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

[David Rheinsberg]

Hi Oleg,

On Fri, Aug 11, 2023, at 1:57 PM, Oleg Nesterov wrote:
>> What code do we need to allow userspace to open a pidfd to a leader pid
>> even if it has already been exited and reaped (without also accidently
>> allowing to open non-lead pid pidfds)?
>
> I'll try to think more, but can you also explain why do we need this?
>
> See my another email. Can't we simply shift the pid_has_task(PIDTYPE_TGID)
> check from pidfd_prepare() to pidfd_create() ? (and then we can kill
> pidfd_prepare and rename __pidfd_prepare to pidfd_prepare).

Yes, the easiest solution would be to use `__pidfd_prepare()` and ensure that the caller only ever calls this on tg-leaders. This would work just fine, imo. And this was my initial approach.

I think Christian preferred an explicit assertion that ensures we do not accidentally hand out pidfds for non-tg-leaders. The question is thus whether there is an easy way to assert this even for reaped tasks? Or whether there is a simple way to flag a pid that was used as tg-leader? Or, ultimately, whether this has limited use and we should just use `__pidfd_prepare()`?

Thanks a lot!
David