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Subject: Re: Crash or hang if SIGSEGV+SIGALRM are nested
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Date: Fri, 30 May 2025 11:04:03 +0200
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From: Christian Franke via Cygwin <cygwin AT cygwin DOT com>
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Takashi Yano via Cygwin wrote:
> On Thu, 29 May 2025 20:01:31 +0200
> Christian Franke wrote:
>> Takashi Yano via Cygwin wrote:
>>> On Thu, 29 May 2025 17:32:19 +0200
>>> Christian Franke wrote:
>>> ...
>>>> I still don't fully understand why a SIGSEGV triggered by an instruction
>>>> could interrupt a SIGALRM handler.
>>>> https://sourceware.org/pipermail/cygwin/2025-May/258145.html
>>>>
>>>> I guess such behavior is valid from the POSIX point of view, but it is
>>>> at least unexpected. If the SIGALRM handler is already running, it
>>>> should have interrupted the thread such that the instruction triggering
>>>> the segfault is not executed until the SIGALRM handler returns.
>>> I try to explain what is happing using the figure below.
>>>
>>>      <<<<<< Main thread >>>>>>                       < Signal Thread >
>>>                                               SIGNAL
>>>     main()   handler1() handler2()             QUEUE    wait_sig()
>>>       |          .          .                    |          |
>>>       |          .          .       ALRM         |          |
>>>       +----------------------------------------->|   ALRM   |
>>>       |          .          .       SEGV         +--------->|
>>>       X----------------------------------------->|          |
>>>       |          .       arm ALRM                |          |
>>>       +----------+ <----------------------------------------+
>>>                  |          .                    |   SEGV   |
>>>                  |          .                    +--------->|
>>>                  |          .                    |          |
>>>                  |          .      arm SEGV      |          |
>>>                  +----------+ <-----------------------------+
>>>                  .          |                    |          |
>>>            longjmp()        |                    |          |
>>>       +---------------------+                    |          |
>>>       |          .          .                    |          |
>>>       |          .          .                    |          |
>>>
>>> The point is the exception handler does not arm SIGSEGV handler
>>> directly. It just pushes the SIGSEGV into the signal queue.
>>> The signal thread reads the queue and process it asynchronously,
>>> and arms the handler().
>>>
>> I see. Thanks for nice explanation!
>>
>> I created an issue at stress-ng upstream because at least the --mprotect
>> test is affected:
>> https://github.com/ColinIanKing/stress-ng/issues/529
> Thanks!
>
> BTW, do you think this is the right thing?
> https://github.com/RISCV-Tools/stress-ng/commit/bf24393357cde137bb8e7b38f917f565946199fd
>
> I think the patch should set SIGSEGV mask on SIGALRM.

Indeed, I missed that, thanks! Reopen of the issue requested.

-- 
Regards,
Christian


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