From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jean Delvare Date: Thu, 20 Dec 2012 11:48:20 +0000 Subject: Re: [lm-sensors] [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors Message-Id: <20121220124820.0069aa66@endymion.delvare> List-Id: References: <1355928015-26212-1-git-send-email-linux@roeck-us.net> <20121219232115.727a2e91@endymion.delvare> <20121219230144.GB26863@roeck-us.net> In-Reply-To: <20121219230144.GB26863@roeck-us.net> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Guenter Roeck Cc: linux-kernel@vger.kernel.org, Andrew Morton , lm-sensors@lm-sensors.org, Juergen Beisert On Wed, 19 Dec 2012 15:01:44 -0800, Guenter Roeck wrote: > On Wed, Dec 19, 2012 at 11:21:15PM +0100, Jean Delvare wrote: > > Hi Guenter, > > > > On Wed, 19 Dec 2012 06:40:15 -0800, Guenter Roeck wrote: > > > Commit 263a523 fixes a warning seen with W=1 due to change in > > > DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations > > > with unsigned divisors to unsigned, even if the dividend is signed and > > > negative (for example, -10 / 5U = 858993457). The C standard says "If one > > > operand has unsigned int type, the other operand is converted to unsigned > > > int", so the compiler is not to blame. > > > > This is surprising to say the least. But if the C standard says so... > > Agreed, but it is how it is. > > > I wouldn't be surprised if there are bugs because of this in the kernel > > and in other projects. > > Might easily be. This might make a good interview question - I suspect many > if not most engineers would fail it. At least I would have until yesterday :). Neither did I. And I'm not sure I'll remember it in one year from now. > > (...) > > Thinking a bit more about this... Documenting the non-working cases is > > great, however I don't really expect all developers to pay attention. I > > can also imagine variable types changing from signed to unsigned later, > > and never thinking this can introduce a bug. > > > > So, is there nothing we can do to spot at least the second issue at > > build time? For regular division there's nothing we can do (although I > > don't understand why gcc doesn't warn...) but here we get the > > opportunity to report the issue, let's take it. > > > > And given that the divisor is almost always a constant, > > maybe we can check for negative divisors too, this would be safer and > > the code size increase would probably be very small in practice. > > Opinions? > > Agreed, though we should fix the problem now and think about reporting > afterwards. Yes, that's a good plan. -- Jean Delvare _______________________________________________ lm-sensors mailing list lm-sensors@lm-sensors.org http://lists.lm-sensors.org/mailman/listinfo/lm-sensors From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752474Ab2LTLsf (ORCPT ); Thu, 20 Dec 2012 06:48:35 -0500 Received: from zoneX.GCU-Squad.org ([194.213.125.0]:44466 "EHLO services.gcu-squad.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751469Ab2LTLs2 (ORCPT ); Thu, 20 Dec 2012 06:48:28 -0500 Date: Thu, 20 Dec 2012 12:48:20 +0100 From: Jean Delvare To: Guenter Roeck Cc: linux-kernel@vger.kernel.org, Andrew Morton , lm-sensors@lm-sensors.org, Juergen Beisert Subject: Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors Message-ID: <20121220124820.0069aa66@endymion.delvare> In-Reply-To: <20121219230144.GB26863@roeck-us.net> References: <1355928015-26212-1-git-send-email-linux@roeck-us.net> <20121219232115.727a2e91@endymion.delvare> <20121219230144.GB26863@roeck-us.net> X-Mailer: Claws Mail 3.7.10 (GTK+ 2.24.7; x86_64-suse-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 19 Dec 2012 15:01:44 -0800, Guenter Roeck wrote: > On Wed, Dec 19, 2012 at 11:21:15PM +0100, Jean Delvare wrote: > > Hi Guenter, > > > > On Wed, 19 Dec 2012 06:40:15 -0800, Guenter Roeck wrote: > > > Commit 263a523 fixes a warning seen with W=1 due to change in > > > DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations > > > with unsigned divisors to unsigned, even if the dividend is signed and > > > negative (for example, -10 / 5U = 858993457). The C standard says "If one > > > operand has unsigned int type, the other operand is converted to unsigned > > > int", so the compiler is not to blame. > > > > This is surprising to say the least. But if the C standard says so... > > Agreed, but it is how it is. > > > I wouldn't be surprised if there are bugs because of this in the kernel > > and in other projects. > > Might easily be. This might make a good interview question - I suspect many > if not most engineers would fail it. At least I would have until yesterday :). Neither did I. And I'm not sure I'll remember it in one year from now. > > (...) > > Thinking a bit more about this... Documenting the non-working cases is > > great, however I don't really expect all developers to pay attention. I > > can also imagine variable types changing from signed to unsigned later, > > and never thinking this can introduce a bug. > > > > So, is there nothing we can do to spot at least the second issue at > > build time? For regular division there's nothing we can do (although I > > don't understand why gcc doesn't warn...) but here we get the > > opportunity to report the issue, let's take it. > > > > And given that the divisor is almost always a constant, > > maybe we can check for negative divisors too, this would be safer and > > the code size increase would probably be very small in practice. > > Opinions? > > Agreed, though we should fix the problem now and think about reporting > afterwards. Yes, that's a good plan. -- Jean Delvare