linux-kernel - Re: [RFC PATCH] pidfd: implement PIDFD_THREAD flag for pidfd

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Message-ID: <CALCETrUFDkt+K9zG8mczxzAFy9t-6Mx5Cz-Sx+it6a4nt+O0pg@mail.gmail.com>
Date: Wed, 31 Jan 2024 10:11:55 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Tycho Andersen <tycho@...ho.pizza>, Christian Brauner <brauner@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-api@...r.kernel.org, Tycho Andersen <tandersen@...flix.com>, 
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [RFC PATCH] pidfd: implement PIDFD_THREAD flag for pidfd_open()

On Mon, Jan 29, 2024 at 3:24 AM Oleg Nesterov <oleg@...hat.com> wrote:
>
> On 01/27, Oleg Nesterov wrote:
> >
> > I'll (hopefully) send v2 on top of
> >
> >       pidfd: cleanup the usage of __pidfd_prepare's flags
> >       pidfd: don't do_notify_pidfd() if !thread_group_empty()
> >
> > on Monday
>
> Sorry, I don't have time to finish v2 today, I need to update the comments
> and write the changelog.
>
> But the patch itself is ready, I am sending it for review.
>
> Tycho, Christian, any comments?

Right now, pidfd_send_signal() sends signals to processes, like so:

 * The syscall currently only signals via PIDTYPE_PID which covers
 * kill(<positive-pid>, <signal>. It does not signal threads or process
 * groups.

This patch adds PIDFD_THREAD which, potentially confusingly, doesn't
change this (AFAICS).  So at least that should be documented loudly
and clearly, IMO.  But I actually just bumped in to this limitation in
pidfd_send_signal(), like so:

https://github.com/systemd/systemd/issues/31093

Specifically, systemd can't properly emulate Ctrl-C using pidfd_send_signal().

I don't know whether implementing the other signal types belongs as
part of this patch, but they're at least thematically related.

--Andy