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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS 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 380E8C10F13 for ; Mon, 8 Apr 2019 22:51:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 02A8F20857 for ; Mon, 8 Apr 2019 22:51:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1554763878; bh=77yxdbS0AK8GnWOtGUqkzsZ3GwU3N3qU2hFQtWbFAAk=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=dULDlRTrs+hj8lB96znL/O62HQdJYYEgU/aMdPCBP/08oPzHWHh7YfsoCHBFOhFZf 6uMmKNfPFjCMEOeWvxtzZpaXvpICfeyYMK3hm7XE08Cz3wxDeCyIsDFVvdSmNmYrin KIjODAfb8Ds4JjxSscKk3MrsmC2HEH7A+7tsG1x4= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726882AbfDHWvQ (ORCPT ); Mon, 8 Apr 2019 18:51:16 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:43732 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726188AbfDHWvQ (ORCPT ); Mon, 8 Apr 2019 18:51:16 -0400 Received: from akpm3.svl.corp.google.com (unknown [104.133.8.65]) by mail.linuxfoundation.org (Postfix) with ESMTPSA id 6D308CBB; Mon, 8 Apr 2019 22:51:15 +0000 (UTC) Date: Mon, 8 Apr 2019 15:51:14 -0700 From: Andrew Morton To: Vadim Pasternak Cc: jacek.anaszewski@gmail.com, pavel@ucw.cz, linux-kernel@vger.kernel.org, linux-leds@vger.kernel.org, idosch@mellanox.com Subject: Re: [PATCH v1 bitops] bitops: Fix UBSAN undefined behavior warning for rotation right Message-Id: <20190408155114.c0031348812a0590662e4bf9@linux-foundation.org> In-Reply-To: <20190407125325.24440-1-vadimp@mellanox.com> References: <20190407125325.24440-1-vadimp@mellanox.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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?