From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755322AbbHBAit (ORCPT ); Sat, 1 Aug 2015 20:38:49 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:36755 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754526AbbHBAeI (ORCPT ); Sat, 1 Aug 2015 20:34:08 -0400 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, "Tejun Heo" , "Jan Kara" , "Jens Axboe" Date: Sun, 02 Aug 2015 01:02:37 +0100 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) Subject: [PATCH 3.2 051/164] writeback: use |1 instead of +1 to protect against div by zero In-Reply-To: X-SA-Exim-Connect-IP: 192.168.4.249 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.2.70-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Tejun Heo commit 464d1387acb94dc43ba772b35242345e3d2ead1b upstream. mm/page-writeback.c has several places where 1 is added to the divisor to prevent division by zero exceptions; however, if the original divisor is equivalent to -1, adding 1 leads to division by zero. There are three places where +1 is used for this purpose - one in pos_ratio_polynom() and two in bdi_position_ratio(). The second one in bdi_position_ratio() actually triggered div-by-zero oops on a machine running a 3.10 kernel. The divisor is x_intercept - bdi_setpoint + 1 == span + 1 span is confirmed to be (u32)-1. It isn't clear how it ended up that but it could be from write bandwidth calculation underflow fixed by c72efb658f7c ("writeback: fix possible underflow in write bandwidth calculation"). At any rate, +1 isn't a proper protection against div-by-zero. This patch converts all +1 protections to |1. Note that bdi_update_dirty_ratelimit() was already using |1 before this patch. Signed-off-by: Tejun Heo Reviewed-by: Jan Kara Signed-off-by: Jens Axboe Signed-off-by: Ben Hutchings --- mm/page-writeback.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -560,7 +560,7 @@ static unsigned long bdi_position_ratio( */ setpoint = (freerun + limit) / 2; x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, - limit - setpoint + 1); + (limit - setpoint) | 1); pos_ratio = x; pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT; pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT; @@ -611,7 +611,7 @@ static unsigned long bdi_position_ratio( * scale global setpoint to bdi's: * bdi_setpoint = setpoint * bdi_thresh / thresh */ - x = div_u64((u64)bdi_thresh << 16, thresh + 1); + x = div_u64((u64)bdi_thresh << 16, thresh | 1); bdi_setpoint = setpoint * (u64)x >> 16; /* * Use span=(8*write_bw) in single bdi case as indicated by @@ -626,7 +626,7 @@ static unsigned long bdi_position_ratio( if (bdi_dirty < x_intercept - span / 4) { pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty), - x_intercept - bdi_setpoint + 1); + (x_intercept - bdi_setpoint) | 1); } else pos_ratio /= 4;