrace_free_task(struct task_struct *)
+	__attribute__((weak));
+bool utrace_interrupt_pending(void)
+	__attribute__((weak));
+void utrace_resume(struct task_struct *, struct pt_regs *)
+	__attribute__((weak));
+void utrace_finish_stop(void)
+	__attribute__((weak));
+int utrace_get_signal(struct task_struct *, struct pt_regs *,
+		      siginfo_t *, struct k_sigaction *)
+	__attribute__((weak));
+void utrace_report_clone(unsigned long, struct task_struct *)
+	__attribute__((weak));
+void utrace_finish_vfork(struct task_struct *)
+	__attribute__((weak));
+void utrace_report_exit(long *exit_code)
+	__attribute__((weak));
+void utrace_report_death(struct task_struct *, struct utrace *, bool, int)
+	__attribute__((weak));
+void utrace_report_jctl(int notify, int type)
+	__attribute__((weak));
+void utrace_report_exec(struct linux_binfmt *, struct linux_binprm *,
+			struct pt_regs *regs)
+	__attribute__((weak));
+bool utrace_report_syscall_entry(struct pt_regs *)
+	__attribute__((weak));
+void utrace_report_syscall_exit(struct pt_regs *)
+	__attribute__((weak));
+void utrace_signal_handler(struct task_struct *, int)
+	__attribute__((weak));
+
+#ifndef CONFIG_UTRACE
+
+/*
+ * <linux/tracehook.h> uses these accessors to avoid #ifdef CONFIG_UTRACE.
+ */
+static inline unsigned long task_utrace_flags(struct task_struct *task)
+{
+	return 0;
+}
+static inline struct utrace *task_utrace_struct(struct task_struct *task)
+{
+	return NULL;
+}
+static inline void utrace_init_task(struct task_struct *child)
+{
+}
+static inline void utrace_release_task(struct task_struct *task)
+{
+}
+
+static inline void task_utrace_proc_status(struct seq_file *m,
+					   struct task_struct *p)
+{
+}
+
+#else  /* CONFIG_UTRACE */
+
+static inline unsigned long task_utrace_flags(struct task_struct *task)
+{
+	return task->utrace_flags;
+}
+
+static inline struct utrace *task_utrace_struct(struct task_struct *task)
+{
+	struct utrace *utrace;
+
+	/*
+	 * This barrier ensures that any prior load of task->utrace_flags
+	 * is ordered before this load of task->utrace.  We use those
+	 * utrace_flags checks in the hot path to decide to call into
+	 * the utrace code.  The first attach installs task->utrace before
+	 * setting task->utrace_flags nonzero, with a barrier between.
+	 * See utrace_task_alloc().
+	 */
+	smp_rmb();
+	utrace = task->utrace;
+
+	smp_read_barrier_depends(); /* See utrace_task_alloc().  */
+	return utrace;
+}
+
+static inline void utrace_init_task(struct task_struct *task)
+{
+	task->utrace_flags = 0;
+	task->utrace = NULL;
+}
+
+void utrace_release_task(struct task_struct *);
+void task_utrace_proc_status(struct seq_file *m, struct task_struct *p);
+
+
+/*
+ * Version number of the API defined in this file.  This will change
+ * whenever a tracing engine's code would need some updates to keep
+ * working.  We maintain this here for the benefit of tracing engine code
+ * that is developed concurrently with utrace API improvements before they
+ * are merged into the kernel, making LINUX_VERSION_CODE checks unwieldy.
+ */
+#define UTRACE_API_VERSION	20090421
+
+/**
+ * enum utrace_resume_action - engine's choice of action for a traced task
+ * @UTRACE_STOP:		Stay quiescent after callbacks.
+ * @UTRACE_INTERRUPT:		Make @report_signal() callback soon.
+ * @UTRACE_REPORT:		Make some callback soon.
+ * @UTRACE_SINGLESTEP:		Resume in user mode for one instruction.
+ * @UTRACE_BLOCKSTEP:		Resume in user mode until next branch.
+ * @UTRACE_RESUME:		Resume normally in user mode.
+ * @UTRACE_DETACH:		Detach my engine (implies %UTRACE_RESUME).
+ *
+ * See utrace_control() for detailed descriptions of each action.  This is
+ * encoded in the @action argument and the return value for every callback
+ * with a &u32 return value.
+ *
+ * The order of these is important.  When there is more than one engine,
+ * each supplies its choice and the smallest value prevails.
+ */
+enum utrace_resume_action {
+	UTRACE_STOP,
+	UTRACE_INTERRUPT,
+	UTRACE_REPORT,
+	UTRACE_SINGLESTEP,
+	UTRACE_BLOCKSTEP,
+	UTRACE_RESUME,
+	UTRACE_DETACH
+};
+#define	UTRACE_RESUME_MASK	0x07
+
+/**
+ * utrace_resume_action - &enum utrace_resume_action from callback action
+ * @action:		&u32 callback @action argument or return value
+ *
+ * This extracts the &enum utrace_resume_action from @action,
+ * which is the @action argument to a &struct utrace_engine_ops
+ * callback or the return value from one.
+ */
+static inline enum utrace_resume_action utrace_resume_action(u32 action)
+{
+	return action & UTRACE_RESUME_MASK;
+}
+
+/**
+ * enum utrace_signal_action - disposition of signal
+ * @UTRACE_SIGNAL_DELIVER:	Deliver according to sigaction.
+ * @UTRACE_SIGNAL_IGN:		Ignore the signal.
+ * @UTRACE_SIGNAL_TERM:		Terminate the process.
+ * @UTRACE_SIGNAL_CORE:		Terminate with core dump.
+ * @UTRACE_SIGNAL_STOP:		Deliver as absolute stop.
+ * @UTRACE_SIGNAL_TSTP:		Deliver as job control stop.
+ * @UTRACE_SIGNAL_REPORT:	Reporting before pending signals.
+ * @UTRACE_SIGNAL_HANDLER:	Reporting after signal handler setup.
+ *
+ * This is encoded in the @action argument and the return value for
+ * a @report_signal() callback.  It says what will happen to the
+ * signal described by the &siginfo_t parameter to the callback.
+ *
+ * The %UTRACE_SIGNAL_REPORT value is used in an @action argument when
+ * a tracing report is being made before dequeuing any pending signal.
+ * If this is immediately after a signal handler has been set up, then
+ * %UTRACE_SIGNAL_HANDLER is used instead.  A @report_signal callback
+ * that uses %UTRACE_SIGNAL_DELIVER|%UTRACE_SINGLESTEP will ensure
+ * it sees a %UTRACE_SIGNAL_HANDLER report.
+ */
+enum utrace_signal_action {
+	UTRACE_SIGNAL_DELIVER	= 0x00,
+	UTRACE_SIGNAL_IGN	= 0x10,
+	UTRACE_SIGNAL_TERM	= 0x20,
+	UTRACE_SIGNAL_CORE	= 0x30,
+	UTRACE_SIGNAL_STOP	= 0x40,
+	UTRACE_SIGNAL_TSTP	= 0x50,
+	UTRACE_SIGNAL_REPORT	= 0x60,
+	UTRACE_SIGNAL_HANDLER	= 0x70
+};
+#define	UTRACE_SIGNAL_MASK	0xf0
+#define UTRACE_SIGNAL_HOLD	0x100 /* Flag, push signal back on queue.  */
+
+/**
+ * utrace_signal_action - &enum utrace_signal_action from callback action
+ * @action:		@report_signal callback @action argument or return value
+ *
+ * This extracts the &enum utrace_signal_action from @action, which
+ * is the @action argument to a @report_signal callback or the
+ * return value from one.
+ */
+static inline enum utrace_signal_action utrace_signal_action(u32 action)
+{
+	return action & UTRACE_SIGNAL_MASK;
+}
+
+/**
+ * enum utrace_syscall_action - disposition of system call attempt
+ * @UTRACE_SYSCALL_RUN:		Run the system call.
+ * @UTRACE_SYSCALL_ABORT:	Don't run the system call.
+ *
+ * This is encoded in the @action argument and the return value for
+ * a @report_syscall_entry callback.
+ */
+enum utrace_syscall_action {
+	UTRACE_SYSCALL_RUN	= 0x00,
+	UTRACE_SYSCALL_ABORT	= 0x10
+};
+#define	UTRACE_SYSCALL_MASK	0xf0
+#define	UTRACE_SYSCALL_RESUMED	0x100 /* Flag, report_syscall_entry() repeats */
+
+/**
+ * utrace_syscall_action - &enum utrace_syscall_action from callback action
+ * @action:		@report_syscall_entry callback @action or return value
+ *
+ * This extracts the &enum utrace_syscall_action from @action, which
+ * is the @action argument to a @report_syscall_entry callback or the
+ * return value from one.
+ */
+static inline enum utrace_syscall_action utrace_syscall_action(u32 action)
+{
+	return action & UTRACE_SYSCALL_MASK;
+}
+
+/*
+ * Flags for utrace_attach_task() and utrace_attach_pid().
+ */
+#define UTRACE_ATTACH_CREATE		0x0010 /* Attach a new engine.  */
+#define UTRACE_ATTACH_EXCLUSIVE		0x0020 /* Refuse if existing match.  */
+#define UTRACE_ATTACH_MATCH_OPS		0x0001 /* Match engines on ops.  */
+#define UTRACE_ATTACH_MATCH_DATA	0x0002 /* Match engines on data.  */
+#define UTRACE_ATTACH_MATCH_MASK	0x000f
+
+/**
+ * struct utrace_engine - per-engine structure
+ * @ops:	&struct utrace_engine_ops pointer passed to utrace_attach_task()
+ * @data:	engine-private &void * passed to utrace_attach_task()
+ * @flags:	event mask set by utrace_set_events() plus internal flag bits
+ *
+ * The task itself never has to worry about engines detaching while
+ * it's doing event callbacks.  These structures are removed from the
+ * task's active list only when it's stopped, or by the task itself.
+ *
+ * utrace_engine_get() and utrace_engine_put() maintain a reference count.
+ * When it drops to zero, the structure is freed.  One reference is held
+ * implicitly while the engine is attached to its task.
+ */
+struct utrace_engine {
+/* private: */
+	struct kref kref;
+	void (*release)(void *);
+	struct list_head entry;
+
+/* public: */
+	const struct utrace_engine_ops *ops;
+	void *data;
+
+	unsigned long flags;
+};
+
+/**
+ * utrace_engine_get - acquire a reference on a &struct utrace_engine
+ * @engine:	&struct utrace_engine pointer
+ *
+ * You must hold a reference on @engine, and you get another.
+ */
+static inline void utrace_engine_get(struct utrace_engine *engine)
+{
+	kref_get(&engine->kref);
+}
+
+void __utrace_engine_release(struct kref *);
+
+/**
+ * utrace_engine_put - release a reference on a &struct utrace_engine
+ * @engine:	&struct utrace_engine pointer
+ *
+ * You must hold a reference on @engine, and you lose that reference.
+ * If it was the last one, @engine becomes an invalid pointer.
+ */
+static inline void utrace_engine_put(struct utrace_engine *engine)
+{
+	kref_put(&engine->kref, __utrace_engine_release);
+}
+
+/**
+ * struct utrace_engine_ops - tracing engine callbacks
+ *
+ * Each @report_*() callback corresponds to an %UTRACE_EVENT(*) bit.
+ * utrace_set_events() calls on @engine choose which callbacks will be made
+ * to @engine from @task.
+ *
+ * Most callbacks take an @action argument, giving the resume action
+ * chosen by other tracing engines.  All callbacks take an @engine
+ * argument, and a @task argument, which is always equal to @current.
+ * For some calls, @action also includes bits specific to that event
+ * and utrace_resume_action() is used to extract the resume action.
+ * This shows what would happen if @engine wasn't there, or will if
+ * the callback's return value uses %UTRACE_RESUME.  This always
+ * starts as %UTRACE_RESUME when no other tracing is being done on
+ * this task.
+ *
+ * All return values contain &enum utrace_resume_action bits.  For
+ * some calls, other bits specific to that kind of event are added to
+ * the resume action bits with OR.  These are the same bits used in
+ * the @action argument.  The resume action returned by a callback
+ * does not override previous engines' choices, it only says what
+ * @engine wants done.  What @task actually does is the action that's
+ * most constrained among the choices made by all attached engines.
+ * See utrace_control() for more information on the actions.
+ *
+ * When %UTRACE_STOP is used in @report_syscall_entry, then @task
+ * stops before attempting the system call.  In this case, another
+ * @report_syscall_entry callback will follow after @task resumes if
+ * %UTRACE_REPORT or %UTRACE_INTERRUPT was returned by some callback
+ * or passed to utrace_control().  In a second or later callback,
+ * %UTRACE_SYSCALL_RESUMED is set in the @action argument to indicate
+ * a repeat callback still waiting to attempt the same system call
+ * invocation.  This repeat callback gives each engine an opportunity
+ * to reexamine registers another engine might have changed while
+ * @task was held in %UTRACE_STOP.
+ *
+ * In other cases, the resume action does not take effect until @task
+ * is ready to check for signals and return to user mode.  If there
+ * are more callbacks to be made, the last round of calls determines
+ * the final action.  A @report_quiesce callback with @event zero, or
+ * a @report_signal callback, will always be the last one made before
+ * @task resumes.  Only %UTRACE_STOP is "sticky"--if @engine returned
+ * %UTRACE_STOP then @task stays stopped unless @engine returns
+ * different from a following callback.
+ *
+ * The report_death() and report_reap() callbacks do not take @action
+ * arguments, and only %UTRACE_DETACH is meaningful in the return value
+ * from a report_death() callback.  None of the resume actions applies
+ * to a dead thread.
+ *
+ * All @report_*() hooks are called with no locks held, in a generally
+ * safe environment when we will be returning to user mode soon (or just
+ * entered the kernel).  It is fine to block for memory allocation and
+ * the like, but all hooks are asynchronous and must not block on
+ * external events!  If you want the thread to block, use %UTRACE_STOP
+ * in your hook's return value; then later wake it up with utrace_control().
+ *
+ * @report_quiesce:
+ *	Requested by %UTRACE_EVENT(%QUIESCE).
+ *	This does not indicate any event, but just that @task (the current
+ *	thread) is in a safe place for examination.  This call is made
+ *	before each specific event callback, except for @report_reap.
