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

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Message-ID: <20240208-erwehren-zeitnah-6701d03a905f@brauner>
Date: Thu, 8 Feb 2024 15:31:47 +0100
From: Christian Brauner <brauner@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
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 Thu, Feb 08, 2024 at 02:53:45PM +0100, Oleg Nesterov wrote:
> 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.

Agreed.

> 
> (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).

Agreed.