Re: [PATCH 4/7] gdb: remove manual frame_info reinflation code in backtrace_command_1

[Bruno Larsen <blarsen@redhat.com>]

On 07/11/2022 16:53, Simon Marchi via Gdb-patches wrote:
> From: Simon Marchi <simon.marchi@efficios.com>
>
> With the following patch applied (gdb: use frame_id_p instead of
> comparing to null_frame_id in frame_info_ptr::reinflate), I would get:
>
>      $ ./gdb -q -nx --data-directory=data-directory testsuite/outputs/gdb.base/bt-selected-frame/bt-selected-frame -ex "b breakpt" -ex r -ex "bt full"
>      Reading symbols from testsuite/outputs/gdb.base/bt-selected-frame/bt-selected-frame...
>      Breakpoint 1 at 0x1131: file /home/smarchi/src/binutils-gdb/gdb/testsuite/gdb.base/bt-selected-frame.c, line 22.
>      Starting program: /home/smarchi/build/binutils-gdb/gdb/testsuite/outputs/gdb.base/bt-selected-frame/bt-selected-frame
>      [Thread debugging using libthread_db enabled]
>      Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
>
>      Breakpoint 1, breakpt () at /home/smarchi/src/binutils-gdb/gdb/testsuite/gdb.base/bt-selected-frame.c:22
>      22      }
>      #0  breakpt () at /home/smarchi/src/binutils-gdb/gdb/testsuite/gdb.base/bt-selected-frame.c:22
>      No locals.
>      /home/smarchi/src/binutils-gdb/gdb/frame-info.c:42: internal-error: reinflate: Assertion `frame_id_p (m_cached_id)' failed.
>
> This is because the code in backtrace_command_1 to manually reinflate
> `fi` steps overs frame_info_ptr's toes.
>
> When calling
>
>      fi.prepare_reinflate ();
>
> `fi` gets properly filled with the cached frame id.  But when this
> happens:
>
>      fi = frame_find_by_id (frame_id);
>
> `fi` gets replaced by a brand new frame_info_ptr that doesn't have a
> cached frame id.  Then this is called without a cached frame id:
>
>      fi.reinflate ();
>
> That doesn't cause any problem currently, since
>
>   - the gdb_assert in the reinflate method doesn't actually do anything
>     (the following patch fixes that)
>   - `fi.m_ptr` will always be non-nullptr, since we just got it from
>     frame_find_by_id, so reinflate will not do anything, it won't try to
>     use m_cached_id
>
> Fix that by removing the code to manually re-fetch the frame.  That
> should be taken care of by frame_info_ptr::reinflate.
>
> Note that the old code checked if we successfully re-inflated the frame
> or not, and if not it did emit a warning.  The equivalent in
> frame_info_ptr::reinflate asserts that the frame has been successfully
> re-inflated.  It's not clear if / when this can happen, but if it can
> happen, we'll need to find a solution to this problem globally
> (everywhere a frame_info_ptr can be re-inflated), not just here.  So I
> propose to leave it like this, until it does become a problem.
>
I think this patch could be squashed with the one you mention at the top 
of the commit message. I think they are both related to the same thing 
(fixing the assert and dealing with fallout) and I don't see much point 
in separating how you did here.

-- 
Cheers,
Bruno