linux-kernel - Re: pidfd design

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Date:   Wed, 20 Mar 2019 13:50:43 -0700
From:   Daniel Colascione <dancol@...gle.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: pidfd design

On Wed, Mar 20, 2019 at 1:47 PM Christian Brauner <christian@...uner.io> wrote:
>
> On Wed, Mar 20, 2019 at 11:39:10PM +0300, Alexey Dobriyan wrote:
> > On Wed, Mar 20, 2019 at 01:14:01PM -0700, Daniel Colascione wrote:
> > > On Wed, Mar 20, 2019 at 1:07 PM Alexey Dobriyan <adobriyan@...il.com> wrote:
> > > > > What would be your opinion to having a
> > > > > /proc/<pid>/handle
> > > > > file instead of having a dirfd.
> > > >
> > > > This is even worse than depending on PROC_FS. Just for the dependency
> > > > pidfd code should be backed out immediately. Forget about /proc.
> > >
> > > We already have pidfds, and we've had them since /proc was added ages
> > > ago.
> >
> > New pidfd code (or whatever the name) should NOT depend on /proc and
> > should not interact with VFS at all at any point (other than probably
> > being a descriptor on a fake filesystem). The reason is that /proc is
> > full of crap and you don't want to spill that into new and hopefully
> > properly designed part of new code.
>
> Yes, I agree. That's why I was thinking that translate_pid() is a good
> candidate to provide that decoupling.

Then again: how do you propose fetching process metadata? If you adopt
a stance that nothing can use procfs and simultaneously adopt a stance
that we don't want to duplicate all the decades of metadata interfaces
in /proc/pid (which are useful, not "crap"), then the overall result
is that we just won't make any progress at all. There's nothing wrong
with taking a dependency on procfs: procfs is how we talk about
processes. It's completely unreasonable to say "no, you can't use the
old thing" and also "no, we can't add a new thing that would duplicate
the old thing".