linux-kernel - Re: [PATCH] pid: allow pidfds for reaped tasks

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Date:   Mon, 14 Aug 2023 17:11:09 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     David Rheinsberg <david@...dahead.eu>,
        linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Kees Cook <keescook@...omium.org>,
        Alexander Mikhalitsyn <alexander@...alicyn.com>,
        Luca Boccassi <bluca@...ian.org>
Subject: Re: [PATCH] pid: allow pidfds for reaped tasks

On Mon, Aug 14, 2023 at 03:20:39PM +0200, Oleg Nesterov wrote:
> On 08/14, David Rheinsberg wrote:
> >
> > Hi Oleg,
> >
> > On Fri, Aug 11, 2023, at 1:57 PM, Oleg Nesterov wrote:
> > >> What code do we need to allow userspace to open a pidfd to a leader pid
> > >> even if it has already been exited and reaped (without also accidently
> > >> allowing to open non-lead pid pidfds)?
> > >
> > > I'll try to think more, but can you also explain why do we need this?
> > >
> > > See my another email. Can't we simply shift the pid_has_task(PIDTYPE_TGID)
> > > check from pidfd_prepare() to pidfd_create() ? (and then we can kill
> > > pidfd_prepare and rename __pidfd_prepare to pidfd_prepare).
> >
> > Yes, the easiest solution would be to use `__pidfd_prepare()` and ensure
> > that the caller only ever calls this on tg-leaders. This would work just
> > fine, imo. And this was my initial approach.
> 
> Great,
> 
> > I think Christian preferred an explicit assertion that ensures we do not
> > accidentally hand out pidfds for non-tg-leaders. The question is thus whether
> > there is an easy way to assert this even for reaped tasks?
> > Or whether there is a simple way to flag a pid that was used as tg-leader?
> 
> I do not see how can we check if a detached pid was a leader pid, and I don't
> think it makes sense to add a new member into struct pid...
> 
> > Or, ultimately, whether this has limited use and we should just use
> > `__pidfd_prepare()`?
> 
> Well, if you confirm that sk->sk_peer_pid and scm->pid are always initialized with
> task_tgid(current), I'd certainly prefer this approach unless Christian objects.

No no, I'm absolutely not objecting. I specifically want you to take the
opinionated lead here. :) Thanks for chiming in!