Ptrace documentation, draft #4

[Denys Vlasenko]

Ptrace discussions repeatedly display a higher than average amount
of misunderstanding and confusion. New ptrace users and even people
who already worked with it are repeatedly confused by details
which are not documented anywhere and knowledge about which exists
mostly in the brains of strace/gdb/other_such_tools developers.

This document is meant as a brain dump of this knowledge.
It assumes that the reader has basic understanding what ptrace is.

The diff from version 3 is attached too.

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		Ptrace

Ptrace API (ab)uses standard Unix parent/child signaling over waitpid.
An unfortunate effect of it is that resulting API is complex and has
subtle quirks. This document aims to describe these quirks.

It is split into two parts. First part focuses exclusively on
userspace-visible API and behavior. Second section describes kernel
internals of ptrace.



		1. Userspace API.

(Note to editors: in this section, do not use kernel concepts and terms
which are not observable through userspace API and user-visible
behavior. Use section 2 for that.)

Debugged processes (tracees) first need to be attached to the debugging
process (tracer). Attachment and subsequent commands are per-thread: in
multi-threaded process, every thread can be individually attached to a
(potentially different) tracer, or left not attached and thus not
debugged. Therefore, "tracee" always means "(one) thread", never "a
(possibly multi-threaded) process". Ptrace commands are always sent to
a specific tracee using ptrace(PTRACE_foo, pid, ...), where pid is a
TID of the corresponding Linux thread.

After attachment, each tracee can be in two states: running or stopped.

There are many kinds of states when tracee is stopped, and in ptrace
discussions they are often conflated. Therefore, it is important to use
precise terms.

In this document, any stopped state in which tracee is ready to accept
ptrace commands from the tracer is called ptrace-stop. Ptrace-stops can
be further subdivided into signal-delivery-stop, group-stop,
syscall-stop and so on. They are described in detail later.


	1.x Death under ptrace.

When a (possibly multi-threaded) process receives a killing signal (a
signal set to SIG_DFL and whose default action is to kill the process),
all threads exit. Tracees report their death to the tracer(s). This is
not a ptrace-stop (because tracer can't query tracee status such as
register contents, cannot restart tracee etc) but the notification
about this event is delivered through waitpid API similarly to
ptrace-stop.

Note that killing signal will first cause signal-delivery-stop (on one
tracee only), and only after it is injected by tracer (or after it was
dispatched to a thread which isn't traced), death from signal will
happen on ALL tracees within multi-threaded process.

SIGKILL operates similarly, with exceptions. No signal-delivery-stop is
generated for SIGKILL and therefore tracer can't suppress it. SIGKILL
kills even within syscalls (syscall-exit-stop is not generated prior to
death by SIGKILL). The net effect is that SIGKILL always kills the
process (all its threads), even if some threads of the process are
ptraced.

Tracer can kill a tracee with ptrace(PTRACE_KILL, pid, 0, 0). This
opeartion is deprecated, use kill/tgkill(SIGKILL) instead.

^^^ Oleg prefers to deprecate it instead of describing (and needing to
support) PTRACE_KILL's quirks.

When tracee executes exit syscall, it reports its death to its tracer.
Other threads are not affected.

When any thread executes exit_group syscall, every tracee in its thread
group reports its death to its tracer.

If PTRACE_O_TRACEEXIT option is on, PTRACE_EVENT_EXIT will happen
before actual death. This applies to both normal exits and signal
deaths (except SIGKILL).

KNOWN BUG: PTRACE_EVENT_EXIT should happen for every tracee in thread
group on exit_group or signal death, but currently (~2.6.38) this is
buggy: some of these stops may be missed.

Tracer cannot assume that ptrace-stopped tracee exists. There are many
scenarios when tracee may die while stopped (such as SIGKILL). There
are cases where tracee disappears without reporting death (such as
execve in multi-threaded process). Therefore, tracer must always be
prepared to handle ESRCH error on any ptrace operation. Unfortunately,
the same error is returned if tracee exists but is not ptrace-stopped
(for commands which require stopped tracee). Tracer needs to keep track
of stopped/running state, and interpret ESRCH as "tracee died
unexpectedly" only if it knows that tracee has been observed to enter
ptrace-stop.

There is no guarantee that waitpid(WNOHANG) will reliably report
tracee's death status if ptrace operation returned ESRCH.
waitpid(WNOHANG) may return 0 instead. IOW: tracee may be "not yet
fully dead" but already refusing ptrace ops.

