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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=unavailable 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 F3842C43381 for ; Mon, 18 Feb 2019 00:53:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C9D702184E for ; Mon, 18 Feb 2019 00:53:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727705AbfBRAtZ (ORCPT ); Sun, 17 Feb 2019 19:49:25 -0500 Received: from ozlabs.org ([203.11.71.1]:36451 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726147AbfBRAtZ (ORCPT ); Sun, 17 Feb 2019 19:49:25 -0500 Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPSA id 442lcd2whwz9s7h; Mon, 18 Feb 2019 11:49:21 +1100 (AEDT) From: Michael Ellerman To: Balbir Singh Cc: Segher Boessenkool , 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() In-Reply-To: <20190217215556.GH31125@350D> References: <20190214062339.7139-1-mpe@ellerman.id.au> <20190216105511.GA31125@350D> <20190216142206.GE14180@gate.crashing.org> <20190217062333.GC31125@350D> <87ef86dd9v.fsf@concordia.ellerman.id.au> <20190217215556.GH31125@350D> Date: Mon, 18 Feb 2019 11:49:18 +1100 Message-ID: <87imxhrkdt.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Balbir Singh writes: > On Sun, Feb 17, 2019 at 07:34:20PM +1100, Michael Ellerman wrote: >> Balbir Singh writes: >> > On Sat, Feb 16, 2019 at 08:22:12AM -0600, Segher Boessenkool wrote: >> >> 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. >> >> >> > >> > Thanks, I am surprised the compiler does not complain about the truncation >> > of bits. I wonder if we are missing -Wconversion >> >> Good luck with that :) >> >> What I should start doing is building with it enabled and then comparing >> the output before and after commits to make sure we're not introducing >> new cases. > > Fair enough, my point was that the compiler can help out. I'll see what > -Wconversion finds on my local build :) I get about 43MB of warnings here :) cheers