Re: [PATCH v2 0/5] pid: add pidfd_open()

[Daniel Colascione]

On Sun, Mar 31, 2019 at 8:05 AM Christian Brauner <christian@xxxxxxxxxx> wrote:
>
> On Sun, Mar 31, 2019 at 07:52:28AM -0700, Linus Torvalds wrote:
> > On Sat, Mar 30, 2019 at 9:47 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> > >
> > > Sure, given a pidfd_clone() syscall, as long as the parent of the
> > > process is giving you a pidfd for it and you don't have to deal with
> > > grandchildren created by fork() calls outside your control, that
> > > works.
> >
> > Don't do pidfd_clone() and pidfd_wait().
> >
> > Both of those existing system calls already get a "flags" argument.
> > Just make a WPIDFD (for waitid) and CLONE_PIDFD (for clone) bit, and
> > make the existing system calls just take/return a pidfd.
>
> Yes, that's one of the options I was considering but was afraid of
> pitching it because of the very massive opposition I got
> regarding"multiplexers". I'm perfectly happy with doing it this way.

This approach is fine by me, FWIW. I like it more than a general-purpose pidctl.

> Btw, the /proc/<pid> race issue that is mentioned constantly is simply
> avoidable by placing the pid that the pidfd has stashed relative to the
> callers' procfs mount's pid namespace in the pidfd's fdinfo. So there's
> not even a need to really go through /proc/<pid> in the first place. A
> caller wanting to get metadata access and avoid a race with pid
> recycling can then simply do:
>
> int pidfd = pidfd_open(pid, 0);
> int pid = parse_fdinfo("/proc/self/fdinfo/<pidfd>");
> int procpidfd = open("/proc/<pid>", ...);

IMHO, it's worth documenting this procedure in the pidfd man page.

> /* Test if process still exists by sending signal 0 through our pidfd. */

Are you planning on officially documenting this incantation in the
pidfd man page?

> int ret = pidfd_send_signal(pid, 0, NULL, PIDFD_SIGNAL_THREAD);
> if (ret < 0 && errno == ESRCH) {
>         /* pid has been recycled and procpidfd refers to another process */
> }

I was going to suggest that WNOHANG also works for this purpose, but
that idea raises another question: normally, you can wait*(2) on a
process only once. Are you imagining waitid on a pidfd succeeding more
than one? ISTM that the pidfd would need to internally store not just
a struct pid, but the exit status as well or some way to get to it.