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

On Sat, Jan 27, 2024 at 11:54:32AM +0100, Oleg Nesterov wrote:

Hi Tycho,

On 01/26, Tycho Andersen wrote:

quoted

On Thu, Jan 25, 2024 at 03:08:31PM +0100, Oleg Nesterov wrote:

quoted

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.

quoted

quoted

+	/* 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.

quoted

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.

quoted

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.

quoted

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.