[SOLVED] making child process die on SIGKILL to parent

olga_chotinun · 02-26-2015, 04:02 PM

Hi!
I am trying to ensure child processes exit when parent is killed. I have registered signal handler for all the "bad" signals to ensure clean exit, but I also want parent to kill children when it gets SIGKILL.

I have tried the following:

polling for getppid() == 1 on child, then exit
prctl(PR_SET_PDEATHSIG, SIGTERM);
setpgid() for child to join parent's process group, called in child process after fork() before exec()

Still, when I kill my parent process with -9, the child processes keep running.

I would appreciate any suggestions.
Thank you!
Olga

genss · 02-26-2015, 04:30 PM

from prctl man page

Code:

       PR_SET_PDEATHSIG (since Linux 2.1.57)
              Set the parent process death signal of the calling process to arg2 (either a signal value in the range
              1..maxsig, or 0 to clear).  This is the signal that the calling process will get when its parent dies.
              This value is cleared for the child of a fork(2) and (since Linux 2.4.36 / 2.6.23)  when  executing  a
              set-user-ID or set-group-ID binary.  This value is preserved across execve(2).

       PR_GET_PDEATHSIG (since Linux 2.3.15)
              Return  the  current  value  of the parent process death signal, in the location pointed to by (int *)
              arg2.

linux only

olga_chotinun · 02-26-2015, 04:34 PM

Thank You! This is the first thing I tried actually, doesn't work for me, I will try to debug.
Thank You!
Olga

genss · 02-26-2015, 05:11 PM

np, np

from what the man page says arg2 to prctl's PR_GET_PDEATHSIG is a pointer to that (childs, in this case) memory
so you would have to periodically check if the value of that int changed

something like (never tried)

int asd = 0;
prctl(PR_GET_PDEATHSIG, &asd);

then check every once in a while if that int holds a SIG (non-null) value

PS thank YOU, didn't know about this call and it seems like fun

olga_chotinun · 02-26-2015, 05:16 PM

Thanks! I am very frustrated because I think PR_SET_PDEATHSIG() alone should have worked for me

when I call it on the child! I will try polling and report here on my success/failure

olga_chotinun · 02-26-2015, 07:02 PM

Allright

prctl(PR_SET_PDEATHSIG, SIGKILL); alone works if I put it right after fork() in child process, and before exec()

Code:

    pid_t cpid = fork();
    if (cpid == 0) {
        ...
        // When parent exists, send SIGKILL to all children
        prctl(PR_SET_PDEATHSIG, SIGKILL);

        // Pass argv[0], arg1, arg2, ..., NULL  to execl()
        int execReturn = execl(_...);
        ...
    }
}

genss · 02-27-2015, 09:29 AM

good that you didn't follow my example, i misunderstood a thing

glad it works but it should work even after exec
i didn't play with this, but that sounds weird