From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762144AbaGRUUg (ORCPT ); Fri, 18 Jul 2014 16:20:36 -0400 Received: from mail-lb0-f181.google.com ([209.85.217.181]:41657 "EHLO mail-lb0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755035AbaGRUUe (ORCPT ); Fri, 18 Jul 2014 16:20:34 -0400 MIME-Version: 1.0 In-Reply-To: <53C98055.5070301@zytor.com> References: <20140718150005.73db8eb9@canb.auug.org.au> <53C97287.5020509@zytor.com> <53C97E0F.6090601@zytor.com> <53C98055.5070301@zytor.com> From: Andy Lutomirski Date: Fri, 18 Jul 2014 13:20:13 -0700 Message-ID: Subject: Re: linux-next: build warning after merge of the tip tree To: "H. Peter Anvin" Cc: Stephen Rothwell , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , "linux-next@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Linus Torvalds Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 18, 2014 at 1:15 PM, H. Peter Anvin wrote: > On 07/18/2014 01:08 PM, Andy Lutomirski wrote: >> >> i isn't an index in to the syms array at all. This code is completely >> wrong. See the patch I sent in reply to Stephen's original email. >> >> But, to your earlier point, presumably this could warn: >> >> for (int i = 0; i < 10; i++) >> if (array[i] > array[5] + 1) >> fail(); >> >> I think that's absurd. There's nothing wrong with that code. A given >> test should have to be always true or always false on *all* loop >> iterations to be flagged, I think. >> > > No, the issue is that gcc is telling you that the code will do the wrong > thing in this case. Yes, only for one iteration, but still. > > The reason this is a concern is that: (x > x + n) and its variants is > often used to mean (x > INT_MAX - n) without the type knowledge, but > that is actually invalid standard C because signed types are not > guaranteed to wrap. Right, but the constant in this case is *much* less than INT_MAX. Anyway, this is moot. I do wonder whether the kind of people who build hardened kernels should enable -fwrapv, though. --Andy > > -hpa > -- Andy Lutomirski AMA Capital Management, LLC