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=-8.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 D5005C433DF for ; Mon, 24 Aug 2020 18:18:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AAF87206BE for ; Mon, 24 Aug 2020 18:18:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726839AbgHXSSG (ORCPT ); Mon, 24 Aug 2020 14:18:06 -0400 Received: from gate.crashing.org ([63.228.1.57]:50505 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726578AbgHXSSD (ORCPT ); Mon, 24 Aug 2020 14:18:03 -0400 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 07OIHSLx029771; Mon, 24 Aug 2020 13:17:28 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 07OIHQLo029770; Mon, 24 Aug 2020 13:17:26 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Mon, 24 Aug 2020 13:17:26 -0500 From: Segher Boessenkool To: Guohua Zhong Cc: christophe.leroy@csgroup.eu, gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, nixiaoming@huawei.com, paulus@samba.org, stable@vger.kernel.org, wangle6@huawei.com Subject: Re: =?utf-8?B?UmXvvJpSZQ==?= =?utf-8?Q?=3A?= [PATCH] powerpc: Fix a bug in __div64_32 if divisor is zero Message-ID: <20200824181726.GR28786@gate.crashing.org> References: <20200823001101.GO28786@gate.crashing.org> <20200824115407.55896-1-zhongguohua1@huawei.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200824115407.55896-1-zhongguohua1@huawei.com> 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 On Mon, Aug 24, 2020 at 07:54:07PM +0800, Guohua Zhong wrote: > >> Yet, I have noticed that there is no checking of 'base' in these functions. > >> But I am not sure how to check is better.As we know that the result is > >> undefined when divisor is zero. It maybe good to print error and dump stack. > >> Let the process to know that the divisor is zero by sending SIGFPE. > > > That is now what the PowerPC integer divide insns do: they just leave > > the result undefined (and they can set the overflow flag then, but no > > one uses that). > > OK ,So just keep the patch as below. If this patch looks good for you, please > help to review. I will send the new patch later. > > Thanks for your reply. > > diff --git a/arch/powerpc/boot/div64.S b/arch/powerpc/boot/div64.S > index 4354928ed62e..1d3561cf16fa 100644 > --- a/arch/powerpc/boot/div64.S > +++ b/arch/powerpc/boot/div64.S > @@ -13,8 +13,10 @@ > > .globl __div64_32 > .globl __div64_32 > __div64_32: > + cmplwi r4,0 # check if divisor r4 is zero > lwz r5,0(r3) # get the dividend into r5/r6 > lwz r6,4(r3) > + beq 5f # jump to label 5 if r4(divisor) is zero Just "beqlr". This instruction scheduling hurts all CPUs that aren't 8xx, fwiw (but likely only in the case where r4 *is* zero, so who cares :-) ) So... What is the *goal* of this patch? It looks like the routine would not get into a loop if r4 is 0, just return the wrong result? But, it *always* will, there *is* no right result? No caller should call it with zero as divisor ever, so in that sense, checking for it in the division routine is just pure wasted work. Segher