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From: "vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug python/31437] New: [gdb/python] Unavailable register breaks
 python unwinding
Date: Fri, 01 Mar 2024 12:24:39 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D31437

            Bug ID: 31437
           Summary: [gdb/python] Unavailable register breaks python
                    unwinding
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: python
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

On arm-linux, until this commit:
...
commit bbb12eb9c84aa2b32480b7c022c494c2469ef717
Author: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Date:   Mon Feb 26 19:11:45 2024 -0300

    gdb/arm: Remove tpidruro register from non-FreeBSD target descriptions
...
I ran into:
...
FAIL: gdb.base/inline-frame-cycle-unwind.exp: cycle at level 5: backtrace w=
hen
the unwind is broken at frame 5
...

Because this was the only reported progression, I decided to investigate a =
bit
further.

I found that the python unwinder fails because this piece of code in
pyuw_sniffer ignores the fact that value can be !entirely_available:
...
    /* Populate registers array.  */
    for (int i =3D 0; i < unwind_info->saved_regs->size (); ++i)
      {
        ...
        memcpy (cached->data.get (), value->contents ().data (), data_size);
      }
  }
...
which throws an unavailable error, which is then caught by
frame_unwind_try_unwinder:
...
      if (ex.error =3D=3D NOT_AVAILABLE_ERROR)
        {
          /* This usually means that not even the PC is available,=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20
             thus most unwinders aren't able to determine if they're=20=20=
=20=20=20=20=20=20=20=20=20=20
             the best fit.  Keep trying.  Fallback prologue unwinders=20=20=
=20=20=20=20=20=20=20=20=20
             should always accept the frame.  */
          return 0;
        }
...

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