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SubjectRe: For review: pidfd_send_signal(2) manual page
On Wed, Sep 25, 2019 at 03:46:26PM +0200, Michael Kerrisk (man-pages) wrote:
> On 9/24/19 11:53 PM, Christian Brauner wrote:
> > On Tue, Sep 24, 2019 at 11:00:03PM +0200, Michael Kerrisk (man-pages) wrote:
> >> Hello Christian,
> >>
> >>>>> If you're the parent of the process you can do this without CLONE_PIDFD:
> >>>>> pid = fork();
> >>>>> pidfd = pidfd_open();
> >>>>> ret = pidfd_send_signal(pidfd, 0, NULL, 0);
> >>>>> if (ret < 0 && errno == ESRCH)
> >>>>> /* pidfd refers to another, recycled process */
> >>>>
> >>>> Although there is still the race between the fork() and the
> >>>> pidfd_open(), right?
> >>>
> >>> Actually no and my code is even too complex.
> >>> If you are the parent, and this is really a sequence that obeys the
> >>> ordering pidfd_open() before waiting:
> >>>
> >>> pid = fork();
> >>> if (pid == 0)
> >>> exit(EXIT_SUCCESS);
> >>> pidfd = pidfd_open(pid, 0);
> >>> waitid(pid, ...);
> >>>
> >>> Then you are guaranteed that pidfd will refer to pid. No recycling can
> >>> happen since the process has not been waited upon yet (That is,
> >>
> >> D'oh! Yes, of course.
> >>
> >>> excluding special cases such as where you have a mainloop where a
> >>> callback reacts to a SIGCHLD event and waits on the child behind your
> >>> back and your next callback in the mainloop calls pidfd_open() while the
> >>> pid has been recycled etc.).
> >>> A race could only appear in sequences where waiting happens before
> >>> pidfd_open():
> >>>
> >>> pid = fork();
> >>> if (pid == 0)
> >>> exit(EXIT_SUCCESS);
> >>> waitid(pid, ...);
> >>> pidfd = pidfd_open(pid, 0);
> >>>
> >>> which honestly simply doesn't make any sense. So if you're the parent
> >>> and you combine fork() + pidfd_open() correctly things should be fine
> >>> without even having to verify via pidfd_send_signal() (I missed that in
> >>> my first mail.).
> >>
> >> Thanks for the additional detail.
> >
> > You're very welcome.
> >
> >>
> >> I added the following to the pidfd_open() page, to
> >> prevent people making the same thinko as me:
> >>
> >> The following code sequence can be used to obtain a file descrip‐
> >> tor for the child of fork(2):
> >>
> >> pid = fork();
> >> if (pid > 0) { /* If parent */
> >> pidfd = pidfd_open(pid, 0);
> >> ...
> >> }
> >>
> >> Even if the child process has already terminated by the time of
> >> the pidfd_open() call, the returned file descriptor is guaranteed
> >> to refer to the child because the parent has not yet waited on the
> >> child (and therefore, the child's ID has not been recycled).
> >
> > Thanks! I'm fine with the example. The code illustrates the basics. If
> > you want to go overboard, you can mention my callback example and put my
> > SIG_IGN code snippet from my earlier mails (cf. [1] and [2]) in there.
> > But imho, that'll complicate the manpage and I'm not sure it's worth it.
>
> I agree that we should not complicate this discussion with more code,
> but how about we refine the text as follows:
>
> The following code sequence can be used to obtain a file descrip‐
> tor for the child of fork(2):
>
> pid = fork();
> if (pid > 0) { /* If parent */
> pidfd = pidfd_open(pid, 0);
> ...
> }
>
> Even if the child has already terminated by the time of the
> pidfd_open() call, its PID will not have been recycled and the
> returned file descriptor will refer to the resulting zombie
> process. Note, however, that this is guaranteed only if the fol‐
> lowing conditions hold true:
>
> * the disposition of SIGCHLD has not been explicitly set to
> SIG_IGN (see sigaction(2)); and

Ugh, I forgot a third one. There's also SA_NOCLDWAIT. When set and
the SIGCHLD handler is set to SIG_DFL then no zombie processes are
created and no SIGCHLD signal is sent. When an explicit handler for
SIGCHLD is set then a SIGCHLD signal is generated but the process will
still not be turned into a zombie...

>
> * the zombie process was not reaped elsewhere in the program
> (e.g., either by an asynchronously executed signal handler or
> by wait(2) or similar in another thread).
>
> If these conditions don't hold true, then the child process should

"If any of these conditions does not hold, the child process..."

That might be clearer. But I leave the call on that to you. :)

Christian

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