Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757484AbcLTLvO (ORCPT ); Tue, 20 Dec 2016 06:51:14 -0500 Received: from ud10.udmedia.de ([194.117.254.50]:48063 "EHLO mail.ud10.udmedia.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752523AbcLTLvN (ORCPT ); Tue, 20 Dec 2016 06:51:13 -0500 Date: Tue, 20 Dec 2016 12:51:09 +0100 From: Markus Trippelsdorf To: "H. Peter Anvin" Cc: tglx@linutronix.de, linux-kernel@vger.kernel.org, mingo@kernel.org, linux-tip-commits@vger.kernel.org Subject: Re: [tip:x86/urgent] x86/tools: Fix gcc-7 warning in relocs.c Message-ID: <20161220115109.GB289@x4> References: <20161215124513.GA289@x4> <20161220100056.GA289@x4> <2966d559-2cea-fd95-76d4-df34192a9b88@zytor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2966d559-2cea-fd95-76d4-df34192a9b88@zytor.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1116 Lines: 32 On 2016.12.20 at 03:10 -0800, H. Peter Anvin wrote: > On 12/20/16 02:00, Markus Trippelsdorf wrote: > > On 2016.12.20 at 01:30 -0800, H. Peter Anvin wrote: > >> I'd strongly prefer a non-data-dependent solution, specifically adding > >> at the top of sort_relocs(): > >> > >> if (!r->count) > >> return; > >> > >> However, by my reading of the C and POSIX standards, this is a gcc > >> error: qsort() should do nothing if the count is zero. > > > > No, it is invoking undefined behavior. > > > Notice the nonnull attribute in /usr/include/stdlib.h: > > > > 739 /* Sort NMEMB elements of BASE, of SIZE bytes each, > > 740 using COMPAR to perform the comparisons. */ > > 741 extern void qsort (void *__base, size_t __nmemb, size_t __size, > > 742 __compar_fn_t __compar) __nonnull ((1, 4)); > > > > But feel free to revert my patch and add your solution. > > Well, s/gcc/glibc/ then. > > > The qsort() function shall sort an array of nel objects, the > > initial element of which is pointed to by base NULL does not point to any object, therefore it is UB. -- Markus