Re: [PATCH] prctl.2: PR_SET_PDEATHSIG by orphan process

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hi Jann,

Your mail is a little cryptic. It would be best to start with
a brief summary of your point--something like the text of your
patch at the end of the mail.

On 01/06/2016 07:23 PM, Jann Horn wrote:
> Proof:
> In kernel/sys.c:
> 
> 	case PR_SET_PDEATHSIG:
> 		if (!valid_signal(arg2)) {
> 			error = -EINVAL;
> 			break;
> 		}
> 		me->pdeath_signal = arg2;
> 		break;

I don't understand how the code above relates to the point you
want to make. (Or maybe you mean: "look, there's no check here
to see that if the parent is already dead"; but it would help
to state that explicitly).

> Testcase:
> 
> #include <sys/prctl.h>
> #include <err.h>
> #include <unistd.h>
> #include <signal.h>
> #include <stdio.h>
> 
> void ponk(int s) {
>   puts("ponk!");
> }
> 
> int main(void) {
>   if (fork() == 0) {
>     if (fork() == 0) {
>       sleep(1);
>       signal(SIGUSR1, ponk);
>       prctl(PR_SET_PDEATHSIG, SIGUSR1, 0, 0, 0);
>       sleep(1);
>       return 0;
>     }
>     return 0;
>   }
> 
>   sleep(3);
>   return 0;
> }
> ---
>  man2/prctl.2 | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/man2/prctl.2 b/man2/prctl.2
> index 5cea3bb..3dce8e9 100644
> --- a/man2/prctl.2
> +++ b/man2/prctl.2
> @@ -670,6 +670,9 @@ In other words, the signal will be sent when that thread terminates
>  (via, for example,
>  .BR pthread_exit (3)),
>  rather than after all of the threads in the parent process terminate.
> +
> +If the parent has already died by the time the parent death signal
> +is set, the new parent death signal will not be sent.

In a way, this seems almost obvious. But perhaps it is better to make the
point explicitly, as you suggest. But, because there may have been a
previous PR_SET_PDEATHSIG, I'd prefer something like this:

[[
If the caller's parent has already died by the time of this
PR_SET_PDEATHSIG operation, the operation shall have no effect.
]]

What do you think?

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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