From mboxrd@z Thu Jan 1 00:00:00 1970 From: u.kleine-koenig@pengutronix.de (Uwe =?iso-8859-1?Q?Kleine-K=F6nig?=) Date: Thu, 19 Sep 2013 14:48:05 +0200 Subject: [PATCH] clockevents: Sanitize ticks to nsec conversion In-Reply-To: References: <20130917100417.GQ12758@n2100.arm.linux.org.uk> <20130917130153.GL26819@ludovic.desroches@atmel.com> <20130918085627.GN24802@pengutronix.de> <20130918150958.GO24802@pengutronix.de> <20130919100239.GS24802@pengutronix.de> Message-ID: <20130919124805.GU24802@pengutronix.de> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hello Thomas, On Thu, Sep 19, 2013 at 12:15:10PM +0200, Thomas Gleixner wrote: > On Thu, 19 Sep 2013, Uwe Kleine-K?nig 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-K?nig | Industrial Linux Solutions | http://www.pengutronix.de/ |