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Date:   Mon, 1 Apr 2019 00:33:56 +0200
From:   Christian Brauner <christian@brauner.io>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     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>,
        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 Sun, Mar 31, 2019 at 03:16:47PM -0700, Linus Torvalds wrote:
> On Sun, Mar 31, 2019 at 3:03 PM Christian Brauner <christian@brauner.io> wrote:
> >
> > Thanks for the input. The problem Jann and I saw with this is that it
> > would be awkward to have the kernel open a file in some procfs instance,
> > since then userspace would have to specify which procfs instance the fd
> > should come from.
> 
> I would actually suggest we just make the rules be that the
> pidfd_open() always return the internal /proc entry regardless of any
> mount-point (or any "hidepid") but also suggest that exactly *because*
> it gives you visibility into the target pid, you'd basically require
> the strictest kind of control of the process you're trying to get the
> pidfd of.
> 
> Ie likely something along the lines of
> 
>         ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS)

I can live with that but I would like to hear what Jann thinks too if
that's ok.

Christian