+ *	The @event argument gives the %UTRACE_EVENT(@which) value for
+ *	the event occurring.  This callback might be made for events @engine
+ *	has not requested, if some other engine is tracing the event;
+ *	calling utrace_set_events() call here can request the immediate
+ *	callback for this occurrence of @event.  @event is zero when there
+ *	is no other event, @task is now ready to check for signals and
+ *	return to user mode, and some engine has used %UTRACE_REPORT or
+ *	%UTRACE_INTERRUPT to request this callback.  For this case,
+ *	if @report_signal is not %NULL, the @report_quiesce callback
+ *	may be replaced with a @report_signal callback passing
+ *	%UTRACE_SIGNAL_REPORT in its @action argument, whenever @task is
+ *	entering the signal-check path anyway.
+ *
+ * @report_signal:
+ *	Requested by %UTRACE_EVENT(%SIGNAL_*) or %UTRACE_EVENT(%QUIESCE).
+ *	Use utrace_signal_action() and utrace_resume_action() on @action.
+ *	The signal action is %UTRACE_SIGNAL_REPORT when some engine has
+ *	used %UTRACE_REPORT or %UTRACE_INTERRUPT; the callback can choose
+ *	to stop or to deliver an artificial signal, before pending signals.
+ *	It's %UTRACE_SIGNAL_HANDLER instead when signal handler setup just
+ *	finished (after a previous %UTRACE_SIGNAL_DELIVER return); this
+ *	serves in lieu of any %UTRACE_SIGNAL_REPORT callback requested by
+ *	%UTRACE_REPORT or %UTRACE_INTERRUPT, and is also implicitly
+ *	requested by %UTRACE_SINGLESTEP or %UTRACE_BLOCKSTEP into the
+ *	signal delivery.  The other signal actions indicate a signal about
+ *	to be delivered; the previous engine's return value sets the signal
+ *	action seen by the the following engine's callback.  The @info data
+ *	can be changed at will, including @info->si_signo.  The settings in
+ *	@return_ka determines what %UTRACE_SIGNAL_DELIVER does.  @orig_ka
+ *	is what was in force before other tracing engines intervened, and
+ *	it's %NULL when this report began as %UTRACE_SIGNAL_REPORT or
+ *	%UTRACE_SIGNAL_HANDLER.  For a report without a new signal, @info
+ *	is left uninitialized and must be set completely by an engine that
+ *	chooses to deliver a signal; if there was a previous @report_signal
+ *	callback ending in %UTRACE_STOP and it was just resumed using
+ *	%UTRACE_REPORT or %UTRACE_INTERRUPT, then @info is left unchanged
+ *	from the previous callback.  In this way, the original signal can
+ *	be left in @info while returning %UTRACE_STOP|%UTRACE_SIGNAL_IGN
+ *	and then found again when resuming @task with %UTRACE_INTERRUPT.
+ *	The %UTRACE_SIGNAL_HOLD flag bit can be OR'd into the return value,
+ *	and might be in @action if the previous engine returned it.  This
+ *	flag asks that the signal in @info be pushed back on @task's queue
+ *	so that it will be seen again after whatever action is taken now.
+ *
+ * @report_clone:
+ *	Requested by %UTRACE_EVENT(%CLONE).
+ *	Event reported for parent, before the new task @child might run.
+ *	@clone_flags gives the flags used in the clone system call,
+ *	or equivalent flags for a fork() or vfork() system call.
+ *	This function can use utrace_attach_task() on @child.  It's guaranteed
+ *	that asynchronous utrace_attach_task() calls will be ordered after
+ *	any calls in @report_clone callbacks for the parent.  Thus
+ *	when using %UTRACE_ATTACH_EXCLUSIVE in the asynchronous calls,
+ *	you can be sure that the parent's @report_clone callback has
+ *	already attached to @child or chosen not to.  Passing %UTRACE_STOP
+ *	to utrace_control() on @child here keeps the child stopped before
+ *	it ever runs in user mode, %UTRACE_REPORT or %UTRACE_INTERRUPT
+ *	ensures a callback from @child before it starts in user mode.
+ *
+ * @report_jctl:
+ *	Requested by %UTRACE_EVENT(%JCTL).
+ *	Job control event; @type is %CLD_STOPPED or %CLD_CONTINUED,
+ *	indicating whether we are stopping or resuming now.  If @notify
+ *	is nonzero, @task is the last thread to stop and so will send
+ *	%SIGCHLD to its parent after this callback; @notify reflects
+ *	what the parent's %SIGCHLD has in @si_code, which can sometimes
+ *	be %CLD_STOPPED even when @type is %CLD_CONTINUED.
+ *
+ * @report_exec:
+ *	Requested by %UTRACE_EVENT(%EXEC).
+ *	An execve system call has succeeded and the new program is about to
+ *	start running.  The initial user register state is handy to be tweaked
+ *	directly in @regs.  @fmt and @bprm gives the details of this exec.
+ *
+ * @report_syscall_entry:
+ *	Requested by %UTRACE_EVENT(%SYSCALL_ENTRY).
+ *	Thread has entered the kernel to request a system call.
+ *	The user register state is handy to be tweaked directly in @regs.
+ *	The @action argument contains an &enum utrace_syscall_action,
+ *	use utrace_syscall_action() to extract it.  The return value
+ *	overrides the last engine's action for the system call.
+ *	If the final action is %UTRACE_SYSCALL_ABORT, no system call
+ *	is made.  The details of the system call being attempted can
+ *	be fetched here with syscall_get_nr() and syscall_get_arguments().
+ *	The parameter registers can be changed with syscall_set_arguments().
+ *	See above about the %UTRACE_SYSCALL_RESUMED flag in @action.
+ *	Use %UTRACE_REPORT in the return value to guarantee you get
+ *	another callback (with %UTRACE_SYSCALL_RESUMED flag) in case
+ *	@task stops with %UTRACE_STOP before attempting the system call.
+ *
+ * @report_syscall_exit:
+ *	Requested by %UTRACE_EVENT(%SYSCALL_EXIT).
+ *	Thread is about to leave the kernel after a system call request.
+ *	The user register state is handy to be tweaked directly in @regs.
+ *	The results of the system call attempt can be examined here using
+ *	syscall_get_error() and syscall_get_return_value().  It is safe
+ *	here to call syscall_set_return_value() or syscall_rollback().
+ *
+ * @report_exit:
+ *	Requested by %UTRACE_EVENT(%EXIT).
+ *	Thread is exiting and cannot be prevented from doing so,
+ *	but all its state is still live.  The @code value will be
+ *	the wait result seen by the parent, and can be changed by
+ *	this engine or others.  The @orig_code value is the real
+ *	status, not changed by any tracing engine.  Returning %UTRACE_STOP
+ *	here keeps @task stopped before it cleans up its state and dies,
+ *	so it can be examined by other processes.  When @task is allowed
+ *	to run, it will die and get to the @report_death callback.
+ *
+ * @report_death:
+ *	Requested by %UTRACE_EVENT(%DEATH).
+ *	Thread is really dead now.  It might be reaped by its parent at
+ *	any time, or self-reap immediately.  Though the actual reaping
+ *	may happen in parallel, a report_reap() callback will always be
+ *	ordered after a report_death() callback.
+ *
+ * @report_reap:
+ *	Requested by %UTRACE_EVENT(%REAP).
+ *	Called when someone reaps the dead task (parent, init, or self).
+ *	This means the parent called wait, or else this was a detached
+ *	thread or a process whose parent ignores SIGCHLD.
+ *	No more callbacks are made after this one.
+ *	The engine is always detached.
+ *	There is nothing more a tracing engine can do about this thread.
+ *	After this callback, the @engine pointer will become invalid.
+ *	The @task pointer may become invalid if get_task_struct() hasn't
+ *	been used to keep it alive.
+ *	An engine should always request this callback if it stores the
+ *	@engine pointer or stores any pointer in @engine->data, so it
+ *	can clean up its data structures.
+ *	Unlike other callbacks, this can be called from the parent's context
+ *	rather than from the traced thread itself--it must not delay the
+ *	parent by blocking.
+ *
+ * @release:
+ *	If not %NULL, this is called after the last utrace_engine_put()
+ *	call for a &struct utrace_engine, which could be implicit after
+ *	a %UTRACE_DETACH return from another callback.  Its argument is
+ *	the engine's @data member.
+ */
+struct utrace_engine_ops {
+	u32 (*report_quiesce)(enum utrace_resume_action action,
+			      struct utrace_engine *engine,
+			      struct task_struct *task,
+			      unsigned long event);
+	u32 (*report_signal)(u32 action,
+			     struct utrace_engine *engine,
+			     struct task_struct *task,
+			     struct pt_regs *regs,
+			     siginfo_t *info,
+			     const struct k_sigaction *orig_ka,
+			     struct k_sigaction *return_ka);
+	u32 (*report_clone)(enum utrace_resume_action action,
+			    struct utrace_engine *engine,
+			    struct task_struct *parent,
+			    unsigned long clone_flags,
+			    struct task_struct *child);
+	u32 (*report_jctl)(enum utrace_resume_action action,
+			   struct utrace_engine *engine,
+			   struct task_struct *task,
+			   int type, int notify);
+	u32 (*report_exec)(enum utrace_resume_action action,
+			   struct utrace_engine *engine,
+			   struct task_struct *task,
+			   const struct linux_binfmt *fmt,
+			   const struct linux_binprm *bprm,
+			   struct pt_regs *regs);
+	u32 (*report_syscall_entry)(u32 action,
+				    struct utrace_engine *engine,
+				    struct task_struct *task,
+				    struct pt_regs *regs);
+	u32 (*report_syscall_exit)(enum utrace_resume_action action,
+				   struct utrace_engine *engine,
+				   struct task_struct *task,
+				   struct pt_regs *regs);
+	u32 (*report_exit)(enum utrace_resume_action action,
+			   struct utrace_engine *engine,
+			   struct task_struct *task,
+			   long orig_code, long *code);
+	u32 (*report_death)(struct utrace_engine *engine,
+			    struct task_struct *task,
+			    bool group_dead, int signal);
+	void (*report_reap)(struct utrace_engine *engine,
+			    struct task_struct *task);
+	void (*release)(void *data);
+};
+
+/**
+ * struct utrace_examiner - private state for using utrace_prepare_examine()
+ *
+ * The members of &struct utrace_examiner are private to the implementation.
+ * This data type holds the state from a call to utrace_prepare_examine()
+ * to be used by a call to utrace_finish_examine().
+ */
+struct utrace_examiner {
+/* private: */
+	long state;
+	unsigned long ncsw;
+};
+
+/*
+ * These are the exported entry points for tracing engines to use.
+ * See kernel/utrace.c for their kerneldoc comments with interface details.
+ */
+struct utrace_engine *utrace_attach_task(struct task_struct *, int,
+					 const struct utrace_engine_ops *,
+					 void *);
+struct utrace_engine *utrace_attach_pid(struct pid *, int,
+					const struct utrace_engine_ops *,
+					void *);
+int __must_check utrace_control(struct task_struct *,
+				struct utrace_engine *,
+				enum utrace_resume_action);
+int __must_check utrace_set_events(struct task_struct *,
+				   struct utrace_engine *,
+				   unsigned long eventmask);
+int __must_check utrace_barrier(struct task_struct *,
+				struct utrace_engine *);
+int __must_check utrace_prepare_examine(struct task_struct *,
+					struct utrace_engine *,
+					struct utrace_examiner *);
+int __must_check utrace_finish_examine(struct task_struct *,
+				       struct utrace_engine *,
+				       struct utrace_examiner *);
+
+/**
+ * utrace_control_pid - control a thread being traced by a tracing engine
+ * @pid:		thread to affect
+ * @engine:		attached engine to affect
+ * @action:		&enum utrace_resume_action for thread to do
+ *
+ * This is the same as utrace_control(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+static inline __must_check int utrace_control_pid(
+	struct pid *pid, struct utrace_engine *engine,
+	enum utrace_resume_action action)
+{
+	/*
+	 * We don't bother with rcu_read_lock() here to protect the
+	 * task_struct pointer, because utrace_control will return
+	 * -ESRCH without looking at that pointer if the engine is
+	 * already detached.  A task_struct pointer can't die before
+	 * all the engines are detached in release_task() first.
+	 */
+	struct task_struct *task = pid_task(pid, PIDTYPE_PID);
+	return unlikely(!task) ? -ESRCH : utrace_control(task, engine, action);
+}
+
+/**
+ * utrace_set_events_pid - choose which event reports a tracing engine gets
+ * @pid:		thread to affect
+ * @engine:		attached engine to affect
+ * @eventmask:		new event mask
+ *
+ * This is the same as utrace_set_events(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+static inline __must_check int utrace_set_events_pid(
+	struct pid *pid, struct utrace_engine *engine, unsigned long eventmask)
+{
+	struct task_struct *task = pid_task(pid, PIDTYPE_PID);
+	return unlikely(!task) ? -ESRCH :
+		utrace_set_events(task, engine, eventmask);
+}
+
+/**
+ * utrace_barrier_pid - synchronize with simultaneous tracing callbacks
+ * @pid:		thread to affect
+ * @engine:		engine to affect (can be detached)
+ *
+ * This is the same as utrace_barrier(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+static inline __must_check int utrace_barrier_pid(struct pid *pid,
+						  struct utrace_engine *engine)
+{
+	struct task_struct *task = pid_task(pid, PIDTYPE_PID);
+	return unlikely(!task) ? -ESRCH : utrace_barrier(task, engine);
+}
+
+#endif	/* CONFIG_UTRACE */
+
+#endif	/* linux/utrace.h */
--- V1/init/Kconfig~14_UTRACE	2009-11-24 20:27:22.000000000 +0100
+++ V1/init/Kconfig	2009-11-24 20:46:05.000000000 +0100
@@ -295,6 +295,15 @@ config AUDIT
 	  logging of avc messages output).  Does not do system-call
 	  auditing without CONFIG_AUDITSYSCALL.
 
+config UTRACE
+	bool "Infrastructure for tracing and debugging user processes"
+	depends on EXPERIMENTAL
+	depends on HAVE_ARCH_TRACEHOOK
+	help
+	  Enable the utrace process tracing interface.  This is an internal
+	  kernel interface exported to kernel modules, to track events in
+	  user threads, extract and change user thread state.
