Re: [PATCH v2 2/2] pidfd: change pidfd_send_signal() to respect PIDFD_THREAD

[Christian Brauner <brauner@kernel.org>]

On Tue, Feb 20, 2024 at 05:22:02PM +0100, Oleg Nesterov wrote:
> On 02/20, Christian Brauner wrote:
> >
> > On Tue, Feb 20, 2024 at 12:00:12PM +0100, Oleg Nesterov wrote:
> > >
> > > Perhaps we can kill the "task_pid(current) != pid" check and just return
> > > EPERM if "kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL", I don't think
> > > anobody needs pidfd_send_send_signal() to signal yourself. See below.
> >
> > Yeah.
> 
> You have my ack in advance
> 
> > > > +       /* Currently unused. */
> > > > +       if (info)
> > > > +               return -EINVAL;
> > >
> > > Well, to me this looks like the unnecessary restriction... And why?
> >
> > Because right now we aren't sure that it's used
> 
> Yes, but...
> 
> > and we aren't sure what use-cases are there.
> 
> the same use-cases as for rt_sigqueueinfo() ?

Specifically for pidfd_send_signal() I mean. To me it seems very
unlikely that anyone would be opening a pidfd to itself and then use
pidfd_send_signal() when they could entirely avoid this - race free - by
simply using *sigqueueinfo(). But I may be wrong of course.

> 
> Christian, I won't really argue but I still disagree.
> 
> Let me first repeat once again, I do not know what people do with pidfd
> and pidfd_send_signal() in particular, so I won't be surprised if this
> change won't cause any regression report.

Fwiw, that's fine as long as we'd fix it up.

> 
> But at the same time, I can easily imagine the following scenario: a
> userspace programmer tries to use pidfd_send_signal(info != NULL), gets
> -EINVAL, decides it can't/shouldn't work, and switches to sigqueueinfo()
> without any report to lkml.
> 
> > Yes, absolutely. That was always the plan. See appended patch I put on top.
> > I put you as author since you did spot this. Let me know if you don't
> > want that.
> 
> Ah. Thanks Christian. I am fine either way, whatever is more convenient
> for you.
> 
> But just in case, I won't mind at all if you simply fold this minor fix
> into your PIDFD_SEND_PROCESS_GROUP patch, I certainly don't care about
> the "From" tag ;)
> 
> A really, really minor/cosmetic nit below, feel free to ignore:
> 
> > -		if ((task_pid(current) != pid) &&
> > +		if (((task_pid(current) != pid) || type > PIDTYPE_TGID) &&
> 
> we can remove the unnecessary parens around "task_pid(current) != pid"
> or add the extra parens aroung "type > PIDTYPE_TGID".
> 
> I mean, the 1st operand of "&&" is
> 
> 	(task_pid(current) != pid) || type > PIDTYPE_TGID
> 
> and this looks a bit inconsistent to me.

Ok, will do.