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 7CEB7C10F0E for ; Tue, 9 Apr 2019 09:50:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4147B20830 for ; Tue, 9 Apr 2019 09:50:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726636AbfDIJux (ORCPT ); Tue, 9 Apr 2019 05:50:53 -0400 Received: from atrey.karlin.mff.cuni.cz ([195.113.26.193]:58698 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726079AbfDIJuw (ORCPT ); Tue, 9 Apr 2019 05:50:52 -0400 Received: by atrey.karlin.mff.cuni.cz (Postfix, from userid 512) id 55CAF80839; Tue, 9 Apr 2019 11:50:41 +0200 (CEST) Date: Tue, 9 Apr 2019 11:50:41 +0200 From: Pavel Machek To: Andrew Morton Cc: Vadim Pasternak , jacek.anaszewski@gmail.com, linux-kernel@vger.kernel.org, linux-leds@vger.kernel.org, idosch@mellanox.com, Andrey Ryabinin Subject: Re: [PATCH v1 bitops] bitops: Fix UBSAN undefined behavior warning for rotation right Message-ID: <20190409095041.GA32344@atrey.karlin.mff.cuni.cz> References: <20190407125325.24440-1-vadimp@mellanox.com> <20190408155217.3f723554ae7fbcb34eeacb30@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190408155217.3f723554ae7fbcb34eeacb30@linux-foundation.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi! > (resend, cc Andrey) > > On Sun, 7 Apr 2019 12:53:25 +0000 Vadim Pasternak wrote: > > > The warning is caused by call to rorXX(), if the second parameters of > > this function "shift" is zero. In such case UBSAN reports the warning > > for the next expression: (word << (XX - shift), where XX is > > 64, 32, 16, 8 for respectively ror64, ror32, ror16, ror8. > > Fix adds validation of this parameter - in case it's equal zero, no > > need to rotate, just original "word" is to be returned to caller. > > > > The UBSAN undefined behavior warning has been reported for call to > > ror32(): > > [ 11.426543] UBSAN: Undefined behaviour in ./include/linux/bitops.h:93:33 > > [ 11.434045] shift exponent 32 is too large for 32-bit type 'unsigned int' > > hm, do we care? > > > ... > > > > > --- a/include/linux/bitops.h > > +++ b/include/linux/bitops.h > > @@ -70,6 +70,9 @@ static inline __u64 rol64(__u64 word, unsigned int shift) > > */ > > static inline __u64 ror64(__u64 word, unsigned int shift) > > { > > + if (!shift) > > + return word; > > + > > return (word >> shift) | (word << (64 - shift)); > > } > > Is there any known architecture or compiler for which UL<<64 doesn't > reliably produce zero? Is there any prospect that this will become a > problem in the future? Compiler is free to assume that shift !=0 after running ror64()... and use that fact in optimalizations. so... if it is not problem today it may easily become problem tommorow. Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html