Tracer can not assume that tracee ALWAYS ends its life by reporting
WIFEXITED(status) or WIFSIGNALED(status). One notable case is execve in
multi-threaded process, which is described later.


	1.x Stopped states.

When running tracee enters ptrace-stop, it notifies its tracer using
waitpid API. Tracer should use waitpid family of syscalls to wait for
tracee to stop. Most of this document assumes that tracer waits with:

	pid = waitpid(pid_or_minus_1, &status, __WALL);

Ptrace-stopped tracees are reported as returns with pid > 0 and
WIFSTOPPED(status) == true.

??? Do we require __WALL usage, or will just using 0 be ok? Are the
rules different if user wants to use waitid? Will waitid require WEXITED?

__WALL value does not include WSTOPPED and WEXITED bits, but implies
their functionality.

Setting of WCONTINUED bit in waitpid flags is not recommended: the
continued state is per-process and consuming it would confuse real
parent of the tracee.

Use of WNOHANG bit in waitpid flags may cause waitpid return 0 ("no
wait results available yet") even if tracer knows there should be a
notification. Example: kill(tracee, SIGKILL); waitpid(tracee, &status,
__WALL | WNOHANG);

??? waitid usage? WNOWAIT?

??? describe how wait notifications queue (or not queue)

The following kinds of ptrace-stops exist: signal-delivery-stops,
group-stop, PTRACE_EVENT stops, syscall-stops [, SINGLESTEP, SYSEMU,
SYSEMU_SINGLESTEP]. They all are reported as waitpid result with
WIFSTOPPED(status) == true. They may be differentiated by checking
(status >> 8) value (note that WSTOPSIG(status) is (status >> 8) &
0xff) and if looking at (status >> 8) value doesn't resolve ambiguity,
by querying PTRACE_GETSIGINFO.


	1.x.x Signal-delivery-stop

When (possibly multi-threaded) process receives any signal except
SIGKILL, kernel selects a thread which handles the signal (if signal is
generated with tgkill, thread selection is done by user). If selected
thread is traced, it enters signal-delivery-stop. By this point, signal
is not yet delivered to the process, and can be suppressed by tracer.
If tracer doesn't suppress the signal, it passes signal to tracee in
the next ptrace request. This is called "signal injection" and will be
described later. Note that if signal is blocked, signal-delivery-stop
doesn't happen until signal is unblocked, with the usual exception that
SIGSTOP can't be blocked.

Signal-delivery-stop is observed by tracer as waitpid returning with
WIFSTOPPED(status) == true, WSTOPSIG(status) == signal. If
WSTOPSIG(status) == SIGTRAP, this may be a different kind of
ptrace-stop - see "Syscall-stops" and "execve" sections below for
details. If WSTOPSIG(status) == stopping signal, this may be a
group-stop - see below.


	1.x.x Signal injection and suppression.

After signal-delivery-stop is observed by tracer, tracer should restart
tracee with
	ptrace(PTRACE_rest, pid, 0, sig)
call, where PTRACE_rest is one of the restarting ptrace ops. If sig is
0, then signal is not delivered. Otherwise, signal sig is delivered.
This operation is called "signal injection", to distinguish it from
signal delivery which causes signal-delivery-stop.

Note that sig value may be different from WSTOPSIG(status) value -
tracer can cause a different signal to be injected.

Note that suppressed signal still causes syscalls to return
prematurely. Restartable syscalls will be restarted (tracer will
observe tracee to execute restart_syscall(2) syscall if tracer uses
PTRACE_SYSCALL), non-restartable syscalls (for example, nanosleep) may
return with -EINTR even though no observable signal is injected to the
tracee.

Note that restarting ptrace commands issued in ptrace-stops other than
signal-delivery-stop are not guaranteed to inject a signal, even if sig
is nonzero. No error is reported, nonzero sig may simply be ignored.
Ptrace users should not try to "create new signal" this way: use
tgkill(2) instead.

This is a cause of confusion among ptrace users. One typical scenario
is that tracer observes group-stop, mistakes it for
signal-delivery-stop, restarts tracee with ptrace(PTRACE_rest, pid, 0,
stopsig) with the intention of injecting stopsig, but stopsig gets
ignored and tracee continues to run.

