Re: [PATCH v2 0/5] pid: add pidfd_open()

[Christian Brauner]

On Mon, Apr 01, 2019 at 08:36:26AM -0700, Linus Torvalds wrote:
> On Mon, Apr 1, 2019 at 4:41 AM Aleksa Sarai <cyphar@xxxxxxxxxx> wrote:
> >
> > Eric pitched a procfs2 which would *just* be the PIDs some time ago (in
> > an attempt to make it possible one day to mount /proc inside a container
> > without adding a bunch of masked paths), though it was just an idea and
> > I don't know if he ever had a patch for it.
> 
> I wonder if we really want a fill procfs2, or maybe we could just make

No, I don't think we want a full procfs2.

> the pidfd readable (yes, it's a directory file descriptor, but we
> could allow reading).

Hm, if I understand this correctly, then the pidfd we return from
pidfd_open() would still be a dirfd but not tied to procfs? So I would
implement a "dummy" procfs anon_procfs that is a kernel internal mount
from which we allocate inodes, stash struct pid and off to userspace we
go?

> 
> What are the *actual* use cases for opening /proc files through it? If
> it's really just for a small subset that android wants to do this
> (getting basic process state like "running" etc), rather than anything
> else, then we could skip the whole /proc linking entirely and go the
> other way instead (ie open_pidfd() would get that limited IO model,
> and we could make the /proc directory node get the same limited IO
> model).

>From the original thread where metadata access was apparently very
important things that were listed:

<quote>
And how do you propose, given one of these handle objects, getting a
process's current priority, or its current oom score, or its list of
memory maps? As I mentioned in my original email, and which nobody has
addressed, if you don't use a dirfd as your process handle or you
don't provide an easy way to get one of these proc directory FDs, you
need to duplicate a lot of metadata access interfaces.
<quote>
( https://lore.kernel.org/lkml/CALCETrUFrFKC2YTLH7ViM_7XPYk3LNmNiaz6s8wtWo1pmJQXzg@xxxxxxxxxxxxxx/ )

Joel can probably speak best to this.