+
 config AUDITSYSCALL
 	bool "Enable system-call auditing support"
 	depends on AUDIT && (X86 || PPC || S390 || IA64 || UML || SPARC64 || SUPERH)
--- V1/kernel/fork.c~14_UTRACE	2009-11-24 20:27:22.000000000 +0100
+++ V1/kernel/fork.c	2009-11-24 20:30:17.000000000 +0100
@@ -152,6 +152,7 @@ void free_task(struct task_struct *tsk)
 	free_thread_info(tsk->stack);
 	rt_mutex_debug_task_free(tsk);
 	ftrace_graph_exit_task(tsk);
+	tracehook_free_task(tsk);
 	free_task_struct(tsk);
 }
 EXPORT_SYMBOL(free_task);
@@ -1018,6 +1019,8 @@ static struct task_struct *copy_process(
 	if (!p)
 		goto fork_out;
 
+	tracehook_init_task(p);
+
 	ftrace_graph_init_task(p);
 
 	rt_mutex_init_task(p);
--- V1/kernel/Makefile~14_UTRACE	2009-11-24 20:30:17.000000000 +0100
+++ V1/kernel/Makefile	2009-11-24 20:30:17.000000000 +0100
@@ -68,6 +68,7 @@ obj-$(CONFIG_IKCONFIG) += configs.o
 obj-$(CONFIG_RESOURCE_COUNTERS) += res_counter.o
 obj-$(CONFIG_STOP_MACHINE) += stop_machine.o
 obj-$(CONFIG_KPROBES_SANITY_TEST) += test_kprobes.o
+obj-$(CONFIG_UTRACE) += utrace.o
 obj-$(CONFIG_UTRACE) += ptrace-utrace.o
 obj-$(CONFIG_AUDIT) += audit.o auditfilter.o audit_watch.o
 obj-$(CONFIG_AUDITSYSCALL) += auditsc.o
--- /dev/null	2009-11-24 16:51:11.614043008 +0100
+++ V1/kernel/utrace.c	2009-11-24 20:30:17.000000000 +0100
@@ -0,0 +1,2430 @@
+/*
+ * utrace infrastructure interface for debugging user processes
+ *
+ * Copyright (C) 2006-2009 Red Hat, Inc.  All rights reserved.
+ *
+ * This copyrighted material is made available to anyone wishing to use,
+ * modify, copy, or redistribute it subject to the terms and conditions
+ * of the GNU General Public License v.2.
+ *
+ * Red Hat Author: Roland McGrath.
+ */
+
+#include <linux/utrace.h>
+#include <linux/tracehook.h>
+#include <linux/regset.h>
+#include <asm/syscall.h>
+#include <linux/ptrace.h>
+#include <linux/err.h>
+#include <linux/sched.h>
+#include <linux/freezer.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/slab.h>
+#include <linux/seq_file.h>
+
+
+/*
+ * Per-thread structure private to utrace implementation.
+ * If task_struct.utrace_flags is nonzero, task_struct.utrace
+ * has always been allocated first.  Once allocated, it is
+ * never freed until free_task().
+ *
+ * The common event reporting loops are done by the task making the
+ * report without ever taking any locks.  To facilitate this, the two
+ * lists @attached and @attaching work together for smooth asynchronous
+ * attaching with low overhead.  Modifying either list requires @lock.
+ * The @attaching list can be modified any time while holding @lock.
+ * New engines being attached always go on this list.
+ *
+ * The @attached list is what the task itself uses for its reporting
+ * loops.  When the task itself is not quiescent, it can use the
+ * @attached list without taking any lock.  Nobody may modify the list
+ * when the task is not quiescent.  When it is quiescent, that means
+ * that it won't run again without taking @lock itself before using
+ * the list.
+ *
+ * At each place where we know the task is quiescent (or it's current),
+ * while holding @lock, we call splice_attaching(), below.  This moves
+ * the @attaching list members on to the end of the @attached list.
+ * Since this happens at the start of any reporting pass, any new
+ * engines attached asynchronously go on the stable @attached list
+ * in time to have their callbacks seen.
+ */
+struct utrace {
+	spinlock_t lock;
+	struct list_head attached, attaching;
+
+	struct task_struct *cloning;
+
+	struct utrace_engine *reporting;
+
+	enum utrace_resume_action resume:3;
+	unsigned int signal_handler:1;
+	unsigned int vfork_stop:1; /* need utrace_stop() before vfork wait */
+	unsigned int death:1;	/* in utrace_report_death() now */
+	unsigned int reap:1;	/* release_task() has run */
+	unsigned int pending_attach:1; /* need splice_attaching() */
+};
+
+static struct kmem_cache *utrace_cachep;
+static struct kmem_cache *utrace_engine_cachep;
+static const struct utrace_engine_ops utrace_detached_ops; /* forward decl */
+
+static int __init utrace_init(void)
+{
+	utrace_cachep = KMEM_CACHE(utrace, SLAB_PANIC);
+	utrace_engine_cachep = KMEM_CACHE(utrace_engine, SLAB_PANIC);
+	return 0;
+}
+module_init(utrace_init);
+
+/*
+ * Set up @task.utrace for the first time.  We can have races
+ * between two utrace_attach_task() calls here.  The task_lock()
+ * governs installing the new pointer.  If another one got in first,
+ * we just punt the new one we allocated.
+ *
+ * This returns false only in case of a memory allocation failure.
+ */
+static bool utrace_task_alloc(struct task_struct *task)
+{
+	struct utrace *utrace = kmem_cache_zalloc(utrace_cachep, GFP_KERNEL);
+	if (unlikely(!utrace))
+		return false;
+	spin_lock_init(&utrace->lock);
+	INIT_LIST_HEAD(&utrace->attached);
+	INIT_LIST_HEAD(&utrace->attaching);
+	utrace->resume = UTRACE_RESUME;
+	task_lock(task);
+	if (likely(!task->utrace)) {
+		/*
+		 * This barrier makes sure the initialization of the struct
+		 * precedes the installation of the pointer.  This pairs
+		 * with smp_read_barrier_depends() in task_utrace_struct().
+		 */
+		smp_wmb();
+		task->utrace = utrace;
+	}
+	task_unlock(task);
+	/*
+	 * That unlock after storing task->utrace acts as a memory barrier
+	 * ordering any subsequent task->utrace_flags store afterwards.
+	 * This pairs with smp_rmb() in task_utrace_struct().
+	 */
+	if (unlikely(task->utrace != utrace))
+		kmem_cache_free(utrace_cachep, utrace);
+	return true;
+}
+
+/*
+ * This is called via tracehook_free_task() from free_task()
+ * when @task is being deallocated.
+ */
+void utrace_free_task(struct task_struct *task)
+{
+	kmem_cache_free(utrace_cachep, task->utrace);
+}
+
+/*
+ * This is called with @utrace->lock held when the task is safely
+ * quiescent, i.e. it won't consult utrace->attached without the lock.
+ * Move any engines attached asynchronously from @utrace->attaching
+ * onto the @utrace->attached list.
+ */
+static void splice_attaching(struct utrace *utrace)
+{
+	list_splice_tail_init(&utrace->attaching, &utrace->attached);
+	utrace->pending_attach = 0;
+}
+
+/*
+ * This is the exported function used by the utrace_engine_put() inline.
+ */
+void __utrace_engine_release(struct kref *kref)
+{
+	struct utrace_engine *engine = container_of(kref, struct utrace_engine,
+						    kref);
+	BUG_ON(!list_empty(&engine->entry));
+	if (engine->release)
+		(*engine->release)(engine->data);
+	kmem_cache_free(utrace_engine_cachep, engine);
+}
+EXPORT_SYMBOL_GPL(__utrace_engine_release);
+
+static bool engine_matches(struct utrace_engine *engine, int flags,
+			   const struct utrace_engine_ops *ops, void *data)
+{
+	if ((flags & UTRACE_ATTACH_MATCH_OPS) && engine->ops != ops)
+		return false;
+	if ((flags & UTRACE_ATTACH_MATCH_DATA) && engine->data != data)
+		return false;
+	return engine->ops && engine->ops != &utrace_detached_ops;
+}
+
+static struct utrace_engine *matching_engine(
+	struct utrace *utrace, int flags,
+	const struct utrace_engine_ops *ops, void *data)
+{
+	struct utrace_engine *engine;
+	list_for_each_entry(engine, &utrace->attached, entry)
+		if (engine_matches(engine, flags, ops, data))
+			return engine;
+	list_for_each_entry(engine, &utrace->attaching, entry)
+		if (engine_matches(engine, flags, ops, data))
+			return engine;
+	return NULL;
+}
+
+/*
+ * Called without locks, when we might be the first utrace engine to attach.
+ * If this is a newborn thread and we are not the creator, we have to wait
+ * for it.  The creator gets the first chance to attach.  The PF_STARTING
+ * flag is cleared after its report_clone hook has had a chance to run.
+ */
+static inline int utrace_attach_delay(struct task_struct *target)
+{
+	if ((target->flags & PF_STARTING) &&
+	    task_utrace_struct(current) &&
+	    task_utrace_struct(current)->cloning != target)
+		do {
+			schedule_timeout_interruptible(1);
+			if (signal_pending(current))
+				return -ERESTARTNOINTR;
+		} while (target->flags & PF_STARTING);
+
+	return 0;
+}
+
+/*
+ * Enqueue @engine, or maybe don't if UTRACE_ATTACH_EXCLUSIVE.
+ */
+static int utrace_add_engine(struct task_struct *target,
+			     struct utrace *utrace,
+			     struct utrace_engine *engine,
+			     int flags,
+			     const struct utrace_engine_ops *ops,
+			     void *data)
+{
+	int ret;
+
+	spin_lock(&utrace->lock);
+
+	ret = -EEXIST;
+	if ((flags & UTRACE_ATTACH_EXCLUSIVE) &&
+	     unlikely(matching_engine(utrace, flags, ops, data)))
+		goto unlock;
+
+	/*
+	 * In case we had no engines before, make sure that
+	 * utrace_flags is not zero.
+	 */
+	ret = -ESRCH;
+	if (!target->utrace_flags) {
+		target->utrace_flags = UTRACE_EVENT(REAP);
+		/*
+		 * If we race with tracehook_prepare_release_task()
+		 * make sure that either it sees utrace_flags != 0
+		 * or we see exit_state == EXIT_DEAD.
+		 */
+		smp_mb();
+		if (unlikely(target->exit_state == EXIT_DEAD)) {
+			target->utrace_flags = 0;
+			goto unlock;
+		}
+	}
+
+	/*
+	 * Put the new engine on the pending ->attaching list.
+	 * Make sure it gets onto the ->attached list by the next
+	 * time it's examined.  Setting ->pending_attach ensures
+	 * that start_report() takes the lock and splices the lists
+	 * before the next new reporting pass.
+	 *
+	 * When target == current, it would be safe just to call
+	 * splice_attaching() right here.  But if we're inside a
+	 * callback, that would mean the new engine also gets
+	 * notified about the event that precipitated its own
+	 * creation.  This is not what the user wants.
+	 */
+	list_add_tail(&engine->entry, &utrace->attaching);
+	utrace->pending_attach = 1;
+	ret = 0;
+unlock:
+	spin_unlock(&utrace->lock);
+
+	return ret;
+}
+
+/**
+ * utrace_attach_task - attach new engine, or look up an attached engine
+ * @target:	thread to attach to
+ * @flags:	flag bits combined with OR, see below
+ * @ops:	callback table for new engine
+ * @data:	engine private data pointer
+ *
+ * The caller must ensure that the @target thread does not get freed,
+ * i.e. hold a ref or be its parent.  It is always safe to call this
+ * on @current, or on the @child pointer in a @report_clone callback.
+ * For most other cases, it's easier to use utrace_attach_pid() instead.
+ *
+ * UTRACE_ATTACH_CREATE:
+ * Create a new engine.  If %UTRACE_ATTACH_CREATE is not specified, you
+ * only look up an existing engine already attached to the thread.
+ *
+ * UTRACE_ATTACH_EXCLUSIVE:
+ * Attempting to attach a second (matching) engine fails with -%EEXIST.
+ *
+ * UTRACE_ATTACH_MATCH_OPS: Only consider engines matching @ops.
+ * UTRACE_ATTACH_MATCH_DATA: Only consider engines matching @data.
+ *
+ * Calls with neither %UTRACE_ATTACH_MATCH_OPS nor %UTRACE_ATTACH_MATCH_DATA
+ * match the first among any engines attached to @target.  That means that
+ * %UTRACE_ATTACH_EXCLUSIVE in such a call fails with -%EEXIST if there
+ * are any engines on @target at all.
+ */
+struct utrace_engine *utrace_attach_task(
+	struct task_struct *target, int flags,
+	const struct utrace_engine_ops *ops, void *data)
+{
+	struct utrace *utrace = task_utrace_struct(target);
+	struct utrace_engine *engine;
+	int ret;
+
+	if (!(flags & UTRACE_ATTACH_CREATE)) {
+		if (unlikely(!utrace))
+			return ERR_PTR(-ENOENT);
+		spin_lock(&utrace->lock);
+		engine = matching_engine(utrace, flags, ops, data);
+		if (engine)
+			utrace_engine_get(engine);
+		spin_unlock(&utrace->lock);
+		return engine ?: ERR_PTR(-ENOENT);
+	}
+
+	if (unlikely(!ops) || unlikely(ops == &utrace_detached_ops))
+		return ERR_PTR(-EINVAL);
+
+	if (unlikely(target->flags & PF_KTHREAD))
+		/*
+		 * Silly kernel, utrace is for users!
+		 */
+		return ERR_PTR(-EPERM);
+
+	if (!utrace) {
+		if (unlikely(!utrace_task_alloc(target)))
+			return ERR_PTR(-ENOMEM);
+		utrace = task_utrace_struct(target);
+	}
+
+	engine = kmem_cache_alloc(utrace_engine_cachep, GFP_KERNEL);
+	if (unlikely(!engine))
+		return ERR_PTR(-ENOMEM);
+
+	/*
+	 * Initialize the new engine structure.  It starts out with two
+	 * refs: one ref to return, and one ref for being attached.