SIGCONT signal has a side effect of waking up (all threads of)
group-stopped process. This side effect happens before
signal-delivery-stop. Tracer can't suppress this side-effect (it can
only suppress signal injection, which only causes SIGCONT handler to
not be executed in the tracee, if such handler is installed). In fact,
waking up from group-stop may be followed by signal-delivery-stop for
signal(s) *other than* SIGCONT, if they were pending when SIGCONT was
delivered. IOW: SIGCONT may be not the first signal observed by the
tracee after it was sent.

Stopping signals cause (all threads of) process to enter group-stop.
This side effect happens after signal injection, and therefore can be
suppressed by tracer.

PTRACE_GETSIGINFO can be used to retrieve siginfo_t structure which
corresponds to delivered signal. PTRACE_SETSIGINFO may be used to
modify it. If PTRACE_SETSIGINFO has been used to alter siginfo_t,
si_signo field and sig parameter in restarting command must match,
otherwise the result is undefined.


	1.x.x Group-stop

When a (possibly multi-threaded) process receives a stopping signal,
all threads stop. If some threads are traced, they enter a group-stop.
Note that stopping signal will first cause signal-delivery-stop (on one
tracee only), and only after it is injected by tracer (or after it was
dispatched to a thread which isn't traced), group-stop will be
initiated on ALL tracees within multi-threaded process. As usual, every
tracee reports its group-stop to corresponding tracer.

Group-stop is observed by tracer as waitpid returning with
WIFSTOPPED(status) == true, WSTOPSIG(status) == signal. The same result
is returned by some other classes of ptrace-stops, therefore the
recommended practice is to perform
	ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)
call. The call can be avoided if signal number is not SIGSTOP, SIGTSTP,
SIGTTIN or SIGTTOU - only these four signals are stopping signals. If
tracer sees something else, it can't be group-stop. Otherwise, tracer
needs to call PTRACE_GETSIGINFO. If PTRACE_GETSIGINFO fails with
EINVAL, then it is definitely a group-stop. (Other failure codes are
possible, such as ESRCH "no such process" if SIGKILL killed the tracee).

As of kernel 2.6.38, after tracer sees tracee ptrace-stop and until it
restarts or kills it, tracee will not run, and will not send
notifications (except SIGKILL death) to tracer, even if tracer enters
into another waitpid call.

Currently, it causes a problem with transparent handling of stopping
signals: if tracer restarts tracee after group-stop, SIGSTOP is
effectively ignored: tracee doesn't remain stopped, it runs. If tracer
doesn't restart tracee before entering into next waitpid, future
SIGCONT will not be reported to the tracer. Which would make SIGCONT to
have no effect.


	1.x.x PTRACE_EVENT stops

If tracer sets TRACE_O_TRACEfoo options, tracee will enter ptrace-stops
called PTRACE_EVENT stops.

PTRACE_EVENT stops are observed by tracer as waitpid returning with
WIFSTOPPED(status) == true, WSTOPSIG(status) == SIGTRAP. Additional bit
is set in a higher byte of status word: value ((status >> 8) & 0xffff)
will be (SIGTRAP | PTRACE_EVENT_foo << 8). The following events exist:

PTRACE_EVENT_VFORK - stop before return from vfork/clone+CLONE_VFORK.
When tracee is continued after this, it will wait for child to
exit/exec before continuing its execution (IOW: usual behavior on
vfork).

PTRACE_EVENT_FORK - stop before return from fork/clone+SIGCHLD

PTRACE_EVENT_CLONE - stop before return from clone

PTRACE_EVENT_VFORK_DONE - stop before return from
vfork/clone+CLONE_VFORK, but after vfork child unblocked this tracee by
exiting or exec'ing.

For all four stops described above: stop occurs in parent, not in newly
created thread. PTRACE_GETEVENTMSG can be used to retrieve new thread's
tid.

PTRACE_EVENT_EXEC - stop before return from exec.

PTRACE_EVENT_EXIT - stop before exit. PTRACE_GETEVENTMSG returns exit
status. Registers can be examined (unlike when "real" exit happens).
The tracee is still alive, it needs to be PTRACE_CONTed to finish exit.

PTRACE_GETSIGINFO on PTRACE_EVENT stops returns si_signo = SIGTRAP,
si_code = (event << 8) | SIGTRAP.


	1.x.x Syscall-stops

If tracee was restarted by PTRACE_SYSCALL, tracee enters
syscall-enter-stop just prior to entering any syscall. If tracer
restarts it with PTRACE_SYSCALL, tracee enters syscall-exit-stop when
syscall is finished, or if it is interrupted by a signal. (That is,
signal-delivery-stop never happens between syscall-enter-stop and
syscall-exit-stop, it happens *after* syscall-exit-stop).

