From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751943AbeBZIqi (ORCPT ); Mon, 26 Feb 2018 03:46:38 -0500 Received: from gate.crashing.org ([63.228.1.57]:35801 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751022AbeBZIqh (ORCPT ); Mon, 26 Feb 2018 03:46:37 -0500 Date: Mon, 26 Feb 2018 02:46:02 -0600 From: Segher Boessenkool To: Christophe LEROY Cc: Mathieu Malaterre , Michael Ellerman , linuxppc-dev@lists.ozlabs.org, Paul Mackerras , linux-kernel@vger.kernel.org, Jiri Slaby Subject: Re: [PATCH 05/21] powerpc: Avoid comparison of unsigned long >= 0 in pfn_valid Message-ID: <20180226084602.GR21977@gate.crashing.org> References: <20180225172236.29650-1-malat@debian.org> <20180225172236.29650-6-malat@debian.org> <55d52f37-c24d-c2da-43c6-73a55403d3b6@c-s.fr> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <55d52f37-c24d-c2da-43c6-73a55403d3b6@c-s.fr> User-Agent: Mutt/1.4.2.3i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 26, 2018 at 07:32:03AM +0100, Christophe LEROY wrote: > Le 25/02/2018 à 18:22, Mathieu Malaterre a écrit : > >-#define pfn_valid(pfn) ((pfn) >= ARCH_PFN_OFFSET && (pfn) < > >max_mapnr) > >+#define pfn_valid(pfn) \ > >+ (((pfn) - ARCH_PFN_OFFSET) < (max_mapnr - ARCH_PFN_OFFSET)) > > What will happen when ARCH_PFN_OFFSET is not nul and pfn is lower than > ARCH_PFN_OFFSET ? It will work fine. Say you are asking for a <= x < b so (in actual integers, no overflow) that is 0 <= x-a < b-a and you also assume x-a overflows, so that we are actually comparing x-a+M < b-a with M = 2**32 or such (the maximum value in the unsigned integer type plus one). This comparison is obviously always false. (It also works if b < a btw). Segher