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Date:   Mon, 15 Apr 2019 12:39:01 -0700
From:   Daniel Colascione <dancol@...gle.com>
To:     Jonathan Kowalski <bl0pbl33p@...il.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jann Horn <jannh@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 2/4] clone: add CLONE_PIDFD

On Mon, Apr 15, 2019 at 10:15 AM Jonathan Kowalski <bl0pbl33p@...il.com> 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
now.

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