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,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 B282AC4360F for ; Thu, 7 Mar 2019 13:24:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8664D20684 for ; Thu, 7 Mar 2019 13:24:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1551965040; bh=ilyNTjSi0mBYdi2D8tTI9r+Wj9kb9qxZfXcbzzMjqmE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=JoYLHhgRy14zREOM3MKCUoa6+yKlNWl6Hhvr+eSTNTWyIV/6hk0UIRQM5F/fAbNVi /VzhHH9bXVaoxdyE3+k30Z0rjlsPVHUOj/6jxzovCGbn21a1hPamF8hxv0NDCUm8Gi gccUj0ba23QuB9h8fhB+5gjw6Q9ddTdZOCjJi+Jk= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726404AbfCGNX5 (ORCPT ); Thu, 7 Mar 2019 08:23:57 -0500 Received: from mx2.suse.de ([195.135.220.15]:55864 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726269AbfCGNX4 (ORCPT ); Thu, 7 Mar 2019 08:23:56 -0500 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 0B34DAAAE; Thu, 7 Mar 2019 13:23:54 +0000 (UTC) Date: Thu, 7 Mar 2019 14:23:53 +0100 From: Michal Hocko To: Pi-Hsun Shih Cc: Andrew Morton , Matthew Wilcox , Johannes Weiner , Vlastimil Babka , Minchan Kim , Omar Sandoval , Huang Ying , Aaron Lu , Tejun Heo , Wei Yang , open list Subject: Re: [PATCH] mm/swap: Avoid undefined behavior in __swapoffset Message-ID: <20190307132353.GB30234@dhcp22.suse.cz> References: <20190307094653.119425-1-pihsun@chromium.org> <20190307122313.GA30234@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 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. > Since the compiler variant for offsetof is used for recent compiler > (GCC >= 4 has support for it), ubsan would not warn if the compiler > variant is used. For old compiler, I guess ubsan would complain on all > offsetof uses. Is this the case for all compilers? If yes then we might want to drop the non-compiler part. Btw. $ git grep "#define offsetof" drivers/gpu/drm/radeon/mkregtable.c:#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) drivers/infiniband/core/uverbs_cmd.c:#define offsetof_after(_struct, _member) \ include/linux/stddef.h:#define offsetof(TYPE, MEMBER) __compiler_offsetof(TYPE, MEMBER) include/linux/stddef.h:#define offsetof(TYPE, MEMBER) ((size_t)&((TYPE *)0)->MEMBER) include/linux/stddef.h:#define offsetofend(TYPE, MEMBER) \ samples/bpf/cookie_uid_helper_example.c:#define offsetof(type, member) __builtin_offsetof(type, member) scripts/kconfig/list.h:#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) tools/include/linux/kernel.h:#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) tools/testing/selftests/bpf/test_select_reuseport_kern.c:#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) tools/usb/usbip/libsrc/list.h:#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) so there is much more to take care of. Not to mention any open coded variants. -- Michal Hocko SUSE Labs