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Date:   Sun, 31 Mar 2019 07:52:28 -0700
From:   Linus Torvalds <>
To:     Jann Horn <>
Cc:     Joel Fernandes <>,
        Daniel Colascione <>,
        Christian Brauner <>,
        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 <>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()

On Sat, Mar 30, 2019 at 9:47 PM Jann Horn <> wrote:
> Sure, given a pidfd_clone() syscall, as long as the parent of the
> process is giving you a pidfd for it and you don't have to deal with
> grandchildren created by fork() calls outside your control, that
> works.

Don't do pidfd_clone() and pidfd_wait().

Both of those existing system calls already get a "flags" argument.
Just make a WPIDFD (for waitid) and CLONE_PIDFD (for clone) bit, and
make the existing system calls just take/return a pidfd.

Side note: we could (should?) also make the default maxpid just be
larger. It needs to fit in an 'int', but MAXINT instead of 65535 would
likely alreadt make a lot of these attacks harder.

There was some really old legacy reason why we actually limited it to
65535 originally.  It was old and crufty even back when..



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