Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754751Ab3CSMx6 (ORCPT ); Tue, 19 Mar 2013 08:53:58 -0400 Received: from mail1.sysgo.com ([176.9.26.183]:38056 "EHLO mail1.sysgo.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753862Ab3CSMx5 (ORCPT ); Tue, 19 Mar 2013 08:53:57 -0400 Message-ID: <51485FE3.1020905@sysgo.com> Date: Tue, 19 Mar 2013 13:53:55 +0100 From: David Engraf User-Agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20130221 Thunderbird/17.0.3 MIME-Version: 1.0 To: Eric Dumazet CC: Thomas Gleixner , John Stultz , linux-kernel@vger.kernel.org Subject: Re: [PATCH] ktime_add_ns() may overflow on 32bit architectures References: <51485A43.4020600@sysgo.com> <1363696716.21184.45.camel@edumazet-glaptop> In-Reply-To: <1363696716.21184.45.camel@edumazet-glaptop> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1642 Lines: 43 Am 19.03.2013 13:38, schrieb Eric Dumazet: > On Tue, 2013-03-19 at 13:29 +0100, David Engraf wrote: >> Hello, >> >> I've triggered an overflow when using ktime_add_ns() on a 32bit >> architecture not supporting CONFIG_KTIME_SCALAR. >> >> When passing a very high value for u64 nsec, e.g. 7881299347898368000 >> the do_div() function converts this value to seconds (7881299347) which >> is still to high to pass to the ktime_set() function as long. The result >> in my case is a negative value. >> >> The problem on my system occurs in the tick-sched.c, >> tick_nohz_stop_sched_tick() when time_delta is set to >> timekeeping_max_deferment(). The check for time_delta < KTIME_MAX is >> valid, thus ktime_add_ns() is called with a too large value resulting in >> a negative expire value. This leads to an endless loop in the ticker code: >> >> time_delta: 7881299347898368000 >> expires = ktime_add_ns(last_update, time_delta) >> expires: negative value >> >> This error doesn't occurs on 64bit or architectures supporting >> CONFIG_KTIME_SCALAR (e.g. ARM, x86-32). >> >> Best regards >> - David >> >> Signed-off-by: David Engraf >> > > But check already exists for 64bit arches in ktime_set() > Yes, but not for 32bit arches. 64-bit arches doesn't run into this problem because ktime_add_ns() can directly calculate the result without calling do_div() and ktime_set(). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/