linux-kernel - Re: [PATCH] pidfd: add ioctl to retrieve pid info

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Message-ID: <CAMw=ZnQB-xsvPX6TrmaXed3KGBR1YO1qYP-iNsUAv8XAOvB=YQ@mail.gmail.com>
Date: Sun, 6 Oct 2024 15:59:42 +0100
From: Luca Boccassi <luca.boccassi@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Christian Brauner <brauner@...nel.org>, Jeff Layton <jlayton@...nel.org>, 
	Josef Bacik <josef@...icpanda.com>, linux-kernel@...r.kernel.org, paul@...l-moore.com, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] pidfd: add ioctl to retrieve pid info

On Sat, 5 Oct 2024 at 12:29, Oleg Nesterov <oleg@...hat.com> wrote:
>
> On 10/04, Luca Boccassi wrote:
> >
> > On Fri, 4 Oct 2024 at 20:30, Oleg Nesterov <oleg@...hat.com> wrote:
> > >
> > > I guess Christian meant you should simply use
> > >
> > >                 info.pid = task_pid_vnr(task);
> > >
> > > task_pid_vnr(task) returns the task's pid in the caller's namespace.
> >
> > Ah I see, I didn't realize there was a difference, sent v3 with the
> > suggested change just now, thanks.
>
> I didn't get v3, I guess I wasn't cc'ed again.
>
> So, just in case, let me add that task_pid_vnr(task) can return 0 if
> this task exits after get_pid_task().
>
> Perhaps this is fine, I do not know. But perhaps you should actually
> use pid_vnr(pid).
>
> Oleg.

I have just sent v5 CC'ing you and adding a final check before the
copy to userspace, that returns ESRCH if the task has exited. This
should solve that issue, and also be future-proof against potential
additions that might slow down processing due to gathering more data
or so.