linux-kernel - Re: [PATCH v2 0/5] pid: add pidfd

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Date:   Sat, 30 Mar 2019 18:12:16 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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>,
        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>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()

On Sat, Mar 30, 2019 at 10:04:33AM -0700, Linus Torvalds wrote:
> On Sat, Mar 30, 2019 at 9:34 AM Daniel Colascione <dancol@...gle.com> wrote:
> >
> > Just to be clear, I'm not proposing granting secret access to procfs,
> > and as far as I can see, nobody else is either. We've been talking
> > about making it easier to avoid races when you happen to want a pidfd
> > and a procfs fd that point to the same process
> 
> So I thought that was the whole point of just opening /proc/<pid>.
> Exactly because that way you can then use openat() from there on.

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 then we internally verify that the struct pid that the pidfd is
refering to, is still the same as the one that /proc/<pid> is refering
to and only then do we return an fd for the process /proc/<pid>
directory which would then allow you to do, e.g.:

int statusfd = openat(procpidfd, "status", O_RDONLY | O_CLOEXEC);

this would provide race-free access to metadat but again, only if /proc
is mounted and available to the user. But if that's an instant NAK we
will definitely *not* do this.