Re: [hackers] Re: [dwm][PATCH v2] Use sigaction(SA_NOCLDWAIT) for SIGCHLD handling

From: NRK <nrk_AT_disroot.org>
Date: Tue, 24 Jan 2023 10:25:11 +0600

On Mon, Jan 23, 2023 at 08:05:19PM +0100, Hiltjo Posthuma wrote:
> Do you perhaps also have some simple way to reproduce where it causes issues in
> some real world use-case?
>
> Ideally some command or script to reproduce the issue.

To trigger the issue, you need to have 3 conditions met:

1. Recieve a SIGCHLD.
2. `waitpid` failing and spoiling the `errno`.
3. Being unlucky that the resuming code gets affected by the `errno`.

Here's a simple dummy program demonstrating the issue. Compile it, run
it and then send some SIGCHLDs to it:

        [/tmp]~> cat sigtest.c
        #include <stdlib.h>
        #include <signal.h>
        #include <errno.h>
        #include <stdio.h>
        #include <sys/wait.h>
        
        void
        sigchld(int unused)
        {
                if (signal(SIGCHLD, sigchld) == SIG_ERR)
                        _Exit(1);
                while (0 < waitpid(-1, NULL, WNOHANG));
        }
        
        int main(void)
        {
                sigchld(0);
                while (1) {
                        errno = 0;
                        char *p = malloc(8);
                        if (p != NULL && errno) {
                                perror("sigtest: ");
                                return 1;
                        }
                        free(p);
                }
        }
        [/tmp]~> cc -o sigtest sigtest.c
        [/tmp]~> ./sigtest &
        [1] 9363
        [/tmp]~> while pidof sigtest >/dev/null; do kill -s SIGCHLD $(pidof sigtest); done
        sigtest: : No child processes

If you're asking for `dwm` in specific - then trigger condition 2 is
difficult (impossible ??) since the window manager will typically have
some children. But if you insert the following, to simulate failure:

                 while (0 < waitpid(-1, NULL, WNOHANG));
        + errno = ECHILD;

then the issue is easily reproducable by sending some SIGCHLDs to dwm
(in quick succession in order to maximize chances of triggering
condition 3):

        var=$(pidof dwm); while :; do kill -s SIGCHLD $var; done

And dwm will crash with a XIO error with in a couple seconds on my
system.

- NRK
Received on Tue Jan 24 2023 - 05:25:11 CET