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=-3.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED 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 25F61C67839 for ; Fri, 14 Dec 2018 08:01:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AB2D62080F for ; Fri, 14 Dec 2018 08:01:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AB2D62080F Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=c-s.fr Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727314AbeLNIBy (ORCPT ); Fri, 14 Dec 2018 03:01:54 -0500 Received: from pegase1.c-s.fr ([93.17.236.30]:47965 "EHLO pegase1.c-s.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726533AbeLNIBy (ORCPT ); Fri, 14 Dec 2018 03:01:54 -0500 Received: from localhost (mailhub1-int [192.168.12.234]) by localhost (Postfix) with ESMTP id 43GNL74xnwz9vGy9; Fri, 14 Dec 2018 09:01:51 +0100 (CET) X-Virus-Scanned: Debian amavisd-new at c-s.fr Received: from pegase1.c-s.fr ([192.168.12.234]) by localhost (pegase1.c-s.fr [192.168.12.234]) (amavisd-new, port 10024) with ESMTP id 1YohcOmq4HKZ; Fri, 14 Dec 2018 09:01:51 +0100 (CET) Received: from messagerie.si.c-s.fr (messagerie.si.c-s.fr [192.168.25.192]) by pegase1.c-s.fr (Postfix) with ESMTP id 43GNL74CS4z9vGy8; Fri, 14 Dec 2018 09:01:51 +0100 (CET) Received: from localhost (localhost [127.0.0.1]) by messagerie.si.c-s.fr (Postfix) with ESMTP id 88AF98B89E; Fri, 14 Dec 2018 09:01:52 +0100 (CET) X-Virus-Scanned: amavisd-new at c-s.fr Received: from messagerie.si.c-s.fr ([127.0.0.1]) by localhost (messagerie.si.c-s.fr [127.0.0.1]) (amavisd-new, port 10023) with ESMTP id gaXjU-IdF9US; Fri, 14 Dec 2018 09:01:52 +0100 (CET) Received: from PO15451 (po15451.idsi0.si.c-s.fr [172.25.231.2]) by messagerie.si.c-s.fr (Postfix) with ESMTP id 48D898B89B; Fri, 14 Dec 2018 09:01:52 +0100 (CET) Subject: Re: [PATCH] powerpc/mm: make NULL pointer deferences explicit on bad page faults. To: Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org References: <42e7e2e9b74af526523fb5a1451fdb29cfe2687a.1539060806.git.christophe.leroy@c-s.fr> <87zht9udqh.fsf@concordia.ellerman.id.au> From: Christophe Leroy Message-ID: <276439ce-aad9-e72f-7bc9-c57fb4a59339@c-s.fr> Date: Fri, 14 Dec 2018 09:01:52 +0100 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.3.3 MIME-Version: 1.0 In-Reply-To: <87zht9udqh.fsf@concordia.ellerman.id.au> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Michael, Le 14/12/2018 à 01:57, Michael Ellerman a écrit : > Hi Christophe, > > You know it's the trivial patches that are going to get lots of review > comments :) I'm so happy to get comments. > > Christophe Leroy writes: >> As several other arches including x86, this patch makes it explicit >> that a bad page fault is a NULL pointer dereference when the fault >> address is lower than PAGE_SIZE > > I'm being pedantic, but it's not necessarily a NULL pointer dereference. > It might just be a direct access to a low address, eg: > > char *p = 0x100; > *p = 0; > > That's not a NULL pointer dereference. > > But other arches do print this so I guess it's OK to add, and in most > cases it will be an actual NULL pointer dereference. > > I wonder though if we should use 4096 rather than PAGE_SIZE, given > that's the actual value other arches are using. We support 256K pages on > some systems, which is getting quite large. Those invalid accesses are catched because the first page is marked non present or non accessible in the page table, so I thing using PAGE_SIZE here is valid regardless of the page size. Looks like the arches have PAGE_SHIFT ranging from 12 to 16 mainly. > >> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c >> index d51cf5f4e45e..501a1eadb3e9 100644 >> --- a/arch/powerpc/mm/fault.c >> +++ b/arch/powerpc/mm/fault.c >> @@ -631,13 +631,16 @@ void bad_page_fault(struct pt_regs *regs, unsigned long address, int sig) >> switch (TRAP(regs)) { >> case 0x300: >> case 0x380: >> - printk(KERN_ALERT "Unable to handle kernel paging request for " >> - "data at address 0x%08lx\n", regs->dar); >> + pr_alert("Unable to handle kernel %s for data at address 0x%08lx\n", >> + regs->dar < PAGE_SIZE ? "NULL pointer dereference" : >> + "paging request", >> + regs->dar); > > This is now too long I think, with printk time you get: > > [ 1096.450711] Unable to handle kernel NULL pointer dereference for data at address 0x00000000 > > Which is 93 columns. It's true on many systems it doesn't really matter > any more, but it would still be good if it was shorter. > > I like that on x86 they prefix it with "BUG:", just to avoid any confusion. > > What if we had for the NULL pointer case: > > BUG: Kernel NULL pointer dereference at 0x00000000 > > And for the normal case: > > BUG: Unable to handle kernel data access at 0x00000000 > > Note on the very next line we print: > Faulting instruction address: 0xc000000000795cc8 > > So there should be no confusion about whether "at" refers to the data > address or the instruction address. Agreed > >> case 0x400: >> case 0x480: >> - printk(KERN_ALERT "Unable to handle kernel paging request for " >> - "instruction fetch\n"); >> + pr_alert("Unable to handle kernel %s for instruction fetch\n", >> + regs->nip < PAGE_SIZE ? "NULL pointer dereference" : >> + "paging request"); > > I don't really like using "NULL pointer dereference" here, that > terminology makes me think of a load/store, I think it confuses things > rather than making it clearer. > > What about: > > BUG: Unable to handle kernel instruction fetch at 0x00000000 I think we still need to make it explicit that we jumped there due to a NULL function pointer, allthought I don't have a good text idea yet for this. > > >> break; >> case 0x600: >> printk(KERN_ALERT "Unable to handle kernel paging request for " > > It would be good to clean up these other cases as well. They seem to be > trying to use the "page request for" terminology which leads to them > being very wordy. I assume that was done to help people grepping kernel > logs for errors, but I think we should not worry about that if we have > the "BUG:" prefix. > > So we have: > printk(KERN_ALERT "Unable to handle kernel paging request for " > "unaligned access at address 0x%08lx\n", regs->dar); > > What about: > > BUG: Unable to handle kernel unaligned access at 0x00000000 > > And: > printk(KERN_ALERT "Unable to handle kernel paging request for " > "unknown fault\n"); > > What about: > > BUG: Unable to handle unknown paging fault at 0x00000000 > > > Thoughts? Looks like good ideas I'll carry on. Christophe