linux-kernel - Re: [PATCH] pidfd: change pidfd_send_signal() to respect PIDFD

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Message-ID: <20240208135344.GD19801@redhat.com>
Date: Thu, 8 Feb 2024 14:53:45 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tycho Andersen <tycho@...ho.pizza>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pidfd: change pidfd_send_signal() to respect PIDFD_THREAD

On 02/08, Christian Brauner wrote:
>
> On Wed, Feb 07, 2024 at 12:45:49PM +0100, Oleg Nesterov wrote:
> > +	type = (f.file->f_flags & PIDFD_THREAD) ? PIDTYPE_PID : PIDTYPE_TGID;
> > +	ret = kill_pid_info_type(sig, &kinfo, pid, type);
>
> If the user doesn't provide siginfo then the kernel fills in the info in
> prepare_kill_siginfo() a few lines above. That sets info->si_code to
> SI_USER even for the PIDFD_THREAD case. Whenever the info is filled in
> by the kernel it's not exactly userspace impersonating anything plus we
> know that what we're sending to is a pidfd by the type of the pidfd. So
> it feels like we should fill in SI_TKILL here as well?

Hmm. Agreed, will do, thanks.

But then I think this needs another preparational 1/2 patch.
prepare_kill_siginfo() should have a new arg so that do_tkill() could
use it too.

(offtopic, but may be the "Only allow sending arbitrary signals to yourself"
 check in pidfd_send_signal() needs another helper, do_rt_sigqueueinfo()
 does the same check).

> I would also suggest we update the obsolete comment on top of
> pidfd_send_signal() along the lines of:

Ah, indeed, thanks.

Oleg.