+	 */
+	kref_set(&engine->kref, 2);
+	engine->flags = 0;
+	engine->ops = ops;
+	engine->data = data;
+	engine->release = ops->release;
+
+	ret = utrace_attach_delay(target);
+	if (likely(!ret))
+		ret = utrace_add_engine(target, utrace, engine,
+					flags, ops, data);
+
+	if (unlikely(ret)) {
+		kmem_cache_free(utrace_engine_cachep, engine);
+		engine = ERR_PTR(ret);
+	}
+
+	return engine;
+}
+EXPORT_SYMBOL_GPL(utrace_attach_task);
+
+/**
+ * utrace_attach_pid - attach new engine, or look up an attached engine
+ * @pid:	&struct pid pointer representing thread to attach to
+ * @flags:	flag bits combined with OR, see utrace_attach_task()
+ * @ops:	callback table for new engine
+ * @data:	engine private data pointer
+ *
+ * This is the same as utrace_attach_task(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+struct utrace_engine *utrace_attach_pid(
+	struct pid *pid, int flags,
+	const struct utrace_engine_ops *ops, void *data)
+{
+	struct utrace_engine *engine = ERR_PTR(-ESRCH);
+	struct task_struct *task = get_pid_task(pid, PIDTYPE_PID);
+	if (task) {
+		engine = utrace_attach_task(task, flags, ops, data);
+		put_task_struct(task);
+	}
+	return engine;
+}
+EXPORT_SYMBOL_GPL(utrace_attach_pid);
+
+/*
+ * When an engine is detached, the target thread may still see it and
+ * make callbacks until it quiesces.  We install a special ops vector
+ * with these two callbacks.  When the target thread quiesces, it can
+ * safely free the engine itself.  For any event we will always get
+ * the report_quiesce() callback first, so we only need this one
+ * pointer to be set.  The only exception is report_reap(), so we
+ * supply that callback too.
+ */
+static u32 utrace_detached_quiesce(enum utrace_resume_action action,
+				   struct utrace_engine *engine,
+				   struct task_struct *task,
+				   unsigned long event)
+{
+	return UTRACE_DETACH;
+}
+
+static void utrace_detached_reap(struct utrace_engine *engine,
+				 struct task_struct *task)
+{
+}
+
+static const struct utrace_engine_ops utrace_detached_ops = {
+	.report_quiesce = &utrace_detached_quiesce,
+	.report_reap = &utrace_detached_reap
+};
+
+/*
+ * The caller has to hold a ref on the engine.  If the attached flag is
+ * true (all but utrace_barrier() calls), the engine is supposed to be
+ * attached.  If the attached flag is false (utrace_barrier() only),
+ * then return -ERESTARTSYS for an engine marked for detach but not yet
+ * fully detached.  The task pointer can be invalid if the engine is
+ * detached.
+ *
+ * Get the utrace lock for the target task.
+ * Returns the struct if locked, or ERR_PTR(-errno).
+ *
+ * This has to be robust against races with:
+ *	utrace_control(target, UTRACE_DETACH) calls
+ *	UTRACE_DETACH after reports
+ *	utrace_report_death
+ *	utrace_release_task
+ */
+static struct utrace *get_utrace_lock(struct task_struct *target,
+				      struct utrace_engine *engine,
+				      bool attached)
+	__acquires(utrace->lock)
+{
+	struct utrace *utrace;
+
+	rcu_read_lock();
+
+	/*
+	 * If this engine was already detached, bail out before we look at
+	 * the task_struct pointer at all.  If it's detached after this
+	 * check, then RCU is still keeping this task_struct pointer valid.
+	 *
+	 * The ops pointer is NULL when the engine is fully detached.
+	 * It's &utrace_detached_ops when it's marked detached but still
+	 * on the list.  In the latter case, utrace_barrier() still works,
+	 * since the target might be in the middle of an old callback.
+	 */
+	if (unlikely(!engine->ops)) {
+		rcu_read_unlock();
+		return ERR_PTR(-ESRCH);
+	}
+
+	if (unlikely(engine->ops == &utrace_detached_ops)) {
+		rcu_read_unlock();
+		return attached ? ERR_PTR(-ESRCH) : ERR_PTR(-ERESTARTSYS);
+	}
+
+	utrace = task_utrace_struct(target);
+	spin_lock(&utrace->lock);
+	if (unlikely(!engine->ops) ||
+	    unlikely(engine->ops == &utrace_detached_ops)) {
+		/*
+		 * By the time we got the utrace lock,
+		 * it had been reaped or detached already.
+		 */
+		spin_unlock(&utrace->lock);
+		utrace = ERR_PTR(-ESRCH);
+		if (!attached && engine->ops == &utrace_detached_ops)
+			utrace = ERR_PTR(-ERESTARTSYS);
+	}
+	rcu_read_unlock();
+
+	return utrace;
+}
+
+/*
+ * Now that we don't hold any locks, run through any
+ * detached engines and free their references.  Each
+ * engine had one implicit ref while it was attached.
+ */
+static void put_detached_list(struct list_head *list)
+{
+	struct utrace_engine *engine, *next;
+	list_for_each_entry_safe(engine, next, list, entry) {
+		list_del_init(&engine->entry);
+		utrace_engine_put(engine);
+	}
+}
+
+/*
+ * Called with utrace->lock held and utrace->reap set.
+ * Notify and clean up all engines, then free utrace.
+ */
+static void utrace_reap(struct task_struct *target, struct utrace *utrace)
+	__releases(utrace->lock)
+{
+	struct utrace_engine *engine, *next;
+
+	/* utrace_add_engine() checks ->utrace_flags != 0 */
+	target->utrace_flags = 0;
+	splice_attaching(utrace);
+
+	/*
+	 * Since we were called with @utrace->reap set, nobody can
+	 * set/clear UTRACE_EVENT(REAP) in @engine->flags or change
+	 * @engine->ops, and nobody can change @utrace->attached.
+	 */
+	spin_unlock(&utrace->lock);
+
+	list_for_each_entry_safe(engine, next, &utrace->attached, entry) {
+		if (engine->flags & UTRACE_EVENT(REAP))
+			engine->ops->report_reap(engine, target);
+
+		engine->ops = NULL;
+		engine->flags = 0;
+		list_del_init(&engine->entry);
+
+		utrace_engine_put(engine);
+	}
+}
+
+
+/*
+ * Called by release_task.
+ */
+void utrace_release_task(struct task_struct *target)
+{
+	struct utrace *utrace = task_utrace_struct(target);
+
+	spin_lock(&utrace->lock);
+
+	utrace->reap = 1;
+
+	/*
+	 * If the target will do some final callbacks but hasn't
+	 * finished them yet, we know because it clears these event
+	 * bits after it's done.  Instead of cleaning up here and
+	 * requiring utrace_report_death() to cope with it, we delay
+	 * the REAP report and the teardown until after the target
+	 * finishes its death reports.
+	 */
+
+	if (target->utrace_flags & _UTRACE_DEATH_EVENTS)
+		spin_unlock(&utrace->lock);
+	else
+		utrace_reap(target, utrace); /* Unlocks.  */
+}
+
+/*
+ * We use an extra bit in utrace_engine.flags past the event bits,
+ * to record whether the engine is keeping the target thread stopped.
+ *
+ * This bit is set in task_struct.utrace_flags whenever it is set in any
+ * engine's flags.  Only utrace_reset() resets it in utrace_flags.
+ */
+#define ENGINE_STOP		(1UL << _UTRACE_NEVENTS)
+
+static void mark_engine_wants_stop(struct task_struct *task,
+				   struct utrace_engine *engine)
+{
+	engine->flags |= ENGINE_STOP;
+	task->utrace_flags |= ENGINE_STOP;
+}
+
+static void clear_engine_wants_stop(struct utrace_engine *engine)
+{
+	engine->flags &= ~ENGINE_STOP;
+}
+
+static bool engine_wants_stop(struct utrace_engine *engine)
+{
+	return (engine->flags & ENGINE_STOP) != 0;
+}
+
+/**
+ * utrace_set_events - choose which event reports a tracing engine gets
+ * @target:		thread to affect
+ * @engine:		attached engine to affect
+ * @events:		new event mask
+ *
+ * This changes the set of events for which @engine wants callbacks made.
+ *
+ * This fails with -%EALREADY and does nothing if you try to clear
+ * %UTRACE_EVENT(%DEATH) when the @report_death callback may already have
+ * begun, if you try to clear %UTRACE_EVENT(%REAP) when the @report_reap
+ * callback may already have begun, or if you try to newly set
+ * %UTRACE_EVENT(%DEATH) or %UTRACE_EVENT(%QUIESCE) when @target is
+ * already dead or dying.
+ *
+ * This can fail with -%ESRCH when @target has already been detached,
+ * including forcible detach on reaping.
+ *
+ * If @target was stopped before the call, then after a successful call,
+ * no event callbacks not requested in @events will be made; if
+ * %UTRACE_EVENT(%QUIESCE) is included in @events, then a
+ * @report_quiesce callback will be made when @target resumes.
+ *
+ * If @target was not stopped and @events excludes some bits that were
+ * set before, this can return -%EINPROGRESS to indicate that @target
+ * may have been making some callback to @engine.  When this returns
+ * zero, you can be sure that no event callbacks you've disabled in
+ * @events can be made.  If @events only sets new bits that were not set
+ * before on @engine, then -%EINPROGRESS will never be returned.
+ *
+ * To synchronize after an -%EINPROGRESS return, see utrace_barrier().
+ *
+ * When @target is @current, -%EINPROGRESS is not returned.  But note
+ * that a newly-created engine will not receive any callbacks related to
+ * an event notification already in progress.  This call enables @events
+ * callbacks to be made as soon as @engine becomes eligible for any
+ * callbacks, see utrace_attach_task().
+ *
+ * These rules provide for coherent synchronization based on %UTRACE_STOP,
+ * even when %SIGKILL is breaking its normal simple rules.
+ */
+int utrace_set_events(struct task_struct *target,
+		      struct utrace_engine *engine,
+		      unsigned long events)
+{
+	struct utrace *utrace;
+	unsigned long old_flags, old_utrace_flags;
+	int ret;
+
+	/*
+	 * We just ignore the internal bit, so callers can use
+	 * engine->flags to seed bitwise ops for our argument.
+	 */
+	events &= ~ENGINE_STOP;
+
+	utrace = get_utrace_lock(target, engine, true);
+	if (unlikely(IS_ERR(utrace)))
+		return PTR_ERR(utrace);
+
+	old_utrace_flags = target->utrace_flags;
+	old_flags = engine->flags & ~ENGINE_STOP;
+
+	if (target->exit_state &&
+	    (((events & ~old_flags) & _UTRACE_DEATH_EVENTS) ||
+	     (utrace->death &&
+	      ((old_flags & ~events) & _UTRACE_DEATH_EVENTS)) ||
+	     (utrace->reap && ((old_flags & ~events) & UTRACE_EVENT(REAP))))) {
+		spin_unlock(&utrace->lock);
+		return -EALREADY;
+	}
+
+	/*
+	 * When setting these flags, it's essential that we really
+	 * synchronize with exit_notify().  They cannot be set after
+	 * exit_notify() takes the tasklist_lock.  By holding the read
+	 * lock here while setting the flags, we ensure that the calls
+	 * to tracehook_notify_death() and tracehook_report_death() will
+	 * see the new flags.  This ensures that utrace_release_task()
+	 * knows positively that utrace_report_death() will be called or
+	 * that it won't.
+	 */
+	if ((events & ~old_utrace_flags) & _UTRACE_DEATH_EVENTS) {
+		read_lock(&tasklist_lock);
+		if (unlikely(target->exit_state)) {
+			read_unlock(&tasklist_lock);
+			spin_unlock(&utrace->lock);
+			return -EALREADY;
+		}
+		target->utrace_flags |= events;
+		read_unlock(&tasklist_lock);
+	}
+
+	engine->flags = events | (engine->flags & ENGINE_STOP);
+	target->utrace_flags |= events;
+
+	if ((events & UTRACE_EVENT_SYSCALL) &&
+	    !(old_utrace_flags & UTRACE_EVENT_SYSCALL))
+		set_tsk_thread_flag(target, TIF_SYSCALL_TRACE);
+
+	ret = 0;
+	if ((old_flags & ~events) && target != current &&
+	    !task_is_stopped_or_traced(target) && !target->exit_state) {
+		/*
+		 * This barrier ensures that our engine->flags changes
+		 * have hit before we examine utrace->reporting,
+		 * pairing with the barrier in start_callback().  If
+		 * @target has not yet hit finish_callback() to clear
+		 * utrace->reporting, we might be in the middle of a
+		 * callback to @engine.
+		 */
+		smp_mb();
+		if (utrace->reporting == engine)
+			ret = -EINPROGRESS;
+	}
+
+	spin_unlock(&utrace->lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_set_events);
+
+/*
+ * Asynchronously mark an engine as being detached.
+ *
+ * This must work while the target thread races with us doing
+ * start_callback(), defined below.  It uses smp_rmb() between checking
+ * @engine->flags and using @engine->ops.  Here we change @engine->ops
+ * first, then use smp_wmb() before changing @engine->flags.  This ensures
+ * it can check the old flags before using the old ops, or check the old
+ * flags before using the new ops, or check the new flags before using the
+ * new ops, but can never check the new flags before using the old ops.
+ * Hence, utrace_detached_ops might be used with any old flags in place.
+ * It has report_quiesce() and report_reap() callbacks to handle all cases.
+ */
+static void mark_engine_detached(struct utrace_engine *engine)
+{
+	engine->ops = &utrace_detached_ops;
+	smp_wmb();
+	engine->flags = UTRACE_EVENT(QUIESCE);
+}
+
+/*
+ * Get @target to stop and return true if it is already stopped now.
+ * If we return false, it will make some event callback soonish.
+ * Called with @utrace locked.
+ */
+static bool utrace_do_stop(struct task_struct *target, struct utrace *utrace)
+{
+	if (task_is_stopped(target)) {
+		/*
+		 * Stopped is considered quiescent; when it wakes up, it will
+		 * go through utrace_finish_stop() before doing anything else.
+		 */
+		spin_lock_irq(&target->sighand->siglock);
+		if (likely(task_is_stopped(target)))
+			__set_task_state(target, TASK_TRACED);
+		spin_unlock_irq(&target->sighand->siglock);
+	} else if (utrace->resume > UTRACE_REPORT) {
+		utrace->resume = UTRACE_REPORT;
+		set_notify_resume(target);
+	}
+
+	return task_is_traced(target);
+}
+
+/*
+ * If the target is not dead it should not be in tracing
+ * stop any more.  Wake it unless it's in job control stop.
+ *
+ * Called with @utrace->lock held and @target in either TASK_TRACED or dead.
+ */
+static void utrace_wakeup(struct task_struct *target, struct utrace *utrace)
+{
+	spin_lock_irq(&target->sighand->siglock);
+	if (target->signal->flags & SIGNAL_STOP_STOPPED ||
+	    target->signal->group_stop_count)
+		target->state = TASK_STOPPED;
+	else
+		wake_up_state(target, __TASK_TRACED);
+	spin_unlock_irq(&target->sighand->siglock);
+}
+
+/*
+ * This is called when there might be some detached engines on the list or
+ * some stale bits in @task->utrace_flags.  Clean them up and recompute the
+ * flags.  Returns true if we're now fully detached.
