From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56650) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Zvssj-0001y3-IJ for qemu-devel@nongnu.org; Mon, 09 Nov 2015 15:17:38 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Zvssg-0005GH-Co for qemu-devel@nongnu.org; Mon, 09 Nov 2015 15:17:37 -0500 Received: from mx1.redhat.com ([209.132.183.28]:44895) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Zvssg-0005G5-7s for qemu-devel@nongnu.org; Mon, 09 Nov 2015 15:17:34 -0500 Date: Mon, 9 Nov 2015 22:17:30 +0200 From: "Michael S. Tsirkin" Message-ID: <20151109221638-mutt-send-email-mst@redhat.com> References: <1447080991-24995-1-git-send-email-peter.maydell@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1447080991-24995-1-git-send-email-peter.maydell@linaro.org> Subject: Re: [Qemu-devel] [PATCH for-2.5] hw/timer/hpet.c: Avoid signed integer overflow which results in bugs on OSX List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Maydell Cc: Aaron Elkins , Paolo Bonzini , qemu-devel@nongnu.org, patches@linaro.org On Mon, Nov 09, 2015 at 02:56:31PM +0000, Peter Maydell wrote: > Signed integer overflow in C is undefined behaviour, and the compiler > is at liberty to assume it can never happen and optimize accordingly. > In particular, the subtractions in hpet_time_after() and hpet_time_after64() > were causing OSX clang to optimize the code such that it was prone to > hangs and complaints about the main loop stalling (presumably because > we were spending all our time trying to service very high frequency > HPET timer callbacks). The clang sanitizer confirms the UB: > > hw/timer/hpet.c:119:26: runtime error: signed integer overflow: -2146967296 - 2147003978 cannot be represented in type 'int' > > Fix this by doing the subtraction as an unsigned operation and then > converting to signed for the comparison. > > Reported-by: Aaron Elkins > Signed-off-by: Peter Maydell Agree, this makes no sense the way it's written. Reviewed-by: Michael S. Tsirkin I'll pick this up in the next pull if Paolo doesn't beat me to it. > --- > hw/timer/hpet.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/hw/timer/hpet.c b/hw/timer/hpet.c > index 3037bef..7f0391c 100644 > --- a/hw/timer/hpet.c > +++ b/hw/timer/hpet.c > @@ -116,12 +116,12 @@ static uint32_t timer_enabled(HPETTimer *t) > > static uint32_t hpet_time_after(uint64_t a, uint64_t b) > { > - return ((int32_t)(b) - (int32_t)(a) < 0); > + return ((int32_t)(b - a) < 0); > } > > static uint32_t hpet_time_after64(uint64_t a, uint64_t b) > { > - return ((int64_t)(b) - (int64_t)(a) < 0); > + return ((int64_t)(b - a) < 0); > } > > static uint64_t ticks_to_ns(uint64_t value) > -- > 2.6.2