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

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Date: Thu, 8 Feb 2024 16:57:31 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Christian Brauner <brauner@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	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, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > Turn kill_pid_info() into kill_pid_info_type(), this allows to pass any
> > pid_type to group_send_sig_info(), despite its name it should work fine
> > even if type = PIDTYPE_PID.
> >
> > Change pidfd_send_signal() to use PIDTYPE_PID or PIDTYPE_TGID depending
> > on PIDFD_THREAD.
> >
> > While at it kill another TODO comment in pidfd_show_fdinfo(). As Christian
> > expains fdinfo reports f_flags, userspace can already detect PIDFD_THREAD.
> >
>
> I have a question here.
>
> Why is this based on group_send_sig_info instead of send_sig_info?

Well. send_sig_info() accepts "struct task_struct *", not "struct pid *",
it doesn't do check_kill_permission(), and it doesn't handle the possible
race with mt-exec.

> In particular I am asking are the intended semantics that the signal is
> sent to a single thread in a thread group and placed in the per thread
> queue, or is the signal sent to the entire thread group and placed
> in the thread group signal queue?

This depends on PIDFD_THREAD. If it is set then the signal goes to
the per thread queue.

> Because honestly right now using group_send_sig_info when
> the intended target of the signal is not the entire thread
> group is very confusing when reading your change.

Agreed, so perhaps it makes sense to rename it later. See

	despite its name it should work fine even if type = PIDTYPE_PID.

in the changelog above.

Oleg.