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Date:   Sat, 30 Mar 2019 19:02:18 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Jonathan Kowalski <bl0pbl33p@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Daniel Colascione <dancol@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        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>,
        "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>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()

On Sat, Mar 30, 2019 at 05:59:34PM +0000, Jonathan Kowalski wrote:
> On Sat, Mar 30, 2019 at 5:52 PM Christian Brauner <christian@...uner.io> wrote:
> >
> > 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@...ux-foundation.org> wrote:
> > > >
> > > > On Sat, Mar 30, 2019 at 10:12 AM Christian Brauner <christian@...uner.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.
> 
> Only if it is a child namespace, or you have the relevant capabilities to setns.
> 
> Currently, if I just put a task in PID namespace, it can see /proc of
> an ancestor PID namespace, and opendir /proc/<PID>, this is accepted
> by pidfd_send_signal.
> 
> If you ever allow signalling across PID namespaces (because file

That's not on the plate for now and pidfd_send_signal() is checking for
that to not allow it.

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