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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 1 Apr 2019 08:36:26 -0700
Message-ID: <CAHk-=wgKqBQznZdTQaM6yQ+_5dcz-+q8=2sbQsAoDh55hQTLMA@mail.gmail.com>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Christian Brauner <christian@brauner.io>,
        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>
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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
the pidfd readable (yes, it's a directory file descriptor, but we
could allow reading).

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).

                  Linus