Re: For review: pidfd_send_signal(2) manual page

On Tue, Sep 24, 2019 at 11:00:03PM +0200, Michael Kerrisk (man-pages) wrote:

Hello Christian,

quoted

quoted

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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. 

quoted

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.

Thanks!
Christian

/* References */
[1]: https://lore.kernel.org/r/20190924195701.7pw2olbviieqsg5q@wittgenstein (local)
[2]: https://lore.kernel.org/r/20190924200735.2dvqhan7ynnmfc7s@wittgenstein (local)