linux-kernel - Re: For review: pidfd_send

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Date:   Tue, 24 Sep 2019 14:08:33 -0700
From:   Daniel Colascione <dancol@...gle.com>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Florian Weimer <fw@...eb.enyo.de>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        linux-man <linux-man@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: For review: pidfd_send_signal(2) manual page

On Tue, Sep 24, 2019 at 2:00 PM Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> 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.