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=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 C9C9FC43381 for ; Tue, 12 Mar 2019 07:03:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 965CB214AF for ; Tue, 12 Mar 2019 07:03:19 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="bKAe8+DL" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727384AbfCLHDR (ORCPT ); Tue, 12 Mar 2019 03:03:17 -0400 Received: from mail-ed1-f67.google.com ([209.85.208.67]:46294 "EHLO mail-ed1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725832AbfCLHDR (ORCPT ); Tue, 12 Mar 2019 03:03:17 -0400 Received: by mail-ed1-f67.google.com with SMTP id n17so1291836edt.13 for ; Tue, 12 Mar 2019 00:03:15 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=VtNK87TX7bSkrrAwYjvU3nK8TW2M2MqixTSikcn74+E=; b=bKAe8+DLs7u151ppsAAmzRyy+tSlM1yZBfV1z0/4SPBllZkO0lcXKE80nUEqzhD6XP cU1efPVKrXUyAcO52wOmUkaT7xQN/dw/Ua7Vq69nse4CRHjWenL5yGXH5vvv9uqjzFUK 1IBaN944XiI2OVLh1d8iFekLY7t4dkdjZAwuY= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=VtNK87TX7bSkrrAwYjvU3nK8TW2M2MqixTSikcn74+E=; b=UyJUD9ge6+BI18V8o17atz8foTpFjQeACfqsNE2RY6ZekKo8YRHBSihVn8/h+bsgWd S22BiXYOnAjupcjbbLxcxXxD61eUOu/iy8onXgqvoy75qHOppQRp3ERdeox73HNwxClo GST+e2LI9wgvpztI91RSR/S9iPCLU3VBYRdV+z4KbD5dCdiThUJp/auWfIA0BfP8gBH/ a/bU7a39f8E9DOOFX1s/HgpRuUnSA9d0HVrzBemoDhIweAIxOXLzfPY08FZFThX+zw3F qbpml5THi/PfqOIxS70dPTOiraXTZ5UVef5vvpHat5v7W6lIi6VUGg4F/Khr//o4RWu6 A79Q== X-Gm-Message-State: APjAAAUWzP37iANt4JOzp2Hg0CXcUrW3CMmuIzfb6DtlTHYs2MH4/vFO DF7H5azncGqeK2yLR/ib15T+etX//OJOov7TyTYTrg== X-Google-Smtp-Source: APXvYqxA/3/WNxFfOyKUuAK0dGNx0JQqL9fcpfLveVq/sS2P/u9AEp7HCaaK65V9IRzr+l+w/bPceRCHRrw/PitKUIE= X-Received: by 2002:a50:bb21:: with SMTP id y30mr2102599ede.156.1552374194791; Tue, 12 Mar 2019 00:03:14 -0700 (PDT) MIME-Version: 1.0 References: <20190307094653.119425-1-pihsun@chromium.org> <20190307122313.GA30234@dhcp22.suse.cz> <20190307132353.GB30234@dhcp22.suse.cz> In-Reply-To: <20190307132353.GB30234@dhcp22.suse.cz> From: Pi-Hsun Shih Date: Tue, 12 Mar 2019 15:02:38 +0800 Message-ID: Subject: Re: [PATCH] mm/swap: Avoid undefined behavior in __swapoffset To: Michal Hocko Cc: Andrew Morton , Matthew Wilcox , Johannes Weiner , Vlastimil Babka , Minchan Kim , Omar Sandoval , Huang Ying , Tejun Heo , Wei Yang , open list Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > 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. Did a "git grep '(0|NULL)\)->'" and there are about 25 of them that can be changed to use offsetof. > -- > Michal Hocko > SUSE Labs