From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965874AbbLRVme (ORCPT ); Fri, 18 Dec 2015 16:42:34 -0500 Received: from mail-pa0-f49.google.com ([209.85.220.49]:33767 "EHLO mail-pa0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965523AbbLRVj1 (ORCPT ); Fri, 18 Dec 2015 16:39:27 -0500 From: John Stultz To: lkml Cc: David Gibson , stable@vger.kernel.org (3.7+ after 4.5-rc2), Thomas Gleixner , Ingo Molnar , Miroslav Lichvar , Prarit Bhargava , Richard Cochran , John Stultz Subject: [PATCH 03/11] time: Avoid signed overflow in timekeeping_get_ns() Date: Fri, 18 Dec 2015 13:39:08 -0800 Message-Id: <1450474756-10144-4-git-send-email-john.stultz@linaro.org> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1450474756-10144-1-git-send-email-john.stultz@linaro.org> References: <1450474756-10144-1-git-send-email-john.stultz@linaro.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: David Gibson 1e75fa8 "time: Condense timekeeper.xtime into xtime_sec" replaced a call to clocksource_cyc2ns() from timekeeping_get_ns() with an open-coded version of the same logic to avoid keeping a semi-redundant struct timespec in struct timekeeper. However, the commit also introduced a subtle semantic change - where clocksource_cyc2ns() uses purely unsigned math, the new version introduces a signed temporary, meaning that if (delta * tk->mult) has a 63-bit overflow the following shift will still give a negative result. The choice of 'maxsec' in __clocksource_updatefreq_scale() means this will generally happen if there's a ~10 minute pause in examining the clocksource. This can be triggered on a powerpc KVM guest by stopping it from qemu for a bit over 10 minutes. After resuming time has jumped backwards several minutes causing numerous problems (jiffies does not advance, msleep()s can be extended by minutes..). It doesn't happen on x86 KVM guests, because the guest TSC is effectively frozen while the guest is stopped, which is not the case for the powerpc timebase. Obviously an unsigned (64 bit) overflow will only take twice as long as a signed, 63-bit overflow. I don't know the time code well enough to know if that will still cause incorrect calculations, or if a 64-bit overflow is avoided elsewhere. Still, an incorrect forwards clock adjustment will cause less trouble than time going backwards. So, this patch removes the potential for intermediate signed overflow. Cc: stable@vger.kernel.org (3.7+ after 4.5-rc2) Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Miroslav Lichvar Cc: Prarit Bhargava Cc: Richard Cochran Suggested-by: Laurent Vivier Tested-by: Laurent Vivier Signed-off-by: David Gibson Signed-off-by: John Stultz --- kernel/time/timekeeping.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index d563c19..99188ee 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -305,8 +305,7 @@ static inline s64 timekeeping_get_ns(struct tk_read_base *tkr) delta = timekeeping_get_delta(tkr); - nsec = delta * tkr->mult + tkr->xtime_nsec; - nsec >>= tkr->shift; + nsec = (delta * tkr->mult + tkr->xtime_nsec) >> tkr->shift; /* If arch requires, add in get_arch_timeoffset() */ return nsec + arch_gettimeoffset(); -- 1.9.1