Other possibilities are that tracee may stop in a PTRACE_EVENT stop,
exit (if it entered exit or exit_group syscall), be killed by SIGKILL,
or die silently (if execve syscall happened in another thread).

Syscall-enter-stop and syscall-exit-stop are observed by tracer as
waitpid returning with WIFSTOPPED(status) == true, WSTOPSIG(status) ==
SIGTRAP. If PTRACE_O_TRACESYSGOOD option was set by tracer, then
WSTOPSIG(status) == (SIGTRAP | 0x80).

Syscall-stops can be distinguished from signal-delivery-stop with
SIGTRAP by querying PTRACE_GETSIGINFO: si_code <= 0 if sent by usual
suspects like [tg]kill/sigqueue/etc; or = SI_KERNEL (0x80) if sent by
kernel, whereas syscall-stops have si_code = SIGTRAP or (SIGTRAP |
0x80). However, syscall-stops happen very often (twice per syscall),
and performing PTRACE_GETSIGINFO for every syscall-stop may be somewhat
expensive.

Some architectures allow to distinguish them by examining registers.
For example, on x86 rax = -ENOSYS in syscall-enter-stop. Since SIGTRAP
(like any other signal) always happens *after* syscall-exit-stop, and
at this point rax almost never contains -ENOSYS, SIGTRAP looks like
"syscall-stop which is not syscall-enter-stop", IOW: it looks like a
"stray syscall-exit-stop" and can be detected this way. But such
detection is fragile and is best avoided.

Using PTRACE_O_TRACESYSGOOD option is a recommended method, since it is
reliable and does not incur performance penalty.

Syscall-enter-stop and syscall-exit-stop are indistinguishable from
each other by tracer. Tracer needs to keep track of the sequence of
ptrace-stops in order to not misinterpret syscall-enter-stop as
syscall-exit-stop or vice versa. The rule is that syscall-enter-stop is
always followed by syscall-exit-stop, PTRACE_EVENT stop or tracee's
death - no other kinds of ptrace-stop can occur in between.

If after syscall-enter-stop tracer uses restarting command other than
PTRACE_SYSCALL, syscall-exit-stop is not generated.

PTRACE_GETSIGINFO on syscall-stops returns si_signo = SIGTRAP, si_code
= SIGTRAP or (SIGTRAP | 0x80).


	1.x.x SINGLESTEP, SYSEMU, SYSEMU_SINGLESTEP

??? document PTRACE_SINGLESTEP, PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP


	1.x Informational and restarting ptrace commands.

Most ptrace commands (all except ATTACH, TRACEME, KILL) require tracee
to be in ptrace-stop, otherwise they fail with ESRCH.

When tracee is in ptrace-stop, tracer can read and write data to tracee
using informational commands. They leave tracee in ptrace-stopped state:

longv = ptrace(PTRACE_PEEKTEXT/PEEKDATA/PEEKUSER, pid, addr, 0);
	ptrace(PTRACE_POKETEXT/POKEDATA/POKEUSER, pid, addr, long_val);
	ptrace(PTRACE_GETREGS/GETFPREGS, pid, 0, &struct);
	ptrace(PTRACE_SETREGS/SETFPREGS, pid, 0, &struct);
	ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo);
	ptrace(PTRACE_SETSIGINFO, pid, 0, &siginfo);
	ptrace(PTRACE_GETEVENTMSG, pid, 0, &long_var);
	ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags);

Note that some errors are not reported. For example, setting siginfo
may have no effect in some ptrace-stops, yet the call may succeed
(return 0 and don't set errno).

ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_flags) affects one tracee.
Current flags are replaced. Flags are inherited by new tracees created
and "auto-attached" via active PTRACE_O_TRACE[V]FORK or
PTRACE_O_TRACECLONE options.

Another group of commands makes ptrace-stopped tracee run. They have
the form:
	ptrace(PTRACE_cmd, pid, 0, sig);
where cmd is CONT, DETACH, SYSCALL, SINGLESTEP, SYSEMU, or
SYSEMU_SINGLESTEP. If tracee is in signal-delivery-stop, sig is the
signal to be injected. Otherwise, sig may be ignored.


