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Date:   Mon, 1 Apr 2019 02:52:46 +0200
From:   Jann Horn <>
To:     Christian Brauner <>
Cc:     Linus Torvalds <>,
        Andy Lutomirski <>,
        Daniel Colascione <>,
        Andrew Lutomirski <>,
        David Howells <>,
        "Serge E. Hallyn" <>,
        Linux API <>,
        Linux List Kernel Mailing <>,
        Arnd Bergmann <>,
        "Eric W. Biederman" <>,
        Konstantin Khlebnikov <>,
        Kees Cook <>,
        Alexey Dobriyan <>,
        Thomas Gleixner <>,
        Michael Kerrisk-manpages <>,
        Jonathan Kowalski <>,
        "Dmitry V. Levin" <>,
        Andrew Morton <>,
        Oleg Nesterov <>,
        Nagarathnam Muthusamy <>,
        Aleksa Sarai <>,
        Al Viro <>,
        Joel Fernandes <>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()

On Mon, Apr 1, 2019 at 12:33 AM Christian Brauner <> wrote:
> On Sun, Mar 31, 2019 at 03:16:47PM -0700, Linus Torvalds wrote:
> > On Sun, Mar 31, 2019 at 3:03 PM Christian Brauner <> 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.

Ah, yes. That seems reasonable. And, as Linus said, pidfd_open() is
less important if you can just do open("/proc/...") on systems with
procfs instead.

One minor detail to keep in mind for the future is that in a
straightforward implementation of this concept, if a non-capable
process is running in a mount namespace, but in the initial network
namespace, without any reachable /proc mount, it will be able to look
at information about other processes' network connections by first
using pidfd_open() on itself or by using clone(CLONE_PIDFD), then
looking at the "net" directory under the resulting file descriptor.

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