+ *
+ * Called with @utrace->lock held, returns with it released.
+ * After this returns, @utrace might be freed if everything detached.
+ */
+static bool utrace_reset(struct task_struct *task, struct utrace *utrace)
+	__releases(utrace->lock)
+{
+	struct utrace_engine *engine, *next;
+	unsigned long flags = 0;
+	LIST_HEAD(detached);
+
+	splice_attaching(utrace);
+
+	/*
+	 * Update the set of events of interest from the union
+	 * of the interests of the remaining tracing engines.
+	 * For any engine marked detached, remove it from the list.
+	 * We'll collect them on the detached list.
+	 */
+	list_for_each_entry_safe(engine, next, &utrace->attached, entry) {
+		if (engine->ops == &utrace_detached_ops) {
+			engine->ops = NULL;
+			list_move(&engine->entry, &detached);
+		} else {
+			flags |= engine->flags | UTRACE_EVENT(REAP);
+		}
+	}
+
+	if (task->exit_state) {
+		/*
+		 * Once it's already dead, we never install any flags
+		 * except REAP.  When ->exit_state is set and events
+		 * like DEATH are not set, then they never can be set.
+		 * This ensures that utrace_release_task() knows
+		 * positively that utrace_report_death() can never run.
+		 */
+		BUG_ON(utrace->death);
+		flags &= UTRACE_EVENT(REAP);
+	} else if (!(flags & UTRACE_EVENT_SYSCALL) &&
+		   test_tsk_thread_flag(task, TIF_SYSCALL_TRACE)) {
+		clear_tsk_thread_flag(task, TIF_SYSCALL_TRACE);
+	}
+
+	if (!flags) {
+		/*
+		 * No more engines, cleared out the utrace.
+		 */
+		utrace->resume = UTRACE_RESUME;
+		utrace->signal_handler = 0;
+	}
+
+	if (task_is_traced(task) && !(flags & ENGINE_STOP))
+		/*
+		 * No more engines want it stopped.  Wake it up.
+		 */
+		utrace_wakeup(task, utrace);
+
+	/*
+	 * In theory spin_lock() doesn't imply rcu_read_lock().
+	 * Once we clear ->utrace_flags this task_struct can go away
+	 * because tracehook_prepare_release_task() path does not take
+	 * utrace->lock when ->utrace_flags == 0.
+	 */
+	rcu_read_lock();
+	task->utrace_flags = flags;
+	spin_unlock(&utrace->lock);
+	rcu_read_unlock();
+
+	put_detached_list(&detached);
+
+	return !flags;
+}
+
+void utrace_finish_stop(void)
+{
+	/*
+	 * If we were task_is_traced() and then SIGKILL'ed, make
+	 * sure we do nothing until the tracer drops utrace->lock.
+	 */
+	if (unlikely(__fatal_signal_pending(current))) {
+		struct utrace *utrace = task_utrace_struct(current);
+		spin_unlock_wait(&utrace->lock);
+	}
+}
+
+/*
+ * Perform %UTRACE_STOP, i.e. block in TASK_TRACED until woken up.
+ * @task == current, @utrace == current->utrace, which is not locked.
+ * Return true if we were woken up by SIGKILL even though some utrace
+ * engine may still want us to stay stopped.
+ */
+static void utrace_stop(struct task_struct *task, struct utrace *utrace,
+			enum utrace_resume_action action)
+{
+relock:
+	spin_lock(&utrace->lock);
+
+	if (action < utrace->resume) {
+		/*
+		 * Ensure a reporting pass when we're resumed.
+		 */
+		utrace->resume = action;
+		if (action == UTRACE_INTERRUPT)
+			set_thread_flag(TIF_SIGPENDING);
+		else
+			set_thread_flag(TIF_NOTIFY_RESUME);
+	}
+
+	/*
+	 * If the ENGINE_STOP bit is clear in utrace_flags, that means
+	 * utrace_reset() ran after we processed some UTRACE_STOP return
+	 * values from callbacks to get here.  If all engines have detached
+	 * or resumed us, we don't stop.  This check doesn't require
+	 * siglock, but it should follow the interrupt/report bookkeeping
+	 * steps (this can matter for UTRACE_RESUME but not UTRACE_DETACH).
+	 */
+	if (unlikely(!(task->utrace_flags & ENGINE_STOP))) {
+		utrace_reset(task, utrace);
+		if (task->utrace_flags & ENGINE_STOP)
+			goto relock;
+		return;
+	}
+
+	/*
+	 * The siglock protects us against signals.  As well as SIGKILL
+	 * waking us up, we must synchronize with the signal bookkeeping
+	 * for stop signals and SIGCONT.
+	 */
+	spin_lock_irq(&task->sighand->siglock);
+
+	if (unlikely(__fatal_signal_pending(task))) {
+		spin_unlock_irq(&task->sighand->siglock);
+		spin_unlock(&utrace->lock);
+		return;
+	}
+
+	__set_current_state(TASK_TRACED);
+
+	/*
+	 * If there is a group stop in progress,
+	 * we must participate in the bookkeeping.
+	 */
+	if (unlikely(task->signal->group_stop_count) &&
+			!--task->signal->group_stop_count)
+		task->signal->flags = SIGNAL_STOP_STOPPED;
+
+	spin_unlock_irq(&task->sighand->siglock);
+	spin_unlock(&utrace->lock);
+
+	/*
+	 * If ptrace is among the reasons for this stop, do its
+	 * notification now.  This could not just be done in
+	 * ptrace's own event report callbacks because it has to
+	 * be done after we are in TASK_TRACED.  This makes the
+	 * synchronization with ptrace_do_wait() work right.
+	 */
+	ptrace_notify_stop(task);
+
+	schedule();
+
+	utrace_finish_stop();
+
+	/*
+	 * While in TASK_TRACED, we were considered "frozen enough".
+	 * Now that we woke up, it's crucial if we're supposed to be
+	 * frozen that we freeze now before running anything substantial.
+	 */
+	try_to_freeze();
+
+	/*
+	 * While we were in TASK_TRACED, complete_signal() considered
+	 * us "uninterested" in signal wakeups.  Now make sure our
+	 * TIF_SIGPENDING state is correct for normal running.
+	 */
+	spin_lock_irq(&task->sighand->siglock);
+	recalc_sigpending();
+	spin_unlock_irq(&task->sighand->siglock);
+}
+
+/*
+ * You can't do anything to a dead task but detach it.
+ * If release_task() has been called, you can't do that.
+ *
+ * On the exit path, DEATH and QUIESCE event bits are set only
+ * before utrace_report_death() has taken the lock.  At that point,
+ * the death report will come soon, so disallow detach until it's
+ * done.  This prevents us from racing with it detaching itself.
+ *
+ * Called with utrace->lock held, when @target->exit_state is nonzero.
+ */
+static inline int utrace_control_dead(struct task_struct *target,
+				      struct utrace *utrace,
+				      enum utrace_resume_action action)
+{
+	if (action != UTRACE_DETACH || unlikely(utrace->reap))
+		return -ESRCH;
+
+	if (unlikely(utrace->death))
+		/*
+		 * We have already started the death report.  We can't
+		 * prevent the report_death and report_reap callbacks,
+		 * so tell the caller they will happen.
+		 */
+		return -EALREADY;
+
+	return 0;
+}
+
+/**
+ * utrace_control - control a thread being traced by a tracing engine
+ * @target:		thread to affect
+ * @engine:		attached engine to affect
+ * @action:		&enum utrace_resume_action for thread to do
+ *
+ * This is how a tracing engine asks a traced thread to do something.
+ * This call is controlled by the @action argument, which has the
+ * same meaning as the &enum utrace_resume_action value returned by
+ * event reporting callbacks.
+ *
+ * If @target is already dead (@target->exit_state nonzero),
+ * all actions except %UTRACE_DETACH fail with -%ESRCH.
+ *
+ * The following sections describe each option for the @action argument.
+ *
+ * UTRACE_DETACH:
+ *
+ * After this, the @engine data structure is no longer accessible,
+ * and the thread might be reaped.  The thread will start running
+ * again if it was stopped and no longer has any attached engines
+ * that want it stopped.
+ *
+ * If the @report_reap callback may already have begun, this fails
+ * with -%ESRCH.  If the @report_death callback may already have
+ * begun, this fails with -%EALREADY.
+ *
+ * If @target is not already stopped, then a callback to this engine
+ * might be in progress or about to start on another CPU.  If so,
+ * then this returns -%EINPROGRESS; the detach happens as soon as
+ * the pending callback is finished.  To synchronize after an
+ * -%EINPROGRESS return, see utrace_barrier().
+ *
+ * If @target is properly stopped before utrace_control() is called,
+ * then after successful return it's guaranteed that no more callbacks
+ * to the @engine->ops vector will be made.
+ *
+ * The only exception is %SIGKILL (and exec or group-exit by another
+ * thread in the group), which can cause asynchronous @report_death
+ * and/or @report_reap callbacks even when %UTRACE_STOP was used.
+ * (In that event, this fails with -%ESRCH or -%EALREADY, see above.)
+ *
+ * UTRACE_STOP:
+ *
+ * This asks that @target stop running.  This returns 0 only if
+ * @target is already stopped, either for tracing or for job
+ * control.  Then @target will remain stopped until another
+ * utrace_control() call is made on @engine; @target can be woken
+ * only by %SIGKILL (or equivalent, such as exec or termination by
+ * another thread in the same thread group).
+ *
+ * This returns -%EINPROGRESS if @target is not already stopped.
+ * Then the effect is like %UTRACE_REPORT.  A @report_quiesce or
+ * @report_signal callback will be made soon.  Your callback can
+ * then return %UTRACE_STOP to keep @target stopped.
+ *
+ * This does not interrupt system calls in progress, including ones
+ * that sleep for a long time.  For that, use %UTRACE_INTERRUPT.
+ * To interrupt system calls and then keep @target stopped, your
+ * @report_signal callback can return %UTRACE_STOP.
+ *
+ * UTRACE_RESUME:
+ *
+ * Just let @target continue running normally, reversing the effect
+ * of a previous %UTRACE_STOP.  If another engine is keeping @target
+ * stopped, then it remains stopped until all engines let it resume.
+ * If @target was not stopped, this has no effect.
+ *
+ * UTRACE_REPORT:
+ *
+ * This is like %UTRACE_RESUME, but also ensures that there will be
+ * a @report_quiesce or @report_signal callback made soon.  If
+ * @target had been stopped, then there will be a callback before it
+ * resumes running normally.  If another engine is keeping @target
+ * stopped, then there might be no callbacks until all engines let
+ * it resume.
+ *
+ * Since this is meaningless unless @report_quiesce callbacks will
+ * be made, it returns -%EINVAL if @engine lacks %UTRACE_EVENT(%QUIESCE).
+ *
+ * UTRACE_INTERRUPT:
+ *
+ * This is like %UTRACE_REPORT, but ensures that @target will make a
+ * @report_signal callback before it resumes or delivers signals.
+ * If @target was in a system call or about to enter one, work in
+ * progress will be interrupted as if by %SIGSTOP.  If another
+ * engine is keeping @target stopped, then there might be no
+ * callbacks until all engines let it resume.
+ *
+ * This gives @engine an opportunity to introduce a forced signal
+ * disposition via its @report_signal callback.
+ *
+ * UTRACE_SINGLESTEP:
+ *
+ * It's invalid to use this unless arch_has_single_step() returned true.
+ * This is like %UTRACE_RESUME, but resumes for one user instruction only.
+ *
+ * Note that passing %UTRACE_SINGLESTEP or %UTRACE_BLOCKSTEP to
+ * utrace_control() or returning it from an event callback alone does
+ * not necessarily ensure that stepping will be enabled.  If there are
+ * more callbacks made to any engine before returning to user mode,
+ * then the resume action is chosen only by the last set of callbacks.
+ * To be sure, enable %UTRACE_EVENT(%QUIESCE) and look for the
+ * @report_quiesce callback with a zero event mask, or the
+ * @report_signal callback with %UTRACE_SIGNAL_REPORT.
+ *
+ * Since this is not robust unless @report_quiesce callbacks will
+ * be made, it returns -%EINVAL if @engine lacks %UTRACE_EVENT(%QUIESCE).
+ *
+ * UTRACE_BLOCKSTEP:
+ *
+ * It's invalid to use this unless arch_has_block_step() returned true.
+ * This is like %UTRACE_SINGLESTEP, but resumes for one whole basic
+ * block of user instructions.
+ *
+ * Since this is not robust unless @report_quiesce callbacks will
+ * be made, it returns -%EINVAL if @engine lacks %UTRACE_EVENT(%QUIESCE).
+ *
+ * %UTRACE_BLOCKSTEP devolves to %UTRACE_SINGLESTEP when another
+ * tracing engine is using %UTRACE_SINGLESTEP at the same time.
+ */
+int utrace_control(struct task_struct *target,
+		   struct utrace_engine *engine,
+		   enum utrace_resume_action action)
+{
+	struct utrace *utrace;
+	bool reset;
+	int ret;
+
+	if (unlikely(action > UTRACE_DETACH))
+		return -EINVAL;
+
+	/*
+	 * This is a sanity check for a programming error in the caller.
+	 * Their request can only work properly in all cases by relying on
+	 * a follow-up callback, but they didn't set one up!  This check
+	 * doesn't do locking, but it shouldn't matter.  The caller has to
+	 * be synchronously sure the callback is set up to be operating the
+	 * interface properly.
+	 */
+	if (action >= UTRACE_REPORT && action < UTRACE_RESUME &&
+	    unlikely(!(engine->flags & UTRACE_EVENT(QUIESCE))))
+		return -EINVAL;
+
+	utrace = get_utrace_lock(target, engine, true);
+	if (unlikely(IS_ERR(utrace)))
+		return PTR_ERR(utrace);
+
+	reset = task_is_traced(target);
+	ret = 0;
+
+	/*
+	 * ->exit_state can change under us, this doesn't matter.
+	 * We do not care about ->exit_state in fact, but we do
+	 * care about ->reap and ->death. If either flag is set,
+	 * we must also see ->exit_state != 0.
