From d17008bc2914d62fd0af6a8f313604ae9f9a102c Mon Sep 17 00:00:00 2001 From: Peter Maydell Date: Mon, 9 Nov 2015 14:56:31 +0000 Subject: [PATCH] hw/timer/hpet.c: Avoid signed integer overflow which results in bugs on OSX 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 Cc: qemu-stable@nongnu.org Signed-off-by: Peter Maydell Message-id: 1447080991-24995-1-git-send-email-peter.maydell@linaro.org --- 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 3037bef72e..7f0391c786 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)