linux-kernel - Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

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Date:   Thu, 7 Dec 2023 18:57:42 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Tycho Andersen <tycho@...ho.pizza>, Oleg Nesterov <oleg@...hat.com>
Cc:     "Eric W . Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Tycho Andersen <tandersen@...flix.com>,
        Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

On Fri, Dec 01, 2023 at 09:31:40AM -0700, Tycho Andersen wrote:
> On Thu, Nov 30, 2023 at 10:57:01AM -0700, Tycho Andersen wrote:
> > On Thu, Nov 30, 2023 at 06:39:39PM +0100, Oleg Nesterov wrote:
> > > I think that wake_up_all(wait_pidfd) should have a single caller,
> > > do_notify_pidfd(). This probably means it should be shiftef from
> > > do_notify_parent() to exit_notify(), I am not sure...
> 
> Indeed, below passes the tests without issue and is much less ugly.

So I think I raised that question on another medium already but what
does the interaction with de_thread() look like?

Say some process creates pidfd for a thread in a non-empty thread-group
is created via CLONE_PIDFD. The pidfd_file->private_data is set to
struct pid of that task. The task the pidfd refers to later exec's.

Once it passed de_thread() the task the pidfd refers to assumes the
struct pid of the old thread-group leader and continues.

At the same time, the old thread-group leader now assumes the struct pid
of the task that just exec'd.

So after de_thread() the pidfd now referes to the old thread-group
leaders struct pid. Any subsequent operation will fail because the
process has already exited.

Basically, the pidfd now refers to the old thread-group leader and any
subsequent operation will fail even though the task still exists.

Conversely, if someone had created a pidfd that referred to the old
thread-group leader task then this pidfd will now suddenly refer to the
new thread-group leader task for the same reason: the struct pid's were
exchanged.

So this also means, iiuc, that the pidfd could now be passed to
waitid(P_PIFD) to retrieve the status of the old thread-group leader
that just got zapped.

And for the case where the pidfd referred to the old thread-group leader
task you would now suddenly _not_ be able to wait on that task anymore.

If these concerns are correct, then I think we need to decide what
semantics we want and how to handle this because that's not ok.