Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753465AbdLHLOY (ORCPT ); Fri, 8 Dec 2017 06:14:24 -0500 Received: from smtp-out4.electric.net ([192.162.216.183]:63193 "EHLO smtp-out4.electric.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753236AbdLHLOT (ORCPT ); Fri, 8 Dec 2017 06:14:19 -0500 From: David Laight To: "'Andrey Ryabinin'" , Andrew Morton CC: Ding Tianhong , "linux-kernel@vger.kernel.org" , LinuxArm Subject: RE: [PATCH v2] ubsan: don't handle misaligned address when support unaligned access Thread-Topic: [PATCH v2] ubsan: don't handle misaligned address when support unaligned access Thread-Index: AQHTcBIjs4rZxZsopkiW0TBvojCGdqM5ShMQ Date: Fri, 8 Dec 2017 11:14:34 +0000 Message-ID: <168669e65cff4bc1b2fbed842cea5dec@AcuMS.aculab.com> References: <5b905d56-609e-3822-096a-3b93b3eb7675@huawei.com> <20171206164916.687d5006b7a49ef1149e7154@linux-foundation.org> <022fdf6b-6e00-fe70-3124-80b8ff50a859@virtuozzo.com> <20171207152453.7aaf8f420ebf69d13c29a761@linux-foundation.org> <8c7cc89e-f976-077b-7b64-1046a1f69569@virtuozzo.com> In-Reply-To: <8c7cc89e-f976-077b-7b64-1046a1f69569@virtuozzo.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.33] Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 X-Outbound-IP: 156.67.243.126 X-Env-From: David.Laight@ACULAB.COM X-Proto: esmtps X-Revdns: X-HELO: AcuMS.aculab.com X-TLS: TLSv1.2:ECDHE-RSA-AES256-SHA384:256 X-Authenticated_ID: X-PolicySMART: 3396946, 3397078 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from base64 to 8bit by nfs id vB8BESmb023439 Content-Length: 939 Lines: 28 From: Andrey Ryabinin > Sent: 08 December 2017 10:49 ... > CONFIG_UBSAN_ALIGNMENT is already disabled by default for HAVE_EFFICIENT_UNALIGNED_ACCESS=y because it's noisy, > but we still allow users to enable it if they want to. > > I don't think we should completely forbid enabling it for HAVE_EFFICIENT_UNALIGNED_ACCESS=y. > Unaligned access is still a bug in non-arch code and outside of sections like #ifdef HAVE_EFFICIENT_UNALIGNED_ACCESS .. #endif . Don't think so. Code that knows that unaligned accesses don't fault can set up pointers that non-arch code dereferences. Happens all the time in the networking stack. ... > And one day, GCC might start doing optimizations based on this, e.g.: > > u64 *ptr; > ... > x = *ptr; > ... > if (ptr & 7) // Compiler can assume that this statement is always false, because 'ptr' was deferenced, so it must be aligned > do_something(); Ugg - shoot the gcc developers :-) David