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Date:   Mon, 15 Apr 2019 12:39:01 -0700
From:   Daniel Colascione <>
To:     Jonathan Kowalski <>
Cc:     Oleg Nesterov <>,
        Christian Brauner <>,
        Linus Torvalds <>,
        Al Viro <>,
        Jann Horn <>,
        David Howells <>,
        Linux API <>,
        linux-kernel <>,
        "Serge E. Hallyn" <>,
        Andy Lutomirski <>,
        Arnd Bergmann <>,
        "Eric W. Biederman" <>,
        Kees Cook <>,
        Thomas Gleixner <>,
        Michael Kerrisk-manpages <>,
        Andrew Morton <>,
        Aleksa Sarai <>,
        Joel Fernandes <>
Subject: Re: [PATCH 2/4] clone: add CLONE_PIDFD

On Mon, Apr 15, 2019 at 10:15 AM Jonathan Kowalski <> wrote:
> > Why else do we want pidfd?
> Apart from what others have already pointed out, there are two other
> things I am looking forward to:

Everything that Christian, Joel, and Jonathan have said is right.

If I can wax philosophical for a bit (as I've been accused to doing
:-)), there's a lot of value in consistency itself, a "more than the
sum of its parts" effect that arises from modeling all kernel-resource
handles as file descriptors. You get lifecycle consistency, API
consistency (e.g., dup, close), introspection consistency (via
/proc/pid/fd and friends), wait consistency, IPC consistency, and tons
of other benefits from using a file descriptor. The alternatives tend
to be very ugly: one of SysV IPC's* biggest mistakes, for example, was
having users manage its resources via non-file-descriptor kernel
handles. The process is, I think, the last major class of kernel
resource that users can't manipulate via file descriptor. Even if
using pidfds didn't provide strong immediate and direct benefits, it'd
*still* be worth moving to a file descriptor resource handle model for
the sake of making the system interface regular and uniform.

* Does anyone know *why* the SysV people didn't use FDs? The
consistency argument I'm making was just as relevant then as it is

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