[RFC PATCH v6 0/6] tracing/probes: uaccess: Add support user-space access

[Masami Hiramatsu]

Hi,

Here is the v6 series of probe-event to support user-space access.

In this version, I replaced user_access_ok() patch with access_ok()
enhancement, which allows user to call access_ok() in IRQ context
if it disables pagefault. In the result of this change, I also
removed user_access_ok() related patch.
This version also extends perf-probe to handle user-space memory.
This is still not automated yet, but it can be done when __user
is encoded in debuginfo.

Changes in v6:
- [1/6]: (New) allow access_ok() in IRQ context if pagefault is
         disabled.
- [3/6]: Remove user_access_ok()
- [5/6]: Add $argN availablity check
- [6/6]: (New) extend perf-probe to handle "@user" attribute which
         allows user to specify "user-space local variable"

V5 series is here;

https://lkml.kernel.org/r/155136974478.2968.3105123100519786079.stgit@devbox

In summary, strncpy_from_user() should work as below

 - strncpy_from_user() can access user memory with set_fs(USER_DS)
   in task context

 - strncpy_from_user() can access kernel memory with set_fs(KERNEL_DS)
   in task context (e.g. devtmpfsd and init)

 - strncpy_from_user() can access user/kernel memory (depends on DS)
   in IRQ context if pagefault is disabled. (both verified)

PeterZ, would you still have any concern about this check?

====
Kprobe event user-space memory access features:

For user-space access extension, this series adds 2 features,
"ustring" type and user-space dereference syntax. "ustring" is
used for recording a null-terminated string in user-space from
kprobe events.

"ustring" type is easy, it is able to use instead of "string"
type, so if you want to record a user-space string via
"__user char *", you can use ustring type instead of string.
For example,

echo 'p do_sys_open path=+0($arg2):ustring' >> kprobe_events

will record the path string from user-space.

The user-space dereference syntax is also simple. Thi just
adds 'u' prefix before an offset value.

   +|-u<OFFSET>(<FETCHARG>)

e.g. +u8(%ax), +u0(+0(%si))

This is more generic. If you want to refer the variable in user-
space from its address or access a field in data structure in
user-space, you need to use this.

For example, if you probe do_sched_setscheduler(pid, policy,
param) and record param->sched_priority, you can add new
probe as below;
    
   p do_sched_setscheduler priority=+u0($arg3)

Actually, with this feature, "ustring" type is not absolutely
necessary, because these are same meanings.

  +0($arg2):ustring == +u0($arg2):string

Note that kprobe event provides these methods, but it doesn't
change it from kernel to user automatically because we do not
know whether the given address is in userspace or kernel on
some arch.


Thank you,

---

Masami Hiramatsu (6):
      x86/uaccess: Allow access_ok() in irq context if pagefault_disabled
      uaccess: Add non-pagefault user-space read functions
      tracing/probe: Add ustring type for user-space string
      tracing/probe: Support user-space dereference
      selftests/ftrace: Add user-memory access syntax testcase
      perf-probe: Add user memory access attribute support


 Documentation/trace/kprobetrace.rst                |   28 ++++-
 Documentation/trace/uprobetracer.rst               |    9 +-
 arch/x86/include/asm/uaccess.h                     |    4 +
 include/linux/uaccess.h                            |   19 +++
 kernel/trace/trace.c                               |    7 +
 kernel/trace/trace_kprobe.c                        |   35 ++++++
 kernel/trace/trace_probe.c                         |   37 +++++-
 kernel/trace/trace_probe.h                         |    3 +
 kernel/trace/trace_probe_tmpl.h                    |   37 +++++-
 kernel/trace/trace_uprobe.c                        |   19 +++
 mm/maccess.c                                       |  117 +++++++++++++++++++-
 tools/perf/Documentation/perf-probe.txt            |    3 -
 tools/perf/util/probe-event.c                      |   11 ++
 tools/perf/util/probe-event.h                      |    2 
 tools/perf/util/probe-file.c                       |    7 +
 tools/perf/util/probe-file.h                       |    1 
 tools/perf/util/probe-finder.c                     |   19 ++-
 .../ftrace/test.d/kprobe/kprobe_args_user.tc       |   32 +++++
 18 files changed, 349 insertions(+), 41 deletions(-)
 create mode 100644 tools/testing/selftests/ftrace/test.d/kprobe/kprobe_args_user.tc

--
Masami Hiramatsu (Linaro) <mhiramat@xxxxxxxxxx>