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Subject: Crash or hang if SIGSEGV+SIGALRM are nested
To: cygwin AT cygwin DOT com
Message-ID: <c4335d1d-5394-2a41-f06b-feb5c9fb1398@t-online.de>
Date: Mon, 19 May 2025 12:55:46 +0200
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From: Christian Franke via Cygwin <cygwin AT cygwin DOT com>
Reply-To: cygwin AT cygwin DOT com
Cc: Christian Franke <Christian DOT Franke AT t-online DOT de>
Sender: "Cygwin" <cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com>

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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.


BTW, this testcase without a second signal now works as expected:
https://sourceware.org/pipermail/cygwin/2025-March/257726.html

-- 
Regards,
Christian


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