On Tue, Sep 24, 2019 at 2:00 PM Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> 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.
You still have a race if you're the parent and you have SIGCHLD set to
SIG_IGN though.
> > 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.).
That's a pretty common case though, especially if you're a library.
> > 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.
>
> 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).
I'd prefer that sample code be robust in all cases.
