linux-kernel - Re: [patch V2 26/50] signal: Get rid of resched

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Date: Fri, 19 Apr 2024 13:06:32 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	John Stultz <jstultz@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Stephen Boyd <sboyd@...nel.org>,
	Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [patch V2 26/50] signal: Get rid of resched_timer logic

On 04/18, Oleg Nesterov wrote:
>
> On 04/18, Oleg Nesterov wrote:
> >
> > On 04/11, Thomas Gleixner wrote:
> > >
> > > There is no reason for handing the *resched pointer argument through
> > > several functions just to check whether the signal is related to a self
> > > rearming posix timer.
> >
> > Agreed, these changes looks good to me.
>
> I meant the intent.
>
> But this is not simple, collect_signal() checks SIGQUEUE_PREALLOC exactly
> because (iiuc) we need to ensure that SI_TIMER didn't come from userspace.
>
> perhaps we should disallow SI_TIMER with _sys_private != 0 from userspace,
> I dunno...
>
> And I don't really understand the "not to be passed to user" comment in
> include/uapi/asm-generic/siginfo.h. copy_siginfo_to_user() just copies
> the whole kernel_siginfo.

OK, si_sys_private is cleared in dequeue_signal() (or in posixtimer_rearm()
with this series).

In the past SI_TIMER was defined as __SI_CODE(__SI_TIMER,-2), it was > 0,
so it could not come from userspace (see the info->si_code >= 0 check in
do_rt_sigqueueinfo).

Today SI_TIMER < 0. We could introduce SI_TIMER_KERNEL > 0 with the minimal
changes, but this can't help because the commit 66dd34ad31e59 allows to send
any siginfo to itself.

Otoh, I have no idea how CRIU restores the posix timers. If a process has
a pending blocked SI_TIMER signal, then I guess it actually needs to enqueue
this signal at restore time, but resched_timer will be never true?

I got lost... Sorry for the noise.

> Confused.
>
> > But,
> >
> > > SI_TIMER is only used by the posix timer code and cannot be queued from
> > > user space.
> >
> > Why? I think sigqueueinfo() can certainly use si_code = SI_TIMER, so
> >
> > > @@ -1011,6 +1001,9 @@ static int __send_signal_locked(int sig,
> > >
> > >  	lockdep_assert_held(&t->sighand->siglock);
> > >
> > > +	if (WARN_ON_ONCE(!is_si_special(info) && info->si_code == SI_TIMER))
> > > +		return 0;
> >
> > this can be easily triggered by userspace and thus looks wrong.
> >
> > Oleg.