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Subject: Re: Crash or hang if SIGSEGV+SIGALRM are nested
To: cygwin AT cygwin DOT com
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Message-ID: <76940263-49e8-1b4a-17f2-11f5545014ab@t-online.de>
Date: Thu, 29 May 2025 17:32:19 +0200
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From: Christian Franke via Cygwin <cygwin AT cygwin DOT com>
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Cc: Christian Franke <Christian DOT Franke AT t-online DOT de>
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Takashi Yano via Cygwin wrote:
> On Wed, 28 May 2025 21:57:07 +0900
> Takashi Yano wrote:
>> Hi Christian,
>>
>> On Mon, 19 May 2025 12:55:46 +0200
>> Christian Franke wrote:
>>> The attached testcase was originally intended to investigate why a
>>> SIGSEGV from non-signal code could interrupt an already running signal
>>> handler.
>>> https://sourceware.org/pipermail/cygwin-patches/2025q2/013703.html
>>>
>>> If run without strace, the testcase may crash silently (with exit status 0):
>>>
>>> $ uname -r
>>> 3.7.0-0.98.gb39b510c1ce6.x86_64
>>>
>>> $ gcc -o sigsegvalrm sigsegvalrm.c
>>>
>>> $ while { ./sigsegvalrm; s=$?; echo exit $s; test $s = 42; }; do :; done
>>> ...
>>> [SEGV during ALRM]
>>> [SEGV]
>>> [ALRM during SEGV]
>>> [ALRM]
>>> 101 total, 24 ALRM during SEGV, 13 SEGV during ALRM
>>> exit 42
>>> ...
>>> [SEGV during ALRM]
>>> [ALRM]
>>> [SEGV]
>>> [ALRM]
>>> [SEGV]
>>> [ALRM during SEGV]
>>> [SEGV]
>>> [ALRM]
>>> [SEGV]
>>> exit 0
>>>
>>>
>>> If the above was run with 'strace ./sigsegvalrm', the result was an
>>> infinte loop:
>>> https://cygwin.com/pipermail/cygwin/2025-May/258144.html
>>>
>>> Fortunately this is fixed since b39b510c. A new result:
>>>
>>> ...
>>> [SEGV during ALRM]
>>>     205  556472 [main] sigsegvalrm 1342 fhandler_console::write: 19 =
>>> fhandler_console::write(...)
>>>      91  556563 [main] sigsegvalrm 1342 write: 19 = write(1, 0x100403020, 19)
>>>      81  556644 [main] sigsegvalrm 1342 clock_nanosleep: clock_nanosleep
>>> (0.001000000)
>>>    8396  565040 [itimer] sigsegvalrm 1342 timer_tracker::thread_func:
>>> 0x7FFE4CC69640 timer expired
>>>     230  565270 [main] sigsegvalrm 1342 clock_nanosleep: 0 =
>>> clock_nanosleep(1, 0, 0.001000000, 0.d)
>>>     123  565393 [itimer] sigsegvalrm 1342 timer_tracker::thread_func:
>>> 0x7FFE4CC69640 sending signal 14
>>>     230  565623 [main] sigsegvalrm 1342 set_signal_mask: setmask 2400,
>>> newmask 0, mask_bits 2400
>>>     147  565770 [main] sigsegvalrm 1342 pthread_sigmask: 0 =
>>> pthread_sigmask(0, 0x100407128, 0x0)
>>>     220  565990 [itimer] sigsegvalrm 1342 sig_send: sendsig 0x158, pid
>>> 1342, signal 14, its_me 1
>>>     278  566268 [main] sigsegvalrm 1342 pthread_sigmask: 0 =
>>> pthread_sigmask(0, 0x0, 0x100407128)
>>> --- Process 148 (pid: 1342), exception c0000005 at 0000000100401287
>>>    1579  567847 [sig] sigsegvalrm 1342 sigpacket::process: signal 14
>>> processing
>>>     189  568036 [sig] sigsegvalrm 1342 init_cygheap::find_tls: sig 14
>>>     235  568271 [sig] sigsegvalrm 1342 sigpacket::process: using tls
>>> 0x7FFFFCE00
>>>     195  568466 [main] sigsegvalrm 1342 exception::handle: In
>>> cygwin_except_handler exception 0xC0000005 at 0x100401287 sp 0x7FFFFCBE0
>>>     131  568597 [sig] sigsegvalrm 1342 sigpacket::process: signal 14,
>>> signal handler 0x100401080
>>>      82  568679 [main] sigsegvalrm 1342 exception::handle: In
>>> cygwin_except_handler signal 11 at 0x100401287
>>>      79  568758 [sig] sigsegvalrm 1342 sigpacket::setup_handler:
>>> suspending thread, tls 0x7FFFFCE00, _main_tls 0x7FFFFCE00
>>> [~30s delay]
>>> --- Process 148 (pid: 1342) thread 14964 created
>>> --- Process 148 (pid: 1342) thread 14048 created
>>> [~30s delay]
>>> --- Process 148 (pid: 1342) thread 5184 exited with status 0x0
>>> --- Process 148 (pid: 1342) thread 5056 exited with status 0x0
>>> [several minutes delay]
>>> --- Process 148 (pid: 1342) thread 9388 created
>>>
>>> The process then ignores SIGKILL.
>> Thanks for reporting this. I finally found the solution.
>> Please test
>> https://cygwin.com/pipermail/cygwin-patches/2025q2/013731.html
>> https://cygwin.com/pipermail/cygwin-patches/2025q2/013732.html
> Updated to v2:
> https://cygwin.com/pipermail/cygwin-patches/2025q2/013731.html
> https://cygwin.com/pipermail/cygwin-patches/2025q2/013733.html
>

Problem does no longer occur if both patches are applied, thanks!

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.

-- 
Thanks,
Christian


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