From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934563AbXK3By4 (ORCPT ); Thu, 29 Nov 2007 20:54:56 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759739AbXK3Byn (ORCPT ); Thu, 29 Nov 2007 20:54:43 -0500 Received: from smtp2.linux-foundation.org ([207.189.120.14]:46269 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763600AbXK3Bym (ORCPT ); Thu, 29 Nov 2007 20:54:42 -0500 Date: Thu, 29 Nov 2007 17:54:40 -0800 From: Andrew Morton To: "H. Peter Anvin" Cc: Linux Kernel Mailing List Subject: Re: [PATCH] Avoid overflows in kernel/time.c Message-Id: <20071129175440.ead82de6.akpm@linux-foundation.org> In-Reply-To: <200711300019.lAU0Jpbr003807@tazenda.hos.anvin.org> References: <200711300019.lAU0Jpbr003807@tazenda.hos.anvin.org> X-Mailer: Sylpheed 2.4.1 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 29 Nov 2007 16:19:51 -0800 "H. Peter Anvin" wrote: > When the conversion factor between jiffies and milli- or microseconds > is not a single multiply or divide, as for the case of HZ == 300, we > currently do a multiply followed by a divide. The intervening > result, however, is subject to overflows, especially since the > fraction is not simplified (for HZ == 300, we multiply by 300 and > divide by 1000). > > This is exposed to the user when passing a large timeout to poll(), > for example. > > This patch replaces the multiply-divide with a reciprocal > multiplication on 32-bit platforms. When the input is an unsigned > long, there is no portable way to do this on 64-bit platforms there is > no portable way to do this since it requires a 128-bit intermediate > result (which gcc does support on 64-bit platforms but may generate > libgcc calls, e.g. on 64-bit s390), but since the output is a 32-bit > integer in the cases affected, just simplify the multiply-divide > (*3/10 instead of *300/1000). > > The reciprocal multiply used can have off-by-one errors in the upper > half of the valid output range. This could be avoided at the expense > of having to deal with a potential 65-bit intermediate result. Since > the intent is to avoid overflow problems and most of the other time > conversions are only semiexact, the off-by-one errors were considered > an acceptable tradeoff. > > NOTE: This patch uses a bc(1) script to compute the appropriate > constants. Does this add the first dependency upon the availability of bc?