+	 */
+	if (unlikely(target->exit_state)) {
+		ret = utrace_control_dead(target, utrace, action);
+		if (ret) {
+			spin_unlock(&utrace->lock);
+			return ret;
+		}
+		reset = true;
+	}
+
+	switch (action) {
+	case UTRACE_STOP:
+		mark_engine_wants_stop(target, engine);
+		if (!reset && !utrace_do_stop(target, utrace))
+			ret = -EINPROGRESS;
+		reset = false;
+		break;
+
+	case UTRACE_DETACH:
+		if (engine_wants_stop(engine))
+			target->utrace_flags &= ~ENGINE_STOP;
+		mark_engine_detached(engine);
+		reset = reset || utrace_do_stop(target, utrace);
+		if (!reset) {
+			/*
+			 * As in utrace_set_events(), this barrier ensures
+			 * that our engine->flags changes have hit before we
+			 * examine utrace->reporting, pairing with the barrier
+			 * in start_callback().  If @target has not yet hit
+			 * finish_callback() to clear utrace->reporting, we
+			 * might be in the middle of a callback to @engine.
+			 */
+			smp_mb();
+			if (utrace->reporting == engine)
+				ret = -EINPROGRESS;
+		}
+		break;
+
+	case UTRACE_RESUME:
+		/*
+		 * This and all other cases imply resuming if stopped.
+		 * There might not be another report before it just
+		 * resumes, so make sure single-step is not left set.
+		 */
+		clear_engine_wants_stop(engine);
+		if (likely(reset))
+			user_disable_single_step(target);
+		break;
+
+	case UTRACE_BLOCKSTEP:
+		/*
+		 * Resume from stopped, step one block.
+		 * We fall through to treat it like UTRACE_SINGLESTEP.
+		 */
+		if (unlikely(!arch_has_block_step())) {
+			WARN_ON(1);
+			action = UTRACE_SINGLESTEP;
+		}
+
+	case UTRACE_SINGLESTEP:
+		/*
+		 * Resume from stopped, step one instruction.
+		 * We fall through to the UTRACE_REPORT case.
+		 */
+		if (unlikely(!arch_has_single_step())) {
+			WARN_ON(1);
+			reset = false;
+			ret = -EOPNOTSUPP;
+			break;
+		}
+
+	case UTRACE_REPORT:
+		/*
+		 * Make the thread call tracehook_notify_resume() soon.
+		 * But don't bother if it's already been interrupted.
+		 * In that case, utrace_get_signal() will be reporting soon.
+		 */
+		clear_engine_wants_stop(engine);
+		if (action < utrace->resume) {
+			utrace->resume = action;
+			set_notify_resume(target);
+		}
+		break;
+
+	case UTRACE_INTERRUPT:
+		/*
+		 * Make the thread call tracehook_get_signal() soon.
+		 */
+		clear_engine_wants_stop(engine);
+		if (utrace->resume == UTRACE_INTERRUPT)
+			break;
+		utrace->resume = UTRACE_INTERRUPT;
+
+		/*
+		 * If it's not already stopped, interrupt it now.  We need
+		 * the siglock here in case it calls recalc_sigpending()
+		 * and clears its own TIF_SIGPENDING.  By taking the lock,
+		 * we've serialized any later recalc_sigpending() after our
+		 * setting of utrace->resume to force it on.
+		 */
+		if (reset) {
+			/*
+			 * This is really just to keep the invariant that
+			 * TIF_SIGPENDING is set with UTRACE_INTERRUPT.
+			 * When it's stopped, we know it's always going
+			 * through utrace_get_signal() and will recalculate.
+			 */
+			set_tsk_thread_flag(target, TIF_SIGPENDING);
+		} else {
+			struct sighand_struct *sighand;
+			unsigned long irqflags;
+			sighand = lock_task_sighand(target, &irqflags);
+			if (likely(sighand)) {
+				signal_wake_up(target, 0);
+				unlock_task_sighand(target, &irqflags);
+			}
+		}
+		break;
+	}
+
+	/*
+	 * Let the thread resume running.  If it's not stopped now,
+	 * there is nothing more we need to do.
+	 */
+	if (reset)
+		utrace_reset(target, utrace);
+	else
+		spin_unlock(&utrace->lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_control);
+
+/**
+ * utrace_barrier - synchronize with simultaneous tracing callbacks
+ * @target:		thread to affect
+ * @engine:		engine to affect (can be detached)
+ *
+ * This blocks while @target might be in the midst of making a callback to
+ * @engine.  It can be interrupted by signals and will return -%ERESTARTSYS.
+ * A return value of zero means no callback from @target to @engine was
+ * in progress.  Any effect of its return value (such as %UTRACE_STOP) has
+ * already been applied to @engine.
+ *
+ * It's not necessary to keep the @target pointer alive for this call.
+ * It's only necessary to hold a ref on @engine.  This will return
+ * safely even if @target has been reaped and has no task refs.
+ *
+ * A successful return from utrace_barrier() guarantees its ordering
+ * with respect to utrace_set_events() and utrace_control() calls.  If
+ * @target was not properly stopped, event callbacks just disabled might
+ * still be in progress; utrace_barrier() waits until there is no chance
+ * an unwanted callback can be in progress.
+ */
+int utrace_barrier(struct task_struct *target, struct utrace_engine *engine)
+{
+	struct utrace *utrace;
+	int ret = -ERESTARTSYS;
+
+	if (unlikely(target == current))
+		return 0;
+
+	do {
+		utrace = get_utrace_lock(target, engine, false);
+		if (unlikely(IS_ERR(utrace))) {
+			ret = PTR_ERR(utrace);
+			if (ret != -ERESTARTSYS)
+				break;
+		} else {
+			/*
+			 * All engine state changes are done while
+			 * holding the lock, i.e. before we get here.
+			 * Since we have the lock, we only need to
+			 * worry about @target making a callback.
+			 * When it has entered start_callback() but
+			 * not yet gotten to finish_callback(), we
+			 * will see utrace->reporting == @engine.
+			 * When @target doesn't take the lock, it uses
+			 * barriers to order setting utrace->reporting
+			 * before it examines the engine state.
+			 */
+			if (utrace->reporting != engine)
+				ret = 0;
+			spin_unlock(&utrace->lock);
+			if (!ret)
+				break;
+		}
+		schedule_timeout_interruptible(1);
+	} while (!signal_pending(current));
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_barrier);
+
+/*
+ * This is local state used for reporting loops, perhaps optimized away.
+ */
+struct utrace_report {
+	u32 result;
+	enum utrace_resume_action action;
+	enum utrace_resume_action resume_action;
+	bool detaches;
+	bool spurious;
+};
+
+#define INIT_REPORT(var)			\
+	struct utrace_report var = {		\
+		.action = UTRACE_RESUME,	\
+		.resume_action = UTRACE_RESUME,	\
+		.spurious = true 		\
+	}
+
+/*
+ * We are now making the report, so clear the flag saying we need one.
+ * When there is a new attach, ->pending_attach is set just so we will
+ * know to do splice_attaching() here before the callback loop.
+ */
+static enum utrace_resume_action start_report(struct utrace *utrace)
+{
+	enum utrace_resume_action resume = utrace->resume;
+	if (utrace->pending_attach ||
+	    (resume > UTRACE_INTERRUPT && resume < UTRACE_RESUME)) {
+		spin_lock(&utrace->lock);
+		splice_attaching(utrace);
+		resume = utrace->resume;
+		if (resume > UTRACE_INTERRUPT)
+			utrace->resume = UTRACE_RESUME;
+		spin_unlock(&utrace->lock);
+	}
+	return resume;
+}
+
+static inline void finish_report_reset(struct task_struct *task,
+				       struct utrace *utrace,
+				       struct utrace_report *report)
+{
+	if (unlikely(report->spurious || report->detaches)) {
+		spin_lock(&utrace->lock);
+		if (utrace_reset(task, utrace))
+			report->action = UTRACE_RESUME;
+	}
+}
+
+/*
+ * Complete a normal reporting pass, pairing with a start_report() call.
+ * This handles any UTRACE_DETACH or UTRACE_REPORT or UTRACE_INTERRUPT
+ * returns from engine callbacks.  If @will_not_stop is true and any
+ * engine's last callback used UTRACE_STOP, we do UTRACE_REPORT here to
+ * ensure we stop before user mode.  If there were no callbacks made, it
+ * will recompute @task->utrace_flags to avoid another false-positive.
+ */
+static void finish_report(struct task_struct *task, struct utrace *utrace,
+			  struct utrace_report *report, bool will_not_stop)
+{
+	enum utrace_resume_action resume = report->action;
+
+	if (resume == UTRACE_STOP)
+		resume = will_not_stop ? UTRACE_REPORT : UTRACE_RESUME;
+
+	if (resume < utrace->resume) {
+		spin_lock(&utrace->lock);
+		utrace->resume = resume;
+		if (resume == UTRACE_INTERRUPT)
+			set_tsk_thread_flag(task, TIF_SIGPENDING);
+		else
+			set_tsk_thread_flag(task, TIF_NOTIFY_RESUME);
+		spin_unlock(&utrace->lock);
+	}
+
+	finish_report_reset(task, utrace, report);
+}
+
+static inline void finish_callback_report(struct task_struct *task,
+					  struct utrace *utrace,
+					  struct utrace_report *report,
+					  struct utrace_engine *engine,
+					  enum utrace_resume_action action)
+{
+	/*
+	 * If utrace_control() was used, treat that like UTRACE_DETACH here.
+	 */
+	if (action == UTRACE_DETACH || engine->ops == &utrace_detached_ops) {
+		engine->ops = &utrace_detached_ops;
+		report->detaches = true;
+		return;
+	}
+
+	if (action < report->action)
+		report->action = action;
+
+	if (action != UTRACE_STOP) {
+		if (action < report->resume_action)
+			report->resume_action = action;
+
+		if (engine_wants_stop(engine)) {
+			spin_lock(&utrace->lock);
+			clear_engine_wants_stop(engine);
+			spin_unlock(&utrace->lock);
+		}
+
+		return;
+	}
+
+	if (!engine_wants_stop(engine)) {
+		spin_lock(&utrace->lock);
+		/*
+		 * If utrace_control() came in and detached us
+		 * before we got the lock, we must not stop now.
+		 */
+		if (unlikely(engine->ops == &utrace_detached_ops))
+			report->detaches = true;
+		else
+			mark_engine_wants_stop(task, engine);
+		spin_unlock(&utrace->lock);
+	}
+}
+
+/*
+ * Apply the return value of one engine callback to @report.
+ * Returns true if @engine detached and should not get any more callbacks.
+ */
+static bool finish_callback(struct task_struct *task, struct utrace *utrace,
+			    struct utrace_report *report,
+			    struct utrace_engine *engine,
+			    u32 ret)
+{
+	report->result = ret & ~UTRACE_RESUME_MASK;
+	finish_callback_report(task, utrace, report, engine,
+			       utrace_resume_action(ret));
+
+	/*
+	 * Now that we have applied the effect of the return value,
+	 * clear this so that utrace_barrier() can stop waiting.
+	 * A subsequent utrace_control() can stop or resume @engine
+	 * and know this was ordered after its callback's action.
+	 *
+	 * We don't need any barriers here because utrace_barrier()
+	 * takes utrace->lock.  If we touched engine->flags above,
+	 * the lock guaranteed this change was before utrace_barrier()
+	 * examined utrace->reporting.
+	 */
+	utrace->reporting = NULL;
+
+	/*
+	 * This is a good place to make sure tracing engines don't
+	 * introduce too much latency under voluntary preemption.
+	 */
+	if (need_resched())
+		cond_resched();
+
+	return engine->ops == &utrace_detached_ops;
+}
+
+/*
+ * Start the callbacks for @engine to consider @event (a bit mask).
+ * This makes the report_quiesce() callback first.  If @engine wants
+ * a specific callback for @event, we return the ops vector to use.
+ * If not, we return NULL.  The return value from the ops->callback
+ * function called should be passed to finish_callback().
+ */
+static const struct utrace_engine_ops *start_callback(
+	struct utrace *utrace, struct utrace_report *report,
+	struct utrace_engine *engine, struct task_struct *task,
+	unsigned long event)
+{
+	const struct utrace_engine_ops *ops;
+	unsigned long want;
+
+	/*
+	 * This barrier ensures that we've set utrace->reporting before
+	 * we examine engine->flags or engine->ops.  utrace_barrier()
+	 * relies on this ordering to indicate that the effect of any
+	 * utrace_control() and utrace_set_events() calls is in place
+	 * by the time utrace->reporting can be seen to be NULL.
+	 */
+	utrace->reporting = engine;
+	smp_mb();
+
+	/*
+	 * This pairs with the barrier in mark_engine_detached().
+	 * It makes sure that we never see the old ops vector with
+	 * the new flags, in case the original vector had no report_quiesce.
+	 */
+	want = engine->flags;
+	smp_rmb();
+	ops = engine->ops;
+
+	if (want & UTRACE_EVENT(QUIESCE)) {
+		if (finish_callback(task, utrace, report, engine,
+				    (*ops->report_quiesce)(report->action,
+							   engine, task,
+							   event)))
+			return NULL;
+
+		/*
+		 * finish_callback() reset utrace->reporting after the
+		 * quiesce callback.  Now we set it again (as above)
+		 * before re-examining engine->flags, which could have
+		 * been changed synchronously by ->report_quiesce or
+		 * asynchronously by utrace_control() or utrace_set_events().
+		 */
+		utrace->reporting = engine;
+		smp_mb();
+		want = engine->flags;
+	}
+
+	if (want & ENGINE_STOP)
+		report->action = UTRACE_STOP;
+
+	if (want & event) {
+		report->spurious = false;
+		return ops;
+	}
+
+	utrace->reporting = NULL;
+	return NULL;
+}
+
+/*
+ * Do a normal reporting pass for engines interested in @event.
+ * @callback is the name of the member in the ops vector, and remaining
+ * args are the extras it takes after the standard three args.
+ */
+#define REPORT(task, utrace, report, event, callback, ...)		      \
+	do {								      \
+		start_report(utrace);					      \
+		REPORT_CALLBACKS(, task, utrace, report, event, callback,     \
+				 (report)->action, engine, current,	      \
+				 ## __VA_ARGS__);  	   		      \
+		finish_report(task, utrace, report, true);		      \
+	} while (0)
+#define REPORT_CALLBACKS(rev, task, utrace, report, event, callback, ...)     \
+	do {								      \
+		struct utrace_engine *engine;				      \
+		const struct utrace_engine_ops *ops;			      \
+		list_for_each_entry##rev(engine, &utrace->attached, entry) {  \
+			ops = start_callback(utrace, report, engine, task,    \
+					     event);			      \
+			if (!ops)					      \
+				continue;				      \
+			finish_callback(task, utrace, report, engine,	      \
+					(*ops->callback)(__VA_ARGS__));	      \
+		}							      \
+	} while (0)
+
+/*
+ * Called iff UTRACE_EVENT(EXEC) flag is set.
