linux-kernel - Re: For review: pidfd_send

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Date:   Tue, 24 Sep 2019 23:00:03 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     mtk.manpages@...il.com, Florian Weimer <fw@...eb.enyo.de>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Daniel Colascione <dancol@...gle.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

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.

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,

Michael

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