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Subject: Re: Segfault if /proc/PID/maps is opened in parallel threads
To: cygwin AT cygwin DOT com
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Date: Sun, 25 May 2025 16:46:49 +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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On Sat, 24 May 2025 15:19:10 +0200, Christian Franke wrote:
> If /proc/PID/maps is opened in parallel threads, the process PID may 
> segfault.
>
> Testcase:
>
> $ uname -r
> 3.7.0-0.98.gb39b510c1ce6.x86_64
>
> $ cat thrdopen.c
> #include <fcntl.h>
> #include <pthread.h>
> #include <stdio.h>
> #include <unistd.h>
>
> static const char * name;
>
> static void *worker(void *arg)
> {
>   for (int i = 0; i < 10000; i++) {
>     putchar((int)(intptr_t)arg); fflush(stdout);
>     int fd = open(name, O_RDONLY);
>     if (fd < 0) {
>       perror(name); break;
>     }
>     close(fd);
>   }
>   return NULL;
> }
>
> int main(int argc, char **argv)
> {
>   if (argc != 2)
>     return 1;
>   name = argv[1];
>   pthread_t t;
>   if (pthread_create(&t, NULL, worker, (void*)'-'))
>     return 2;
>   worker((void*)'+');
>   pthread_join(t, NULL);
>   return 0;
> }
>
> $ gcc -o thrdopen thrdopen.c
>
> $ cygstart mintty - # start 2nd terminal
>
> $ pstree -p
> ?(1)─┬─mintty(1146)───bash(1147)
>      └─mintty(992)───bash(993)───pstree(1152)
>
> $ ./thrdopen /proc/1147/maps # 2nd terminal closes
> +-+-+-+-+--+-++--+-+-+-+-+-++-+-+-+-+-+-+-+/proc/1147/maps: No such 
> file or directory
> -/proc/1147/maps: No such file or directory
>
> $ pstree -p
> ?(1)───mintty(992)───bash(993)───pstree(1154)
>
>
> If the bash is run in strace, output is like this:
> ...
>   155  536982 [main] bash 1179 select_stuff::wait: m 3, us 
> 18446744073709551615, wmfo_timeout -1
> [testcase run here]
> --- Process 14992 (pid: 1179) thread 15200 exited with status 0xc0000005
> --- Process 14992 thread 15024 exited with status 0xc0000005
> --- Process 14992 thread 9340 exited with status 0xc0000005
> --- Process 14992 thread 2504 exited with status 0xc0000005
> --- Process 14992 thread 2136 exited with status 0xc0000005
> --- Process 14992 thread 880 exited with status 0xc0000005
> --- Process 14992 thread 6484 exited with status 0xc0000005
> --- Process 14992 thread 16516 exited with status 0xc0000005
> --- Process 14992 exited with status 0xc0000005
>
> The last line may not appear and strace hangs then.
>
> Problem is not reproducible with any of the other /proc/PID/* files.
>

A closer look shows that the problem is unrelated to thread safety. The 
segfault also occurs when the following command is run simultaneously in 
two terminals:

$ while cat /proc/1234/maps > /dev/null; do printf .; done
.............cat: /proc/1234/maps: No such file or directory

-- 
Regards,
Christian


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