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Date:   Sun, 31 Mar 2019 08:21:39 -0700
From:   Daniel Colascione <dancol@...gle.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andrew Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        Jonathan Kowalski <bl0pbl33p@...il.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Nagarathnam Muthusamy <nagarathnam.muthusamy@...cle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()

On Sun, Mar 31, 2019 at 8:05 AM Christian Brauner <christian@...uner.io> 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@...gle.com> 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.

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