From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44483) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Zvns9-0004qY-4l for qemu-devel@nongnu.org; Mon, 09 Nov 2015 09:56:41 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Zvns8-00068R-7s for qemu-devel@nongnu.org; Mon, 09 Nov 2015 09:56:41 -0500 Received: from mnementh.archaic.org.uk ([2001:8b0:1d0::1]:35353) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Zvns8-00066p-12 for qemu-devel@nongnu.org; Mon, 09 Nov 2015 09:56:40 -0500 From: Peter Maydell Date: Mon, 9 Nov 2015 14:56:31 +0000 Message-Id: <1447080991-24995-1-git-send-email-peter.maydell@linaro.org> Subject: [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: qemu-devel@nongnu.org Cc: Aaron Elkins , Paolo Bonzini , "Michael S. Tsirkin" , patches@linaro.org 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); } 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