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

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Date: Thu, 8 Feb 2024 15:34:07 +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, 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.

Cough... lets forget this patch for the moment.

Is prepare_kill_siginfo() correct when we send a signal to the child
pid namespace? si_pid = task_tgid_vnr(current) doesn't look right in
this case but perhaps I am totally confused.

And why do we need it at all? Can't sys_kill() and pidfd_send_signal()
just use SEND_SIG_NOINFO?

OK, I am sure I missed something. Will read this code tomorrow.

Oleg.