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

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Date: Fri, 9 Feb 2024 10:26:06 +0100
From: Christian Brauner <brauner@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, 
	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 Thu, Feb 08, 2024 at 04:57:31PM +0100, Oleg Nesterov wrote:
> 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

Agreed. The function seems misnamed and incorrectly documented. It just
seems that it's never been used with PIDTYPE_PID but it's perfectly
capable of doing that. So maybe just put a patch on top renaming it to
send_sig_info_type() and remove the old comment. But I can live without
renaming it for now as well.