	1.x Attaching and detaching

A thread can be attached to tracer using ptrace(PTRACE_ATTACH, pid, 0,
0) call. This also sends SIGSTOP to this thread. If tracer wants this
SIGSTOP to have no effect, it needs to suppress it. Note that if other
signals are concurrently sent to this thread during attach, tracer may
see tracee enter signal-delivery-stop with other signal(s) first! The
usual practice is to reinject these signals until SIGSTOP is seen, then
suppress SIGSTOP injection. The design bug here is that attach and
concurrent SIGSTOP are racing and SIGSTOP may be lost.

??? Describe how to attach to a thread which is already group-stopped.

Since attaching sends SIGSTOP and tracer usually suppresses it, this
may cause stray EINTR return from the currently executing syscall in
the tracee, as described in "signal injection and suppression" section.

ptrace(PTRACE_TRACEME, 0, 0, 0) request turns current thread into a
tracee. It continues to run (doesn't enter ptrace-stop). A common
practice is follow ptrace(PTRACE_TRACEME) with raise(SIGSTOP) and allow
parent (which is our tracer now) to observe our signal-delivery-stop.

If PTRACE_O_TRACE[V]FORK or PTRACE_O_TRACECLONE options are in effect,
then children created by (vfork or clone(CLONE_VFORK)), (fork or
clone(SIGCHLD)) and (other kinds of clone) respectively are
automatically attached to the same tracer which traced their parent.
SIGSTOP is delivered to them, causing them to enter
signal-delivery-stop after they exit syscall which created them.

Detaching of tracee is performed by ptrace(PTRACE_DETACH, pid, 0, sig).
PTRACE_DETACH is a restarting operation, therefore it requires tracee
to be in ptrace-stop. If tracee is in signal-delivery-stop, signal can
be injected. Othervice, sig parameter may be silently ignored.

If tracee is running when tracer wants to detach it, the usual solution
is to send SIGSTOP (using tgkill, to make sure it goes to the correct
thread), wait for tracee to stop in signal-delivery-stop for SIGSTOP
and then detach it (suppressing SIGSTOP injection). Design bug is that
this can race with concurrent SIGSTOPs. Another complication is that
tracee may enter other ptrace-stops and needs to be restarted and
waited for again, until SIGSTOP is seen. Yet another complication is to
be sure that tracee is not already ptrace-stopped, because no signal
delivery happens while it is - not even SIGSTOP.

??? Describe how to detach from a group-stopped tracee so that it
    doesn't run, but continues to wait for SIGCONT.

If tracer dies, all tracees are automatically detached and restarted,
unless they were in group-stop. Handling of restart from group-stop is
currently buggy, but "as planned" behavior is to leave tracee stopped
and waiting for SIGCONT. If tracee is restarted from
signal-delivery-stop, pending signal is injected.


	1.x execve under ptrace.

During execve, kernel destroys all other threads in the process, and
resets execve'ing thread tid to tgid (process id). This looks very
confusing to tracers:

All other threads "disappear" - that is, they terminate their execution
without returning any waitpid notifications to anyone, even if they are
currently traced.

The execve-ing tracee changes its pid while it is in execve syscall.
(Remember, under ptrace 'pid' returned from waitpid, or fed into ptrace
calls, is tracee's tid). That is, pid is reset to process id, which
coincides with thread group leader tid.

If thread group leader has reported its death by this time, for tracer
this looks like dead thread leader "reappears from nowhere". If thread
group leader was still alive, for tracer this may look as if thread
group leader returns from a different syscall than it entered, or even
"returned from syscall even though it was not in any syscall". If
thread group leader was not traced (or was traced by a different
tracer), during execve it will appear as if it has become a tracee of
the tracer of execve'ing tracee. All these effects are the artifacts of
pid change.

PTRACE_O_TRACEEXEC option is the recommended tool for dealing with this
case. It enables PTRACE_EVENT_EXEC stop which occurs before execve
syscall return.

Pid change happens before PTRACE_EVENT_EXEC stop, not after.

When tracer receives PTRACE_EVENT_EXEC stop notification, it is
guaranteed that except this tracee, no other threads from the process
are alive. Moreover, it is guaranteed that tracer will not receive any
"buffered" death reports from any of them, even if some threads were
racing with execve'ing tracee, for example were entering exit syscall.

On receiving this notification, tracer should clean up all its internal
data structures about all threads of this process, and retain only one
data structure, one which describes single still running tracee, with
pid = tgid = process id.

