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

[Simon Marchi <simon.marchi@polymtl.ca>]

On 11/8/22 05:14, Bruno Larsen wrote:
> 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.

I think it would be fine to have them merged or have them separate, but
at this point I'd prefer to keep it like this, since it's done.

Simon