From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 09478C43381 for ; Sat, 16 Feb 2019 14:25:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D1156222E0 for ; Sat, 16 Feb 2019 14:25:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728398AbfBPOZq (ORCPT ); Sat, 16 Feb 2019 09:25:46 -0500 Received: from gate.crashing.org ([63.228.1.57]:44288 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725294AbfBPOZp (ORCPT ); Sat, 16 Feb 2019 09:25:45 -0500 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id x1GEMUfJ030078; Sat, 16 Feb 2019 08:22:30 -0600 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id x1GEMCDe030050; Sat, 16 Feb 2019 08:22:12 -0600 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Sat, 16 Feb 2019 08:22:12 -0600 From: Segher Boessenkool To: Balbir Singh Cc: Michael Ellerman , erhard_f@mailbox.org, jack@suse.cz, linuxppc-dev@ozlabs.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, aneesh.kumar@linux.vnet.ibm.com Subject: Re: [PATCH] powerpc/64s: Fix possible corruption on big endian due to pgd/pud_present() Message-ID: <20190216142206.GE14180@gate.crashing.org> References: <20190214062339.7139-1-mpe@ellerman.id.au> <20190216105511.GA31125@350D> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190216105511.GA31125@350D> User-Agent: Mutt/1.4.2.3i Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi all, On Sat, Feb 16, 2019 at 09:55:11PM +1100, Balbir Singh wrote: > On Thu, Feb 14, 2019 at 05:23:39PM +1100, Michael Ellerman wrote: > > In v4.20 we changed our pgd/pud_present() to check for _PAGE_PRESENT > > rather than just checking that the value is non-zero, e.g.: > > > > static inline int pgd_present(pgd_t pgd) > > { > > - return !pgd_none(pgd); > > + return (pgd_raw(pgd) & cpu_to_be64(_PAGE_PRESENT)); > > } > > > > Unfortunately this is broken on big endian, as the result of the > > bitwise && is truncated to int, which is always zero because (Bitwise "&" of course). > Not sure why that should happen, why is the result an int? What > causes the casting of pgd_t & be64 to be truncated to an int. Yes, it's not obvious as written... It's simply that the return type of pgd_present is int. So it is truncated _after_ the bitwise and. Segher