+ */
+void utrace_report_exec(struct linux_binfmt *fmt, struct linux_binprm *bprm,
+			struct pt_regs *regs)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(EXEC),
+	       report_exec, fmt, bprm, regs);
+}
+
+static inline u32 do_report_syscall_entry(struct pt_regs *regs,
+					  struct task_struct *task,
+					  struct utrace *utrace,
+					  struct utrace_report *report,
+					  u32 resume_report)
+{
+	start_report(utrace);
+	REPORT_CALLBACKS(_reverse, task, utrace, report,
+			 UTRACE_EVENT(SYSCALL_ENTRY), report_syscall_entry,
+			 resume_report | report->result | report->action,
+			 engine, current, regs);
+	finish_report(task, utrace, report, false);
+
+	if (report->action != UTRACE_STOP)
+		return 0;
+
+	utrace_stop(task, utrace, report->resume_action);
+
+	if (fatal_signal_pending(task)) {
+		/*
+		 * We are continuing despite UTRACE_STOP because of a
+		 * SIGKILL.  Don't let the system call actually proceed.
+		 */
+		report->result = UTRACE_SYSCALL_ABORT;
+	} else if (utrace->resume <= UTRACE_REPORT) {
+		/*
+		 * If we've been asked for another report after our stop,
+		 * go back to report (and maybe stop) again before we run
+		 * the system call.  The second (and later) reports are
+		 * marked with the UTRACE_SYSCALL_RESUMED flag so that
+		 * engines know this is a second report at the same
+		 * entry.  This gives them the chance to examine the
+		 * registers anew after they might have been changed
+		 * while we were stopped.
+		 */
+		report->detaches = false;
+		report->spurious = true;
+		report->action = report->resume_action = UTRACE_RESUME;
+		return UTRACE_SYSCALL_RESUMED;
+	}
+
+	return 0;
+}
+
+/*
+ * Called iff UTRACE_EVENT(SYSCALL_ENTRY) flag is set.
+ * Return true to prevent the system call.
+ */
+bool utrace_report_syscall_entry(struct pt_regs *regs)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+	u32 resume_report = 0;
+
+	do {
+		resume_report = do_report_syscall_entry(regs, task, utrace,
+							&report, resume_report);
+	} while (resume_report);
+
+	return utrace_syscall_action(report.result) == UTRACE_SYSCALL_ABORT;
+}
+
+/*
+ * Called iff UTRACE_EVENT(SYSCALL_EXIT) flag is set.
+ */
+void utrace_report_syscall_exit(struct pt_regs *regs)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(SYSCALL_EXIT),
+	       report_syscall_exit, regs);
+}
+
+/*
+ * Called iff UTRACE_EVENT(CLONE) flag is set.
+ * This notification call blocks the wake_up_new_task call on the child.
+ * So we must not quiesce here.  tracehook_report_clone_complete will do
+ * a quiescence check momentarily.
+ */
+void utrace_report_clone(unsigned long clone_flags, struct task_struct *child)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	/*
+	 * We don't use the REPORT() macro here, because we need
+	 * to clear utrace->cloning before finish_report().
+	 * After finish_report(), utrace can be a stale pointer
+	 * in cases when report.action is still UTRACE_RESUME.
+	 */
+	start_report(utrace);
+	utrace->cloning = child;
+
+	REPORT_CALLBACKS(, task, utrace, &report,
+			 UTRACE_EVENT(CLONE), report_clone,
+			 report.action, engine, task, clone_flags, child);
+
+	utrace->cloning = NULL;
+	finish_report(task, utrace, &report, !(clone_flags & CLONE_VFORK));
+
+	/*
+	 * For a vfork, we will go into an uninterruptible block waiting
+	 * for the child.  We need UTRACE_STOP to happen before this, not
+	 * after.  For CLONE_VFORK, utrace_finish_vfork() will be called.
+	 */
+	if (report.action == UTRACE_STOP && (clone_flags & CLONE_VFORK)) {
+		spin_lock(&utrace->lock);
+		utrace->vfork_stop = 1;
+		spin_unlock(&utrace->lock);
+	}
+}
+
+/*
+ * We're called after utrace_report_clone() for a CLONE_VFORK.
+ * If UTRACE_STOP was left from the clone report, we stop here.
+ * After this, we'll enter the uninterruptible wait_for_completion()
+ * waiting for the child.
+ */
+void utrace_finish_vfork(struct task_struct *task)
+{
+	struct utrace *utrace = task_utrace_struct(task);
+
+	if (utrace->vfork_stop) {
+		spin_lock(&utrace->lock);
+		utrace->vfork_stop = 0;
+		spin_unlock(&utrace->lock);
+		utrace_stop(task, utrace, UTRACE_RESUME); /* XXX */
+	}
+}
+
+/*
+ * Called iff UTRACE_EVENT(JCTL) flag is set.
+ *
+ * Called with siglock held.
+ */
+void utrace_report_jctl(int notify, int what)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	spin_unlock_irq(&task->sighand->siglock);
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(JCTL),
+	       report_jctl, what, notify);
+
+	spin_lock_irq(&task->sighand->siglock);
+}
+
+/*
+ * Called iff UTRACE_EVENT(EXIT) flag is set.
+ */
+void utrace_report_exit(long *exit_code)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+	long orig_code = *exit_code;
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(EXIT),
+	       report_exit, orig_code, exit_code);
+
+	if (report.action == UTRACE_STOP)
+		utrace_stop(task, utrace, report.resume_action);
+}
+
+/*
+ * Called iff UTRACE_EVENT(DEATH) or UTRACE_EVENT(QUIESCE) flag is set.
+ *
+ * It is always possible that we are racing with utrace_release_task here.
+ * For this reason, utrace_release_task checks for the event bits that get
+ * us here, and delays its cleanup for us to do.
+ */
+void utrace_report_death(struct task_struct *task, struct utrace *utrace,
+			 bool group_dead, int signal)
+{
+	INIT_REPORT(report);
+
+	BUG_ON(!task->exit_state);
+
+	/*
+	 * We are presently considered "quiescent"--which is accurate
+	 * inasmuch as we won't run any more user instructions ever again.
+	 * But for utrace_control and utrace_set_events to be robust, they
+	 * must be sure whether or not we will run any more callbacks.  If
+	 * a call comes in before we do, taking the lock here synchronizes
+	 * us so we don't run any callbacks just disabled.  Calls that come
+	 * in while we're running the callbacks will see the exit.death
+	 * flag and know that we are not yet fully quiescent for purposes
+	 * of detach bookkeeping.
+	 */
+	spin_lock(&utrace->lock);
+	BUG_ON(utrace->death);
+	utrace->death = 1;
+	utrace->resume = UTRACE_RESUME;
+	splice_attaching(utrace);
+	spin_unlock(&utrace->lock);
+
+	REPORT_CALLBACKS(, task, utrace, &report, UTRACE_EVENT(DEATH),
+			 report_death, engine, task, group_dead, signal);
+
+	spin_lock(&utrace->lock);
+
+	/*
+	 * After we unlock (possibly inside utrace_reap for callbacks) with
+	 * this flag clear, competing utrace_control/utrace_set_events calls
+	 * know that we've finished our callbacks and any detach bookkeeping.
+	 */
+	utrace->death = 0;
+
+	if (utrace->reap)
+		/*
+		 * utrace_release_task() was already called in parallel.
+		 * We must complete its work now.
+		 */
+		utrace_reap(task, utrace);
+	else
+		utrace_reset(task, utrace);
+}
+
+/*
+ * Finish the last reporting pass before returning to user mode.
+ */
+static void finish_resume_report(struct task_struct *task,
+				 struct utrace *utrace,
+				 struct utrace_report *report)
+{
+	finish_report_reset(task, utrace, report);
+
+	switch (report->action) {
+	case UTRACE_STOP:
+		utrace_stop(task, utrace, report->resume_action);
+		break;
+
+	case UTRACE_INTERRUPT:
+		if (!signal_pending(task))
+			set_tsk_thread_flag(task, TIF_SIGPENDING);
+		break;
+
+	case UTRACE_BLOCKSTEP:
+		if (likely(arch_has_block_step())) {
+			user_enable_block_step(task);
+			break;
+		}
+
+		/*
+		 * This means some callback is to blame for failing
+		 * to check arch_has_block_step() itself.  Warn and
+		 * then fall through to treat it as SINGLESTEP.
+		 */
+		WARN_ON(1);
+
+	case UTRACE_SINGLESTEP:
+		if (likely(arch_has_single_step()))
+			user_enable_single_step(task);
+		else
+			/*
+			 * This means some callback is to blame for failing
+			 * to check arch_has_single_step() itself.  Spew
+			 * about it so the loser will fix his module.
+			 */
+			WARN_ON(1);
+		break;
+
+	case UTRACE_REPORT:
+	case UTRACE_RESUME:
+	default:
+		user_disable_single_step(task);
+		break;
+	}
+}
+
+/*
+ * This is called when TIF_NOTIFY_RESUME had been set (and is now clear).
+ * We are close to user mode, and this is the place to report or stop.
+ * When we return, we're going to user mode or into the signals code.
+ */
+void utrace_resume(struct task_struct *task, struct pt_regs *regs)
+{
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+	struct utrace_engine *engine;
+
+	/*
+	 * Some machines get here with interrupts disabled.  The same arch
+	 * code path leads to calling into get_signal_to_deliver(), which
+	 * implicitly reenables them by virtue of spin_unlock_irq.
+	 */
+	local_irq_enable();
+
+	/*
+	 * If this flag is still set it's because there was a signal
+	 * handler setup done but no report_signal following it.  Clear
+	 * the flag before we get to user so it doesn't confuse us later.
+	 */
+	if (unlikely(utrace->signal_handler)) {
+		spin_lock(&utrace->lock);
+		utrace->signal_handler = 0;
+		spin_unlock(&utrace->lock);
+	}
+
+	/*
+	 * Update our bookkeeping even if there are no callbacks made here.
+	 */
+	report.action = start_report(utrace);
+
+	switch (report.action) {
+	case UTRACE_RESUME:
+		/*
+		 * Anything we might have done was already handled by
+		 * utrace_get_signal(), or this is an entirely spurious
+		 * call.  (The arch might use TIF_NOTIFY_RESUME for other
+		 * purposes as well as calling us.)
+		 */
+		return;
+	case UTRACE_REPORT:
+		if (unlikely(!(task->utrace_flags & UTRACE_EVENT(QUIESCE))))
+			break;
+		/*
+		 * Do a simple reporting pass, with no specific
+		 * callback after report_quiesce.
+		 */
+		report.action = UTRACE_RESUME;
+		list_for_each_entry(engine, &utrace->attached, entry)
+			start_callback(utrace, &report, engine, task, 0);
+		break;
+	default:
+		/*
+		 * Even if this report was truly spurious, there is no need
+		 * for utrace_reset() now.  TIF_NOTIFY_RESUME was already
+		 * cleared--it doesn't stay spuriously set.
+		 */
+		report.spurious = false;
+		break;
+	}
+
+	/*
+	 * Finish the report and either stop or get ready to resume.
+	 * If utrace->resume was not UTRACE_REPORT, this applies its
+	 * effect now (i.e. step or interrupt).
+	 */
+	finish_resume_report(task, utrace, &report);
+}
+
+/*
+ * Return true if current has forced signal_pending().
+ *
+ * This is called only when current->utrace_flags is nonzero, so we know
+ * that current->utrace must be set.  It's not inlined in tracehook.h
+ * just so that struct utrace can stay opaque outside this file.
+ */
+bool utrace_interrupt_pending(void)
+{
+	return task_utrace_struct(current)->resume == UTRACE_INTERRUPT;
+}
+
+/*
+ * Take the siglock and push @info back on our queue.
+ * Returns with @task->sighand->siglock held.
+ */
+static void push_back_signal(struct task_struct *task, siginfo_t *info)
+	__acquires(task->sighand->siglock)
+{
+	struct sigqueue *q;
+
+	if (unlikely(!info->si_signo)) { /* Oh, a wise guy! */
+		spin_lock_irq(&task->sighand->siglock);
+		return;
+	}
+
+	q = sigqueue_alloc();
+	if (likely(q)) {
+		q->flags = 0;
+		copy_siginfo(&q->info, info);
+	}
+
+	spin_lock_irq(&task->sighand->siglock);
+
+	sigaddset(&task->pending.signal, info->si_signo);
+	if (likely(q))
+		list_add(&q->list, &task->pending.list);
+
+	set_tsk_thread_flag(task, TIF_SIGPENDING);
+}
+
+/*
+ * This is the hook from the signals code, called with the siglock held.
+ * Here is the ideal place to stop.  We also dequeue and intercept signals.
+ */
+int utrace_get_signal(struct task_struct *task, struct pt_regs *regs,
+		      siginfo_t *info, struct k_sigaction *return_ka)
+	__releases(task->sighand->siglock)
+	__acquires(task->sighand->siglock)
+{
+	struct utrace *utrace;
+	struct k_sigaction *ka;
+	INIT_REPORT(report);
+	struct utrace_engine *engine;
+	const struct utrace_engine_ops *ops;
+	unsigned long event, want;
+	u32 ret;
+	int signr;
+
+	utrace = task_utrace_struct(task);
+	if (utrace->resume < UTRACE_RESUME ||
+	    utrace->pending_attach || utrace->signal_handler) {
+		enum utrace_resume_action resume;
+
+		/*
+		 * We've been asked for an explicit report before we
+		 * even check for pending signals.
+		 */
+
+		spin_unlock_irq(&task->sighand->siglock);
+
+		spin_lock(&utrace->lock);
+
+		splice_attaching(utrace);
+
+		report.result = utrace->signal_handler ?
+			UTRACE_SIGNAL_HANDLER : UTRACE_SIGNAL_REPORT;
+		utrace->signal_handler = 0;
+
+		resume = utrace->resume;
+		utrace->resume = UTRACE_RESUME;
+
+		spin_unlock(&utrace->lock);
+
+		/*
+		 * Make sure signal_pending() only returns true
+		 * if there are real signals pending.
+		 */
+		if (signal_pending(task)) {
+			spin_lock_irq(&task->sighand->siglock);
+			recalc_sigpending();
+			spin_unlock_irq(&task->sighand->siglock);
+		}
+
+		if (resume > UTRACE_REPORT) {
+			/*
+			 * We only got here to process utrace->resume.
+			 * Despite no callbacks, this report is not spurious.
