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Date:   Mon, 1 Apr 2019 17:47:37 +0200
From:   Christian Brauner <christian@brauner.io>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Aleksa Sarai <cyphar@cyphar.com>,
        Andy Lutomirski <luto@amacapital.net>,
        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>,
        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>,
        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 Mon, Apr 01, 2019 at 08:36:26AM -0700, Linus Torvalds wrote:
> On Mon, Apr 1, 2019 at 4:41 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
> >
> > Eric pitched a procfs2 which would *just* be the PIDs some time ago (in
> > an attempt to make it possible one day to mount /proc inside a container
> > without adding a bunch of masked paths), though it was just an idea and
> > I don't know if he ever had a patch for it.
> 
> I wonder if we really want a fill procfs2, or maybe we could just make

No, I don't think we want a full procfs2.

> the pidfd readable (yes, it's a directory file descriptor, but we
> could allow reading).

Hm, if I understand this correctly, then the pidfd we return from
pidfd_open() would still be a dirfd but not tied to procfs? So I would
implement a "dummy" procfs anon_procfs that is a kernel internal mount
from which we allocate inodes, stash struct pid and off to userspace we
go?

> 
> What are the *actual* use cases for opening /proc files through it? If
> it's really just for a small subset that android wants to do this
> (getting basic process state like "running" etc), rather than anything
> else, then we could skip the whole /proc linking entirely and go the
> other way instead (ie open_pidfd() would get that limited IO model,
> and we could make the /proc directory node get the same limited IO
> model).

From the original thread where metadata access was apparently very
important things that were listed:

<quote>
And how do you propose, given one of these handle objects, getting a
process's current priority, or its current oom score, or its list of
memory maps? As I mentioned in my original email, and which nobody has
addressed, if you don't use a dirfd as your process handle or you
don't provide an easy way to get one of these proc directory FDs, you
need to duplicate a lot of metadata access interfaces.
<quote>
( https://lore.kernel.org/lkml/CALCETrUFrFKC2YTLH7ViM_7XPYk3LNmNiaz6s8wtWo1pmJQXzg@mail.gmail.com/ )

Joel can probably speak best to this.