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

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Date:   Fri, 11 Aug 2023 13:29:11 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <brauner@...nel.org>
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

Hi Christian,

Sorry for delay, I've just returned from vacation and I am slowly
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...

Oleg.