Re: [PATCH] gdb: fix an issue with thread list corruption

[Simon Marchi <simark@simark.ca>]

On 2026-05-05 01:04, Metzger, Markus T wrote:
> Hello Simon,
> 
>> -----Original Message-----
>> From: Simon Marchi <simark@simark.ca>
>> Sent: Monday, May 4, 2026 5:30 PM
>> To: Metzger, Markus T <markus.t.metzger@intel.com>; gdb-
>> patches@sourceware.org
>> Subject: Re: [PATCH] gdb: fix an issue with thread list corruption
>>
>> On 5/4/26 3:16 AM, Markus Metzger wrote:
>>> When resuming a target in non-stop mode with 'c -a', the continue
>> command
>>> uses for_each_thread() to proceed each stopped thread individually.  This
>>> uses an all_threads_safe() iteration.
>>>
>>> If one of the stopped threads does an inline step-over, since the target
>>> is non-stop, we stop_all_threads(), which involves update_thread_list(),
>>> which, in turn, may delete_thread().
>>>
>>> If this deleted the thread pointed to by the m_next safe iterator member,
>>> the above all_threads_safe() iteration will be corrupted.
>>>
>>> The thread we're proceeding is stopped and there is no reason to delete
>>> it.  Consequently, there is no reason for all_threads_safe(), which isn't
>>> that safe in this scenario.
>>>
>>> Iterate using all_threads() and inline proceed_thread_callback().
>>
>> Just to try to make sure I understand the circumstances that lead the
>> the failure correcty (and make sure this doesn't just cover up other
>> problems):
>>
>> - thread 1 is stopped on a breakpoint
>> - thread 2 is executing
>> - displaced stepping is disabled
>> - the target doesn't report thread events to the core
>> - you "continue -a"
>> - thread 1 starts an inline step-over, which calls stop_all_threads,
>>   which calls update_thread_list
>> - meanwhile, thread 2 exits
>> - update_thread_list causes thread 2 to be deleted
>> - the safe iterator's m_next field now points to a deleted thread info

Can you comment on whether the sequence of events above sounds correct?
If it is, I think it would be possible to write a CPU test to reproduce
it, that would be helpful along with the fix.

Simon