??? How tracer knows which of its many tracees _are_ threads of that
particular process? (It may trace more than one process; it may even
don't keep track of its tracees' thread group relations at all...)

??? what happens if two threads execve at the same time? Clearly, only
one of them succeeds, but *which* one? Think "strace -f" or
multi-threaded process here:

  ** we get death notification: leader died: **
 PID0 exit(0)                            = ?
  ** we get syscall-entry-stop in thread 1: **
 PID1 execve("/bin/foo", "foo" <unfinished ...>
  ** we get syscall-entry-stop in thread 2: **
 PID2 execve("/bin/bar", "bar" <unfinished ...>
  ** we get PTRACE_EVENT_EXEC for PID0, we issue PTRACE_SYSCALL **
  ** we get syscall-exit-stop for PID0: **
 PID0 <... execve resumed> )             = 0

??? Question: WHICH execve succeeded? Can tracer figure it out?

If PTRACE_O_TRACEEXEC option is NOT in effect for the execve'ing
tracee, kernel delivers an extra SIGTRAP to tracee after execve syscall
returns. This is an ordinary signal (similar to one which can be
generated by "kill -TRAP"), not a special kind of ptrace-stop.
GETSIGINFO on it has si_code = 0 (SI_USER). It can be blocked by signal
mask, and thus can happen (much) later.

Usually, tracer (for example, strace) would not want to show this extra
post-execve SIGTRAP signal to the user, and would suppress its delivery
to the tracee (if SIGTRAP is set to SIG_DFL, it is a killing signal).
However, determining *which* SIGTRAP to suppress is not easy. Setting
PTRACE_O_TRACEEXEC option and thus suppressing this extra SIGTRAP is
the recommended approach.


	1.x Real parent

Ptrace API (ab)uses standard Unix parent/child signaling over waitpid.
This used to cause real parent of the process to stop receiving several
kinds of waitpid notifications when child process is traced by some
other process.

Many of these bugs have been fixed, but as of 2.6.38 several still
exist.

As of 2.6.38, the following is believed to work correctly:

- exit/death by signal is reported first to tracer, then, when tracer
consumes waitpid result, to real parent (to real parent only when the
whole multi-threaded process exits). If they are the same process, the
report is sent only once.

- ??? add more docs

Following bugs still exist:

- group-stop notifications are sent to tracer, but not to real parent.

- If thread group leader it is traced and exits, do_wait(WEXITED)
doesn't work (until all threads exit) for its the tracer.

??? add more known bugs here



	2. Linux kernel implementation

TODO
--- DOCa	2011-05-20 21:52:24.152964068 +0200
+++ DOCb.txt	2011-05-30 16:03:34.878790225 +0200
@@ -105,16 +105,39 @@
 When running tracee enters ptrace-stop, it notifies its tracer using
 waitpid API. Tracer should use waitpid family of syscalls to wait for
 tracee to stop. Most of this document assumes that tracer waits with:
+
 	pid = waitpid(pid_or_minus_1, &status, __WALL);
+
 Ptrace-stopped tracees are reported as returns with pid > 0 and
 WIFSTOPPED(status) == true.
 
-??? any pitfalls with WNOHANG (I remember that there are bugs in this
-    area)? effects of WSTOPPED, WEXITED, WCONTINUED bits? Are they ok?
-    waitid usage? WNOWAIT?
+??? Do we require __WALL usage, or will just using 0 be ok? Are the
+rules different if user wants to use waitid?
+
+__WALL value does not include WSTOPPED and WEXITED bits, but implies
+their functionality.
+
+Setting of WCONTINUED bit in waitpid flags is not recommended: the
+continued state is per-process and consuming it would confuse real
+parent of the tracee.
+
+Use of WNOHANG bit in waitpid flags may cause waitpid return 0 ("no
+wait results available yet") even if tracer knows there should be a
+notification. Example: kill(tracee, SIGKILL); waitpid(tracee, &status,
+__WALL | WNOHANG);
+
+??? waitid usage? WNOWAIT?
 