+			 */
+			report.action = resume;
+			report.spurious = false;
+			finish_resume_report(task, utrace, &report);
+			return -1;
+		} else if (!(task->utrace_flags & UTRACE_EVENT(QUIESCE))) {
+			/*
+			 * We only got here to clear utrace->signal_handler.
+			 */
+			return -1;
+		}
+
+		/*
+		 * Do a reporting pass for no signal, just for EVENT(QUIESCE).
+		 * The engine callbacks can fill in *info and *return_ka.
+		 * We'll pass NULL for the @orig_ka argument to indicate
+		 * that there was no original signal.
+		 */
+		event = 0;
+		ka = NULL;
+		memset(return_ka, 0, sizeof *return_ka);
+	} else if (!(task->utrace_flags & UTRACE_EVENT_SIGNAL_ALL) ||
+		   unlikely(task->signal->group_stop_count)) {
+		/*
+		 * If no engine is interested in intercepting signals or
+		 * we must stop, let the caller just dequeue them normally
+		 * or participate in group-stop.
+		 */
+		return 0;
+	} else {
+		/*
+		 * Steal the next signal so we can let tracing engines
+		 * examine it.  From the signal number and sigaction,
+		 * determine what normal delivery would do.  If no
+		 * engine perturbs it, we'll do that by returning the
+		 * signal number after setting *return_ka.
+		 */
+		signr = dequeue_signal(task, &task->blocked, info);
+		if (signr == 0)
+			return signr;
+		BUG_ON(signr != info->si_signo);
+
+		ka = &task->sighand->action[signr - 1];
+		*return_ka = *ka;
+
+		/*
+		 * We are never allowed to interfere with SIGKILL.
+		 * Just punt after filling in *return_ka for our caller.
+		 */
+		if (signr == SIGKILL)
+			return signr;
+
+		if (ka->sa.sa_handler == SIG_IGN) {
+			event = UTRACE_EVENT(SIGNAL_IGN);
+			report.result = UTRACE_SIGNAL_IGN;
+		} else if (ka->sa.sa_handler != SIG_DFL) {
+			event = UTRACE_EVENT(SIGNAL);
+			report.result = UTRACE_SIGNAL_DELIVER;
+		} else if (sig_kernel_coredump(signr)) {
+			event = UTRACE_EVENT(SIGNAL_CORE);
+			report.result = UTRACE_SIGNAL_CORE;
+		} else if (sig_kernel_ignore(signr)) {
+			event = UTRACE_EVENT(SIGNAL_IGN);
+			report.result = UTRACE_SIGNAL_IGN;
+		} else if (signr == SIGSTOP) {
+			event = UTRACE_EVENT(SIGNAL_STOP);
+			report.result = UTRACE_SIGNAL_STOP;
+		} else if (sig_kernel_stop(signr)) {
+			event = UTRACE_EVENT(SIGNAL_STOP);
+			report.result = UTRACE_SIGNAL_TSTP;
+		} else {
+			event = UTRACE_EVENT(SIGNAL_TERM);
+			report.result = UTRACE_SIGNAL_TERM;
+		}
+
+		/*
+		 * Now that we know what event type this signal is, we
+		 * can short-circuit if no engines care about those.
+		 */
+		if ((task->utrace_flags & (event | UTRACE_EVENT(QUIESCE))) == 0)
+			return signr;
+
+		/*
+		 * We have some interested engines, so tell them about
+		 * the signal and let them change its disposition.
+		 */
+		spin_unlock_irq(&task->sighand->siglock);
+	}
+
+	/*
+	 * This reporting pass chooses what signal disposition we'll act on.
+	 */
+	list_for_each_entry(engine, &utrace->attached, entry) {
+		/*
+		 * See start_callback() comment about this barrier.
+		 */
+		utrace->reporting = engine;
+		smp_mb();
+
+		/*
+		 * This pairs with the barrier in mark_engine_detached(),
+		 * see start_callback() comments.
+		 */
+		want = engine->flags;
+		smp_rmb();
+		ops = engine->ops;
+
+		if ((want & (event | UTRACE_EVENT(QUIESCE))) == 0) {
+			utrace->reporting = NULL;
+			continue;
+		}
+
+		if (ops->report_signal)
+			ret = (*ops->report_signal)(
+				report.result | report.action, engine, task,
+				regs, info, ka, return_ka);
+		else
+			ret = (report.result | (*ops->report_quiesce)(
+				       report.action, engine, task, event));
+
+		/*
+		 * Avoid a tight loop reporting again and again if some
+		 * engine is too stupid.
+		 */
+		switch (utrace_resume_action(ret)) {
+		default:
+			break;
+		case UTRACE_INTERRUPT:
+		case UTRACE_REPORT:
+			ret = (ret & ~UTRACE_RESUME_MASK) | UTRACE_RESUME;
+			break;
+		}
+
+		finish_callback(task, utrace, &report, engine, ret);
+	}
+
+	/*
+	 * We express the chosen action to the signals code in terms
+	 * of a representative signal whose default action does it.
+	 * Our caller uses our return value (signr) to decide what to
+	 * do, but uses info->si_signo as the signal number to report.
+	 */
+	switch (utrace_signal_action(report.result)) {
+	case UTRACE_SIGNAL_TERM:
+		signr = SIGTERM;
+		break;
+
+	case UTRACE_SIGNAL_CORE:
+		signr = SIGQUIT;
+		break;
+
+	case UTRACE_SIGNAL_STOP:
+		signr = SIGSTOP;
+		break;
+
+	case UTRACE_SIGNAL_TSTP:
+		signr = SIGTSTP;
+		break;
+
+	case UTRACE_SIGNAL_DELIVER:
+		signr = info->si_signo;
+
+		if (return_ka->sa.sa_handler == SIG_DFL) {
+			/*
+			 * We'll do signr's normal default action.
+			 * For ignore, we'll fall through below.
+			 * For stop/death, break locks and returns it.
+			 */
+			if (likely(signr) && !sig_kernel_ignore(signr))
+				break;
+		} else if (return_ka->sa.sa_handler != SIG_IGN &&
+			   likely(signr)) {
+			/*
+			 * Complete the bookkeeping after the report.
+			 * The handler will run.  If an engine wanted to
+			 * stop or step, then make sure we do another
+			 * report after signal handler setup.
+			 */
+			if (report.action != UTRACE_RESUME)
+				report.action = UTRACE_INTERRUPT;
+			finish_report(task, utrace, &report, true);
+
+			if (unlikely(report.result & UTRACE_SIGNAL_HOLD))
+				push_back_signal(task, info);
+			else
+				spin_lock_irq(&task->sighand->siglock);
+
+			/*
+			 * We do the SA_ONESHOT work here since the
+			 * normal path will only touch *return_ka now.
+			 */
+			if (unlikely(return_ka->sa.sa_flags & SA_ONESHOT)) {
+				return_ka->sa.sa_flags &= ~SA_ONESHOT;
+				if (likely(valid_signal(signr))) {
+					ka = &task->sighand->action[signr - 1];
+					ka->sa.sa_handler = SIG_DFL;
+				}
+			}
+
+			return signr;
+		}
+
+		/* Fall through for an ignored signal.  */
+
+	case UTRACE_SIGNAL_IGN:
+	case UTRACE_SIGNAL_REPORT:
+	default:
+		/*
+		 * If the signal is being ignored, then we are on the way
+		 * directly back to user mode.  We can stop here, or step,
+		 * as in utrace_resume(), above.  After we've dealt with that,
+		 * our caller will relock and come back through here.
+		 */
+		finish_resume_report(task, utrace, &report);
+
+		if (unlikely(fatal_signal_pending(task))) {
+			/*
+			 * The only reason we woke up now was because of a
+			 * SIGKILL.  Don't do normal dequeuing in case it
+			 * might get a signal other than SIGKILL.  That would
+			 * perturb the death state so it might differ from
+			 * what the debugger would have allowed to happen.
+			 * Instead, pluck out just the SIGKILL to be sure
+			 * we'll die immediately with nothing else different
+			 * from the quiescent state the debugger wanted us in.
+			 */
+			sigset_t sigkill_only;
+			siginitsetinv(&sigkill_only, sigmask(SIGKILL));
+			spin_lock_irq(&task->sighand->siglock);
+			signr = dequeue_signal(task, &sigkill_only, info);
+			BUG_ON(signr != SIGKILL);
+			*return_ka = task->sighand->action[SIGKILL - 1];
+			return signr;
+		}
+
+		if (unlikely(report.result & UTRACE_SIGNAL_HOLD)) {
+			push_back_signal(task, info);
+			spin_unlock_irq(&task->sighand->siglock);
+		}
+
+		return -1;
+	}
+
+	/*
+	 * Complete the bookkeeping after the report.
+	 * This sets utrace->resume if UTRACE_STOP was used.
+	 */
+	finish_report(task, utrace, &report, true);
+
+	return_ka->sa.sa_handler = SIG_DFL;
+
+	if (unlikely(report.result & UTRACE_SIGNAL_HOLD))
+		push_back_signal(task, info);
+	else
+		spin_lock_irq(&task->sighand->siglock);
+
+	if (sig_kernel_stop(signr))
+		task->signal->flags |= SIGNAL_STOP_DEQUEUED;
+
+	return signr;
+}
+
+/*
+ * This gets called after a signal handler has been set up.
+ * We set a flag so the next report knows it happened.
+ * If we're already stepping, make sure we do a report_signal.
+ * If not, make sure we get into utrace_resume() where we can
+ * clear the signal_handler flag before resuming.
+ */
+void utrace_signal_handler(struct task_struct *task, int stepping)
+{
+	struct utrace *utrace = task_utrace_struct(task);
+
+	spin_lock(&utrace->lock);
+
+	utrace->signal_handler = 1;
+	if (utrace->resume > UTRACE_INTERRUPT) {
+		if (stepping) {
+			utrace->resume = UTRACE_INTERRUPT;
+			set_tsk_thread_flag(task, TIF_SIGPENDING);
+		} else if (utrace->resume == UTRACE_RESUME) {
+			set_tsk_thread_flag(task, TIF_NOTIFY_RESUME);
+		}
+	}
+
+	spin_unlock(&utrace->lock);
+}
+
+/**
+ * utrace_prepare_examine - prepare to examine thread state
+ * @target:		thread of interest, a &struct task_struct pointer
+ * @engine:		engine pointer returned by utrace_attach_task()
+ * @exam:		temporary state, a &struct utrace_examiner pointer
+ *
+ * This call prepares to safely examine the thread @target using
+ * &struct user_regset calls, or direct access to thread-synchronous fields.
+ *
+ * When @target is current, this call is superfluous.  When @target is
+ * another thread, it must be held stopped via %UTRACE_STOP by @engine.
+ *
+ * This call may block the caller until @target stays stopped, so it must
+ * be called only after the caller is sure @target is about to unschedule.
+ * This means a zero return from a utrace_control() call on @engine giving
+ * %UTRACE_STOP, or a report_quiesce() or report_signal() callback to
+ * @engine that used %UTRACE_STOP in its return value.
+ *
+ * Returns -%ESRCH if @target is dead or -%EINVAL if %UTRACE_STOP was
+ * not used.  If @target has started running again despite %UTRACE_STOP
+ * (for %SIGKILL or a spurious wakeup), this call returns -%EAGAIN.
+ *
+ * When this call returns zero, it's safe to use &struct user_regset
+ * calls and task_user_regset_view() on @target and to examine some of
+ * its fields directly.  When the examination is complete, a
+ * utrace_finish_examine() call must follow to check whether it was
+ * completed safely.
+ */
+int utrace_prepare_examine(struct task_struct *target,
+			   struct utrace_engine *engine,
+			   struct utrace_examiner *exam)
+{
+	int ret = 0;
+
+	if (unlikely(target == current))
+		return 0;
+
+	rcu_read_lock();
+	if (unlikely(!engine_wants_stop(engine)))
+		ret = -EINVAL;
+	else if (unlikely(target->exit_state))
+		ret = -ESRCH;
+	else {
+		exam->state = target->state;
+		if (unlikely(exam->state == TASK_RUNNING))
+			ret = -EAGAIN;
+		else
+			get_task_struct(target);
+	}
+	rcu_read_unlock();
+
+	if (likely(!ret)) {
+		exam->ncsw = wait_task_inactive(target, exam->state);
+		put_task_struct(target);
+		if (unlikely(!exam->ncsw))
+			ret = -EAGAIN;
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_prepare_examine);
+
+/**
+ * utrace_finish_examine - complete an examination of thread state
+ * @target:		thread of interest, a &struct task_struct pointer
+ * @engine:		engine pointer returned by utrace_attach_task()
+ * @exam:		pointer passed to utrace_prepare_examine() call
+ *
+ * This call completes an examination on the thread @target begun by a
+ * paired utrace_prepare_examine() call with the same arguments that
+ * returned success (zero).
+ *
+ * When @target is current, this call is superfluous.  When @target is
+ * another thread, this returns zero if @target has remained unscheduled
+ * since the paired utrace_prepare_examine() call returned zero.
+ *
+ * When this returns an error, any examination done since the paired
+ * utrace_prepare_examine() call is unreliable and the data extracted
+ * should be discarded.  The error is -%EINVAL if @engine is not
+ * keeping @target stopped, or -%EAGAIN if @target woke up unexpectedly.
+ */
+int utrace_finish_examine(struct task_struct *target,
+			  struct utrace_engine *engine,
+			  struct utrace_examiner *exam)
+{
+	int ret = 0;
+
+	if (unlikely(target == current))
+		return 0;
+
+	rcu_read_lock();
+	if (unlikely(!engine_wants_stop(engine)))
+		ret = -EINVAL;
+	else if (unlikely(target->state != exam->state))
+		ret = -EAGAIN;
+	else
+		get_task_struct(target);
+	rcu_read_unlock();
+
+	if (likely(!ret)) {
+		unsigned long ncsw = wait_task_inactive(target, exam->state);
+		if (unlikely(ncsw != exam->ncsw))
+			ret = -EAGAIN;
+		put_task_struct(target);
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_finish_examine);
+
+/*
+ * This is declared in linux/regset.h and defined in machine-dependent
+ * code.  We put the export here to ensure no machine forgets it.
+ */
+EXPORT_SYMBOL_GPL(task_user_regset_view);
+
+/*
+ * Called with rcu_read_lock() held.
+ */
+void task_utrace_proc_status(struct seq_file *m, struct task_struct *p)
+{
+	seq_printf(m, "Utrace:\t%lx\n", p->utrace_flags);
+}

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