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

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Date: Thu, 18 Apr 2024 20:18:21 +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 04/11, 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.

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.