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From:   Alexander Mikhalitsyn <alexander@mihalicyn.com>
Date:   Mon, 14 Aug 2023 15:34:22 +0200
Message-ID: <CAJqdLrr4-Jzw7v5n8gXp54RM918-tCUa-rEg0XsnxOJ2bNKt7A@mail.gmail.com>
Subject: Re: [PATCH] pid: allow pidfds for reaped tasks
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     David Rheinsberg <david@readahead.eu>,
        Christian Brauner <brauner@kernel.org>,
        linux-kernel@vger.kernel.org, Jan Kara <jack@suse.cz>,
        Kees Cook <keescook@chromium.org>,
        Luca Boccassi <bluca@debian.org>
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On Mon, Aug 14, 2023 at 3:21=E2=80=AFPM Oleg Nesterov <oleg@redhat.com> wro=
te:
>
> On 08/14, David Rheinsberg wrote:
> >
> > 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 accident=
ly
> > >> 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 ensur=
e
> > that the caller only ever calls this on tg-leaders. This would work jus=
t
> > fine, imo. And this was my initial approach.
>
> Great,
>
> > I think Christian preferred an explicit assertion that ensures we do no=
t
> > accidentally hand out pidfds for non-tg-leaders. The question is thus w=
hether
> > 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-lead=
er?
>
> I do not see how can we check if a detached pid was a leader pid, and I d=
on't
> think it makes sense to add a new member into struct pid...
>
> > Or, ultimately, whether this has limited use and we should just use
> > `__pidfd_prepare()`?
>
> Well, if you confirm that sk->sk_peer_pid and scm->pid are always initial=
ized with
> task_tgid(current), I'd certainly prefer this approach unless Christian o=
bjects.

Dear colleagues,

I can confirm that sk->sk_peer_pid and scm->pid are always thread-group lea=
ders.

Kind regards,
Alex

>
> Oleg.
>