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From:   Jann Horn <jannh@google.com>
Date:   Sat, 30 Mar 2019 19:00:03 +0100
Message-ID: <CAG48ez0F0FLcp_cd_aBVxp+M9k6RJYzOqKCjnD+oUBvt8+VM2w@mail.gmail.com>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Christian Brauner <christian@brauner.io>,
        Daniel Colascione <dancol@google.com>,
        Andrew Lutomirski <luto@kernel.org>,
        David Howells <dhowells@redhat.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Linux API <linux-api@vger.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Kees Cook <keescook@chromium.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Michael Kerrisk-manpages <mtk.manpages@gmail.com>,
        Jonathan Kowalski <bl0pbl33p@gmail.com>,
        "Dmitry V. Levin" <ldv@altlinux.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Joel Fernandes <joel@joelfernandes.org>
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On Sat, Mar 30, 2019 at 6:24 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Sat, Mar 30, 2019 at 10:12 AM Christian Brauner <christian@brauner.io> wrote:
> > To clarify, what the Android guys really wanted to be part of the api is
> > a way to get race-free access to metadata associated with a given pidfd.
> > And the idea was that *if and only if procfs is mounted* you could do:
> >
> > int pidfd = pidfd_open(1234, 0);
> >
> > int procfd = open("/proc", O_RDONLY | O_CLOEXEC);
> > int procpidfd = ioctl(pidfd, PIDFD_TO_PROCFD, procfd);
>
> And my claim is that this is three system calls - one of them very
> hacky - to just do
>
>     int pidfd = open("/proc/%d", O_PATH);
>
> and you're done. It acts as the pidfd _and_ the way to get the
> associated status files etc.
>
> So there is absolutely zero advantage to going through pidfd_open().
>
> No. No. No.
>
> So the *only* reason for "pidfd_open()" is if you don't have /proc in
> the first place. In which case the whole PIDFD_TO_PROCFD is bogus.

So if, in the future, there is some sort of "create a new task and
return an fd to it" syscall, do you think it should always return
pidfds, or do you think it should return fds to /proc if procfs is
available? And if it should return fds to /proc, does that mean that
this "create a task" API should take an extra argument with a file
descriptor to the procfs instance you want to use?
(This can't always be implemented easily in userspace on top of normal
clone(), because if you create a task without a termination signal -
like a thread -, its PID can be recycled under you.)
An API like this would have less complexity stuffed into a single
syscall if it always returns pidfds, and if you then actually want an
fd to procfs, you can do the conversion that requires specifying a
procfs instance separately.

Of course, if you think that we shouldn't add an API for
pidfd-to-procfs conversion before we have an API for
clone()-with-an-fd-retval, that's understandable.