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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED, 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 E8A99C43381 for ; Tue, 12 Mar 2019 08:07:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B1E3D214AE for ; Tue, 12 Mar 2019 08:07:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1552378071; bh=Tvjet4ftyHbZxfuOm8lUk4sy6NygoiCOBU/oc/z3J0k=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=Ho/DyOGba7vR1k35rFC19id9yhWmmlxtf8pPofVVAGYN2WRpBZZYho68TdHN2LPhI WKMu93T1OF9OwDPqMa/aCXqx15P4bmDaiX0dblcEEo6rhxKjO0d0+9pQx7Lsl1bk+6 BQo204FtZBsTswXXiTGDwxplYGjc1aVc+b8pVElQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727541AbfCLIHu (ORCPT ); Tue, 12 Mar 2019 04:07:50 -0400 Received: from mx2.suse.de ([195.135.220.15]:42968 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725906AbfCLIHt (ORCPT ); Tue, 12 Mar 2019 04:07:49 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id A97BEB609; Tue, 12 Mar 2019 08:07:47 +0000 (UTC) Date: Tue, 12 Mar 2019 09:07:46 +0100 From: Michal Hocko To: Pi-Hsun Shih Cc: Andrew Morton , Matthew Wilcox , Johannes Weiner , Vlastimil Babka , Minchan Kim , Omar Sandoval , Huang Ying , Tejun Heo , Wei Yang , open list Subject: Re: [PATCH] mm/swap: Avoid undefined behavior in __swapoffset Message-ID: <20190312080746.GF5721@dhcp22.suse.cz> References: <20190307094653.119425-1-pihsun@chromium.org> <20190307122313.GA30234@dhcp22.suse.cz> <20190307132353.GB30234@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 12-03-19 15:02:38, Pi-Hsun Shih wrote: > On Thu, Mar 7, 2019 at 9:23 PM Michal Hocko wrote: > > > > On Thu 07-03-19 20:47:52, Pi-Hsun Shih wrote: > > > On Thu, Mar 7, 2019 at 8:23 PM Michal Hocko wrote: > > > > > > > > On Thu 07-03-19 17:46:50, Pi-Hsun Shih wrote: > > > > > Use offsetof to calculate offset of a field to avoid UBSAN warning like: > > > > > > > > > > =================================================================== > > > > > UBSAN: Undefined behaviour in mm/swapfile.c:3010:38 > > > > > member access within null pointer of type 'union swap_header' > > > > > CPU: 6 PID: 1833 Comm: swapon Tainted: G S 4.19.23 #43 > > > > > Call trace: > > > > > dump_backtrace+0x0/0x194 > > > > > show_stack+0x20/0x2c > > > > > __dump_stack+0x20/0x28 > > > > > dump_stack+0x70/0x94 > > > > > ubsan_epilogue+0x14/0x44 > > > > > ubsan_type_mismatch_common+0xf4/0xfc > > > > > __ubsan_handle_type_mismatch_v1+0x34/0x54 > > > > > __se_sys_swapon+0x654/0x1084 > > > > > __arm64_sys_swapon+0x1c/0x24 > > > > > el0_svc_common+0xa8/0x150 > > > > > el0_svc_compat_handler+0x2c/0x38 > > > > > el0_svc_compat+0x8/0x18 > > > > > ================================================================== > > > > > > > > Could you be more specific about what exactly is undefined here and > > > > why offsetof is any better. AFAIR it uses the same construct unless a > > > > compiler defines a built in. > > > > > > > > I do not object the change itself because it is cleaner to use the > > > > existing helper but I am wondering why this is fixing ubsan. Is ubsan > > > > defining the compiler variant and consider it safe? > > > > > > > > > > The undefined behavior is from trying to accessing a member of NULL, > > > even not using it value but only use the address. > > > > Hmm, we've been using this trick for ages and I do not remember any > > compiler to complain as there is no real access. I am not sure what the > > C standard has to tell about that but I presume reasonable compilers > > will not abuse the UB here. > > > > Some more testing shows that GCC optimize the > ((size_t)&((type*)0)->member) to a constant in the result binary, and > never emit any UBSAN checks on the statement. > Clang doesn't optimize it to a constant in -O0, optimize it to a > constant in -O1 or above, and always emit the > __ubsan_handle_type_mismatch check when "-fsanitize=undefined" is > given. > So this UBSAN warning only happens when kernel is compiled by clang, not GCC. > > From what I've found, it's a UB from C standard view point > (https://software.intel.com/en-us/blogs/2015/04/20/null-pointer-dereferencing-causes-undefined-behavior), > but I agree that probably no reasonable compilers would abuse the UB > here. I really do not want to go and lawyering about the standard here but getting an address of an offset based on NULL ptr is not really dereferencing of a NULL ptr. At least this was not the case for ages and no compiler can afford to change it because there is quite a lot of userspace to rely on this construct. But as I've said using offseoff is nicer so I completely support a patch that get's read of a custom redefinition of it or open code directly. But calling it an UB is a bit of stretch. -- Michal Hocko SUSE Labs