Received-SPF: pass (google.com: domain of linux-kernel+bounces-58195-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Date: Thu, 8 Feb 2024 14:53:45 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Christian Brauner <brauner@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Tycho Andersen <tycho@tycho.pizza>, linux-api@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] pidfd: change pidfd_send_signal() to respect PIDFD_THREAD
Message-ID: <20240208135344.GD19801@redhat.com>
References: <20240207114549.GA12697@redhat.com>
 <20240208-fragt-prospekt-7866333b15f0@brauner>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240208-fragt-prospekt-7866333b15f0@brauner>
User-Agent: Mutt/1.5.24 (2015-08-30)

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.