Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

[Christian Brauner]

On Sat, Jan 27, 2024 at 11:54:32AM +0100, Oleg Nesterov wrote:
> Hi Tycho,
> 
> On 01/26, Tycho Andersen wrote:
> >
> > On Thu, Jan 25, 2024 at 03:08:31PM +0100, Oleg Nesterov wrote:
> > > What do you think?
> >
> > Thank you, it passes all my tests.
> 
> Great, thanks!
> 
> OK, I'll make v2 on top of the recent
> "pidfd: cleanup the usage of __pidfd_prepare's flags"
> 
> but we need to finish our discussion with Christian about the
> usage of O_EXCL.

Just write the patch exactly like you want. If it's as a little a detail
as the uapi flag value we can just always change that when applying.
There's no reason to introduce artificial delays because of my
preference here..

> 
> As for clone(CLONE_PIDFD | CLONE_THREAD), this is trivial but
> I think this needs another discussion too, lets do this later.
> 
> > > +	/* unnecessary if do_notify_parent() was already called,
> > > +	   we can do better */
> > > +	do_notify_pidfd(tsk);
> >
> > "do better" here could be something like,
> >
> > [...snip...]
> 
> No, no, please see below.
> 
> For the moment, please forget about PIDFD_THREAD, lets discuss
> the current behaviour.
> 
> > but even with that, there's other calls in the tree to
> > do_notify_parent() that might double notify.
> 
> Yes, and we can't avoid this. Well, perhaps do_notify_parent()
> can do something like
> 
> 	if (ptrace_reparented())
> 		do_notify_pidfd();
> 
> so that only the "final" do_notify_parent() does do_notify_pidfd()
> but this needs another discussion and in fact I don't think this
> would be right or make much sense. Lets forget this for now.
> 
> Now. Even without PIDFD_THREAD, I think it makes sense to change
> do_notify_parent() to do
> 
> 	if (thread_group_empty(tsk))
> 		do_notify_pidfd(tsk);
> 
> thread_group_empty(tsk) can only be true if tsk is a group leader
> and it is the last thread. And this is exactly what pidfd_poll()
> currently needs.
> 
> In fact I'd even prefer to do this in a separate patch for the
> documentation purposes.
> 
> Now, PIDFD_THREAD can just add
> 
> 	if (!thread_group_empty(tsk))
> 		do_notify_pidfd(tsk);
> 
> right after "tsk->exit_state = EXIT_ZOMBIE", that is all.

Sounds good.

> 
> This also preserves the do_notify_pidfd/__wake_up_parent ordering.
> Not that I think this is important, just for consistency.
> 
> > This brings up another interesting behavior that I noticed while
> > testing this, if you do a poll() on pidfd, followed quickly by a
> > pidfd_getfd() on the same thread you just got an event on, you can
> > sometimes get an EBADF from __pidfd_fget() instead of the more
> > expected ESRCH higher up the stack.
> 
> exit_notify() is called after exit_files(). pidfd_getfd() returns
> ESRCH if the exiting thread completes release_task(), otherwise it
> returns EBADF because ->files == NULL. This too doesn't really
> depend on PIDFD_THREAD.
> 
> > I wonder if it makes sense to abuse ->f_flags to add a PIDFD_NOTIFIED?
> > Then we can refuse further pidfd syscall operations in a sane way, and
> 
> But how? We only have "struct pid *", how can we find all files
> "attached" to this pid?

We can't. There's some use-cases that would make this desirable but that
would mean we would need another data structure to track this. I once
toyed with a patch for this but never got so excited about it to
actually finish it.