linux-kernel - Re: [PATCH] posix-timers: Do not modify an already queued timer signal

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Message-ID: <20080721152352.GA182@tv-sign.ru>
Date:	Mon, 21 Jul 2008 19:23:52 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Roland McGrath <roland@...hat.com>
Cc:	Mark McLoughlin <markmc@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] posix-timers: Do not modify an already queued timer signal

On 07/20, Roland McGrath wrote:
>
> > Yes, thanks, I see. But does it have any meaning for the user-space?
> [si_sys_private]
>
> No, it's not part of the user ABI.  It's not even copied out (see
> copy_siginfo_to_user).

Heh, I didn't know, thanks.

> > Let me repeat. Can't we make a simple fix for now for this nasty and
> > ancient bug, before we do the more clever changes,
>
> You do need to clear si_overrun there to be correct in the usual case
> (not already queued).

Indeed, I missed that. Can't we do this in send_sigqueue() ?

> It would be a perfectly fine and worthwhile optimization/cleanup on its
> own just to move all the initialization of sigq->info (everything but
> si_sys_private) to alloc_posix_timer.

Yes, we can do this in sys_timer_create(). But this is not very trivial to
do without uglifying the code futher, note this "if (timer_event_spec)".
And we can't do this after "->it_process = process", the timer is already
visible to sys_timer_settime().

> Even if it's a fine stopgap, I'm not comfortable calling this a real "fix".
> ...
> I don't find it easy to be sure there aren't more bad
> problems caused by trying to re-send the same sigqueue entry.

Yes, yes, I agree. I propose this change as a first step only.

> It seems likely this is the good choice for the stable branch.

So, what do you and Mark think about the patch below?

> > The thread which does dequeue_signal() continues and re-schedules the
> > timer while ->sigq is queued. Then it copies sigq->info to the user space.
>
> The thread that dequeued the first timer signal had ceased all reference
> to sigq by the time it unlocked siglock.  When its do_schedule_next_timer
> call gets it_lock, it can do bookkeeping in struct k_itimer to figure out
> what posix_timer_event or timer_settime has done lately.

Yes, this should work.

Oleg.

--- a/kernel/posix-timers.c
+++ b/kernel/posix-timers.c
@@ -298,12 +298,10 @@ void do_schedule_next_timer(struct sigin
 
 int posix_timer_event(struct k_itimer *timr,int si_private)
 {
-	memset(&timr->sigq->info, 0, sizeof(siginfo_t));
 	timr->sigq->info.si_sys_private = si_private;
 	/* Send signal to the process that owns this timer.*/
 
 	timr->sigq->info.si_signo = timr->it_sigev_signo;
-	timr->sigq->info.si_errno = 0;
 	timr->sigq->info.si_code = SI_TIMER;
 	timr->sigq->info.si_tid = timr->it_id;
 	timr->sigq->info.si_value = timr->it_sigev_value;
@@ -435,6 +433,7 @@ static struct k_itimer * alloc_posix_tim
 		kmem_cache_free(posix_timers_cache, tmr);
 		tmr = NULL;
 	}
+	memset(&tmr->sigq->info, 0, sizeof(siginfo_t));
 	return tmr;
 }
 
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1310,6 +1310,7 @@ int send_sigqueue(struct sigqueue *q, st
 		q->info.si_overrun++;
 		goto out;
 	}
+	q->info.si_overrun = 0;
 
 	signalfd_notify(t, sig);
 	pending = group ? &t->signal->shared_pending : &t->pending;

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