Re: [PATCH] pid: allow pidfds for reaped tasks

[Oleg Nesterov <oleg@redhat.com>]

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.