[Bug backtrace/15558] New: "set backtrace limit" can cause assertion failure

[Bug backtrace/15558] New: "set backtrace limit" can cause assertion failure

[tromey at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15558

            Bug ID: 15558
           Summary: "set backtrace limit" can cause assertion failure
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: backtrace
          Assignee: unassigned at sourceware dot org
          Reporter: tromey at redhat dot com

"set backtrace limit" can cause an assertion failure.
If the limit is reached at an inline frame, then this
assertion in inline_frame_this_id will fail:

  gdb_assert (frame_id_p (*this_id));

One simple workaround is to change get_prev_frame to ignore the
backtrace limit for INLINE_FRAME.  This isn't completely ideal,
though.

Pedro suggests that most internals code should use get_prev_frame_1
instead, and bypass all the checks that correspond to user settings.
E.g., gcore.c or infrun.c:stepped_in_from.  The idea would be to
use get_prev_frame only in cases like "bt" or the MI equivalent.

Another wrinkle is that the Python code should probably use
the _1 variant -- but then this will mean that "backtrace limit"
won't be in effect when a Python frame filter is found.  So, some
extra code will be needed here.

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[Bug backtrace/15558] "set backtrace limit" can cause assertion failure

[tromey at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15558

Tom Tromey <tromey at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at sourceware dot org   |tromey at redhat dot com

--- Comment #1 from Tom Tromey <tromey at redhat dot com> ---
Testing a patch.

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[Bug backtrace/15558] "set backtrace limit" can cause assertion failure

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=15558

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https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=51d481464ec03be1f5479ed648cc38fb944d7fc0

commit 51d481464ec03be1f5479ed648cc38fb944d7fc0
Author: Pedro Alves <palves@redhat.com>
Date:   Fri Apr 18 10:15:21 2014 +0100

    Fix PR backtrace/15558

    This PR is about an assertion failure in GDB that can be triggered by
    setting "backtrace limit" to a value that causes GDB to stop unwinding
    after an inline frame.  In this case, an assertion in
    inline_frame_this_id will trigger:

      /* We need a valid frame ID, so we need to be based on a valid
         frame.  (...).  */
      gdb_assert (frame_id_p (*this_id));

    Looking at the function:

     static void
     inline_frame_this_id (struct frame_info *this_frame,
                   void **this_cache,
                   struct frame_id *this_id)
     {
       struct symbol *func;

       /* In order to have a stable frame ID for a given inline function,
          we must get the stack / special addresses from the underlying
          real frame's this_id method.  So we must call get_prev_frame.
          Because we are inlined into some function, there must be previous
          frames, so this is safe - as long as we're careful not to
          create any cycles.  */
       *this_id = get_frame_id (get_prev_frame (this_frame));

    we see we're computing the frame id for the inline frame.  If this is
    an inline frame, which is a virtual frame constructed based on debug
    info, on top of a real stack frame, we should _always_ be able to find
    where the frame was inlined into, as that ultimately just means
    peeling off the virtual frames on top of the real stack frame.  If
    there ultimately was no prev (real) stack frame, then we wouldn't have
    been able to construct the inline frame either, by design.  That's
    what the assertion catches.

    So we have an inline frame, we should _always_ be able to compute its
    ID, even if that means bypassing the user backtrace limits to get at
    the real stack frame's info.  The problem is that inline_frame_id
    calls get_prev_frame, and that takes user backtrace limits into
    account.  Code that wants to bypass the limits calls get_prev_frame_1
    instead.

    Note how get_prev_frame_1 already skips all checks for inline frames:

       /* If we are unwinding from an inline frame, all of the below tests
          were already performed when we unwound from the next non-inline
          frame.  We must skip them, since we can not get THIS_FRAME's ID
          until we have unwound all the way down to the previous non-inline
          frame.  */
       if (get_frame_type (this_frame) == INLINE_FRAME)
         return get_prev_frame_if_no_cycle (this_frame);

    And note how the related frame_unwind_caller_id function also uses
    get_prev_frame_1:

     struct frame_id
     frame_unwind_caller_id (struct frame_info *next_frame)
     {
       struct frame_info *this_frame;

       /* Use get_prev_frame_1, and not get_prev_frame.  The latter will
truncate
          the frame chain, leading to this function unintentionally
          returning a null_frame_id (e.g., when a caller requests the frame
          ID of "main()"s caller.  */

       next_frame = skip_artificial_frames (next_frame);
       this_frame = get_prev_frame_1 (next_frame);
       if (this_frame)
         return get_frame_id (skip_artificial_frames (this_frame));
       else
         return null_frame_id;
     }

    get_prev_frame_1 is currently static in frame.c.  As a _1 suffix is
    not a good name for an extern function, I've renamed it.

    Tested on x86-64 Fedora 17.

    gdb/
    2014-04-18  Pedro alves  <palves@redhat.com>
            Tom Tromey  <tromey@redhat.com>

        PR backtrace/15558
        * frame.c (get_prev_frame_1): Rename to ...
        (get_prev_frame_always): ... this, and make extern.  Adjust.
        (skip_artificial_frames): Use get_prev_frame_always.
        (frame_unwind_caller_id, frame_pop, get_prev_frame)
        (get_frame_unwind_stop_reason): Adjust to rename.
        * frame.h (get_prev_frame_always): Declare.
        * inline-frame.c: Include frame.h.
        (inline_frame_this_id): Use get_prev_frame_always.

    gdb/testsuite/
    2014-04-18  Tom Tromey  <palves@redhat.com>
            Pedro alves  <tromey@redhat.com>

        PR backtrace/15558
        * gdb.opt/inline-bt.exp: Test backtracing from an inline function
        with a backtrace limit.
        * gdb.python/py-frame-inline.exp: Test running to an inline
        function with a backtrace limit, and printing the newest frame.
        * gdb.python/py-frame-inline.c (main): Call f.

-----------------------------------------------------------------------

Summary of changes:
 gdb/ChangeLog                                |   13 +++++++++++
 gdb/frame.c                                  |   31 +++++++++++++++-----------
 gdb/frame.h                                  |    7 ++++++
 gdb/inline-frame.c                           |   11 +++++----
 gdb/testsuite/ChangeLog                      |   10 ++++++++
 gdb/testsuite/gdb.opt/inline-bt.exp          |   16 +++++++++++++
 gdb/testsuite/gdb.python/py-frame-inline.c   |    4 ++-
 gdb/testsuite/gdb.python/py-frame-inline.exp |   14 +++++++++++
 8 files changed, 87 insertions(+), 19 deletions(-)

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[Bug backtrace/15558] "set backtrace limit" can cause assertion failure

[palves at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=15558

Pedro Alves <palves at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |palves at redhat dot com
         Resolution|---                         |FIXED
   Target Milestone|---                         |7.8

--- Comment #3 from Pedro Alves <palves at redhat dot com> ---
Fixed.

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