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.

As for clone(CLONE_PIDFD | CLONE_THREAD), this is trivial but
I think this needs another discussion too, lets do this later.

quoted

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+	/* 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.

It seems like (and the current pidfd_test enforces for some cases) we
want exactly one notification for a task dying. I don't understand
how we guarantee this now, with all of these calls.

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.

Yup, understood. It just seems like an inconsistency we might want to
fix.

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?

Yeah, we'd need some other linkage as Christian points out. But if
there is a predicate we can write that says whether this task has been
notified or not, it's not necessary. I just don't understand what that
is. But maybe your patch will make it clearer.

Tycho