From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:51092) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZvoJz-0006xB-54 for qemu-devel@nongnu.org; Mon, 09 Nov 2015 10:25:31 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZvoJu-0004gp-Ur for qemu-devel@nongnu.org; Mon, 09 Nov 2015 10:25:27 -0500 References: <1447080991-24995-1-git-send-email-peter.maydell@linaro.org> From: Paolo Bonzini Message-ID: <5640BAD8.9040005@redhat.com> Date: Mon, 9 Nov 2015 16:25:12 +0100 MIME-Version: 1.0 In-Reply-To: <1447080991-24995-1-git-send-email-peter.maydell@linaro.org> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit 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 , qemu-devel@nongnu.org Cc: Aaron Elkins , "Michael S. Tsirkin" , qemu-stable , patches@linaro.org On 09/11/2015 15:56, 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 > --- > 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); Does checkpatch complain about the outer parentheses? > } > > 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); > } Cc: qemu-stable@nongnu.org Looks good, thanks! Can you apply it yourself? Paolo > static uint64_t ticks_to_ns(uint64_t value) >