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Date:   Sat, 30 Mar 2019 18:52:42 +0100
From:   Christian Brauner <christian@brauner.io>
To:     Jonathan Kowalski <bl0pbl33p@gmail.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Daniel Colascione <dancol@google.com>,
        Jann Horn <jannh@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>,
        "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>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()
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On Sat, Mar 30, 2019 at 05:50:20PM +0000, Jonathan Kowalski wrote:
> On Sat, Mar 30, 2019 at 5:24 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.
> >
> > Yeah, yeah, if you want to avoid going through the pathname
> > translation, that's one thing, but if that's your aim, then you again
> > should also just admit that PIDFD_TO_PROCFD is disgusting and wrong,
> > and you're basically saying "ok, I'm not going to do /proc at all".
> >
> > So I'm ok with the whole "simpler, faster, no-proc pidfd", but then it
> > really has to be *SIMPLER* and *NO PROCFS*.
> >
> 
> (Resending because accidently it wasn't a reply-all)
> 
> If you go with pidfd_open, that should also mean you remove the
> ability to be able to use /proc/<PID> dir fds in pidfd_send_signal.
> 
> Otherwise the semantics are hairy: I can only pidfd_open a task
> reachable from my active namespace, but somehow also be able to open a

You can easily setns() to another pid namespace and get a pidfd there.
That's how most namespace interactions work right now. We already had
that discussion.