linux-kernel - Re: [PATCH] clockevents: Sanitize ticks to nsec conversion

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Message-ID: <20130919124805.GU24802@pengutronix.de>
Date:	Thu, 19 Sep 2013 14:48:05 +0200
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ludovic Desroches <ludovic.desroches@...el.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Marc Kleine-Budde <mkl@...gutronix.de>,
	nicolas.ferre@...el.com, LKML <linux-kernel@...r.kernel.org>,
	Marc Pignat <marc.pignat@...s.ch>, john.stultz@...aro.org,
	kernel@...gutronix.de, Ronald Wahl <ronald.wahl@...itan.com>,
	LAK <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] clockevents: Sanitize ticks to nsec conversion

Hello Thomas,

On Thu, Sep 19, 2013 at 12:15:10PM +0200, Thomas Gleixner wrote:
> On Thu, 19 Sep 2013, Uwe Kleine-König wrote:
> > On Thu, Sep 19, 2013 at 12:01:25AM +0200, Thomas Gleixner wrote:
> > > Versus the 64bit overflow check, we need to be even more careful. We
> > > need to check for overflowing (1 << 63) - 1 (i.e. the max positive
> > > value which fits into a s64). See clockevents_program_event().
> > 
> > That is because you interpret times < 0 as in the past, right? But note
> > that the interim result we're talking about here is still to be divided
> > by evt->mult. So assuming mult > 1, that check is too strict unless you
> > move it below the do_div in clockevent_delta2ns. For sure it makes sense
> > to use the same value for a and b in the handling:
> 
> No, it's not too strict.
> 
>     nsec = (latch << shift) / mult;
> 
> Now the backwards conversion does:
> 
>     latch = (nsec * mult) >> shift;
>
> So we want nsec * mult to be in the positive range of s64. Which
> means, that latch << shift must be in that range as well.
The backwards conversion is in clockevents_program_event(), right? There
is:

	clc = ((unsigned long long) delta * dev->mult) >> dev->shift;

So I don't see a problem if nsec * mult overflows (1 << 63) - 1 as long
as it still fits into an unsigned long long (i.e. a 64 bit value).

What am I missing?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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