 ??? describe how wait notifications queue (or not queue)
 
+The following kinds of ptrace-stops exist: signal-delivery-stops,
+group-stop, PTRACE_EVENT stops, syscall-stops [, SINGLESTEP, SYSEMU,
+SYSEMU_SINGLESTEP]. They all are reported as waitpid result with
+WIFSTOPPED(status) == true. They may be differentiated by checking
+(status >> 8) value (note that WSTOPSIG(status) is (status >> 8) &
+0xff) and if looking at (status >> 8) value doesn't resolve ambiguity,
+by querying PTRACE_GETSIGINFO.
+
 
 	1.x.x Signal-delivery-stop
 
@@ -186,7 +209,8 @@
 PTRACE_GETSIGINFO can be used to retrieve siginfo_t structure which
 corresponds to delivered signal. PTRACE_SETSIGINFO may be used to
 modify it. If PTRACE_SETSIGINFO has been used to alter siginfo_t,
-si_signo field and sig parameter in restarting command must match.
+si_signo field and sig parameter in restarting command must match,
+otherwise the result is undefined.
 
 
 	1.x.x Group-stop
@@ -207,8 +231,9 @@
 call. The call can be avoided if signal number is not SIGSTOP, SIGTSTP,
 SIGTTIN or SIGTTOU - only these four signals are stopping signals. If
 tracer sees something else, it can't be group-stop. Otherwise, tracer
-needs to call PTRACE_GETSIGINFO. If PTRACE_GETSIGINFO fails, then it is
-definitely a group-stop.
+needs to call PTRACE_GETSIGINFO. If PTRACE_GETSIGINFO fails with
+EINVAL, then it is definitely a group-stop. (Other failure codes are
+possible, such as ESRCH "no such process" if SIGKILL killed the tracee).
 
 As of kernel 2.6.38, after tracer sees tracee ptrace-stop and until it
 restarts or kills it, tracee will not run, and will not send
@@ -278,20 +303,24 @@
 SIGTRAP. If PTRACE_O_TRACESYSGOOD option was set by tracer, then
 WSTOPSIG(status) == (SIGTRAP | 0x80).
 
-There is no portable way to distinguish them from signal-delivery-stop
-with SIGTRAP. Some architectures allow to distinguish them by examining
-registers. For example, on x86 rax = -ENOSYS in syscall-enter-stop.
-Since SIGTRAP (like any other signal) always happens *after*
-syscall-exit-stop, and at this point rax almost never contains -ENOSYS,
-SIGTRAP looks like "syscall-stop which is not syscall-enter-stop", IOW:
-it looks like a "stray syscall-exit-stop" and can be detected this way.
-But such detection is fragile and is best avoided. Using
-PTRACE_O_TRACESYSGOOD option is a recommended method.
-
-??? can be distinguished by PTRACE_GETSIGINFO, si_code <= 0 if sent by
-usual suspects like [t]kill, sigqueue; or = SI_KERNEL (0x80) if sent by
+Syscall-stops can be distinguished from signal-delivery-stop with
+SIGTRAP by querying PTRACE_GETSIGINFO: si_code <= 0 if sent by usual
+suspects like [tg]kill/sigqueue/etc; or = SI_KERNEL (0x80) if sent by
 kernel, whereas syscall-stops have si_code = SIGTRAP or (SIGTRAP |
-0x80). Right? Should this be documented?
+0x80). However, syscall-stops happen very often (twice per syscall),
+and performing PTRACE_GETSIGINFO for every syscall-stop may be somewhat
+expensive.
+
+Some architectures allow to distinguish them by examining registers.
+For example, on x86 rax = -ENOSYS in syscall-enter-stop. Since SIGTRAP
+(like any other signal) always happens *after* syscall-exit-stop, and
+at this point rax almost never contains -ENOSYS, SIGTRAP looks like
+"syscall-stop which is not syscall-enter-stop", IOW: it looks like a
+"stray syscall-exit-stop" and can be detected this way. But such
+detection is fragile and is best avoided.
+
+Using PTRACE_O_TRACESYSGOOD option is a recommended method, since it is
+reliable and does not incur performance penalty.
 
 Syscall-enter-stop and syscall-exit-stop are indistinguishable from
 each other by tracer. Tracer needs to keep track of the sequence of
@@ -387,11 +416,9 @@
 this can race with concurrent SIGSTOPs. Another complication is that
 tracee may enter other ptrace-stops and needs to be restarted and
 waited for again, until SIGSTOP is seen. Yet another complication is to
-be sure that tracee is not already group-stopped, because no signal
+be sure that tracee is not already ptrace-stopped, because no signal
 delivery happens while it is - not even SIGSTOP.
 
-??? is above accurate?
-
 ??? Describe how to detach from a group-stopped tracee so that it
     doesn't run, but continues to wait for SIGCONT.