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

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Date:   Fri, 11 Aug 2023 13:40:41 +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 Fri, Aug 11, 2023 at 01:29:11PM +0200, Oleg Nesterov wrote:
> Hi Christian,
> 
> Sorry for delay, I've just returned from vacation and I am slowly

Absolutely no problem! Thanks for getting back to us.

> crawling my email backlog.
> 
> 
> 
> On 08/07, Christian Brauner wrote:
> >
> > >  int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)
> > >  {
> > > -	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
> > > +	if (!pid)
> > > +		return -EINVAL;
> > > +
> > > +	/*
> > > +	 * Non thread-group leaders cannot have pidfds, but we allow them for
> > > +	 * reaped thread-group leaders.
> > > +	 */
> > > +	if (pid_has_task(pid, PIDTYPE_PID) && !pid_has_task(pid, PIDTYPE_TGID))
> > >  		return -EINVAL;
> >
> > TL;DR userspace wants to be able to get a pidfd to an already reaped
> > thread-group leader. I don't see any issues with this.
> 
> I guess I need to read the whole thread carefully, but right now
> I don't understand this patch and the problem...
> 
> OK, suppose we have a group leader L with pid 100 and its sub-thread
> T with pid 101.
> 
> With this patch pidfd_open(101) can succeed if T exits right after
> find_get_pid(101) because pid_has_task(pid, PIDTYPE_PID) above will
> fail, right?
> 
> This looks wrong, 101 was never a leader pid...

Well, let me simplify the question:

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 hope that clarifies?