Hang or crash after multiple SIGILL or SIGSEGV and siglongjmp

[Takashi Yano]

On Tue, 25 Mar 2025 14:38:35 +0100
Christian Franke wrote:
> Found because 'stress-ng --priv-instr ...' hangs and then requires 
> '/bin/kill --force ...':
> 
> Testcase with
>    [PATCH v2] Cygwin: signal: Copy context to alternate stack in the 
> SA_ONSTACK case
> already applied:
> 
> $ uname -r
> 3.7.0-dev-16-g2ef1a37e7823-dirty.x86_64
> 
> $ cat loopsigill.c
> #include <setjmp.h>
> #include <signal.h>
> #include <stdio.h>
> #include <unistd.h>
> 
> static volatile sig_atomic_t sigcnt;
> static sigjmp_buf sjb;
> 
> static void sighandler(int sig)
> {
>    (void)sig;
>    ++sigcnt;
>    siglongjmp(sjb, 1);
>    write(1, "[FAIL]\n", 7);
> }
> 
> int main()
> {
>    signal(SIGILL, sighandler);
>    printf("pid=%d\n", (int)getpid());
> 
>    while (sigsetjmp(sjb, 1))
>      ;
> 
>    // loop:
>    if (sigcnt < 10 || !(sigcnt % 1000))
>      printf("%06d\n", sigcnt);
>    if (sigcnt >= 100000)
>        return 42;
>    asm volatile ("invd"); // goto loop;
> 
>    return 13; // NOT REACHED
> }
> 
> $ gcc -o loopsigill loopsigill.c
> 
> $ ./loopsigill # may succeed ...
> pid=122
> 000000
> 000001
> ...
> 099000
> 100000
> 
> $ echo $?
> 42
> 
> $ ./loopsigill # ... or crash silently ...
> pid=130
> 000000
> 000001
> ...
> 026000
> 027000
> 
> $ echo $?
> 0
> 
> $ ./loopsigill # ... or hang
> pid=135
> 000000
> 000001
> ...
> 037000
> 038000
> [requires '/bin/kill --force ...']
> 
> $ strace -o trace.log ./loopsigill # run '/bin/kill --force ...' ASAP!
> pid=142
> 000000
> [always hangs after first signal and fills trace.log quickly]
> 
> $ less trace.log
> ...
>     25   25501 [main] loopsigill 142 write: 7 = write(1, 0xA00017710, 7)
> --- Process 6856 (pid: 142), exception c0000096 at 00000001004011b9
>    142   25643 [main] loopsigill 142 exception::handle: In 
> cygwin_except_handler exception 0xC0000096 at 0x1004011B9 sp 0x7FFFFCBE0
>     26   25669 [main] loopsigill 142 exception::handle: In 
> cygwin_except_handler signal 4 at 0x1004011B9
>     38   25707 [main] loopsigill 142 break_here: break here
> --- Process 6856 (pid: 142), exception c0000096 at 00000001004011b9
> --- Process 6856 (pid: 142), exception c0000096 at 00000001004011b9
> ... likely repeated until disk is full or time_t wraps around...
> --- Process 6856 (pid: 142), exception c0000096 at 00000001004011b9
> 
> 
> Problem also occurs
> - without the mentioned patch,
> - with get/setcontext() instead of sig*jmp(),
> - with nullptr access and SIGSEGV handler,
> - with Cygwin 3.5.7-1.
> 
> I agree that this is not a common use case :-)

Thanks for the report. I'm sorry for keeping you waiting so long.
I finally could fix the issue. I'll push the patch shortly.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>