From: Anton Blanchard Subject: Re: [PATCH 1/2] ext4: fix warning about stack corruption Date: Tue, 22 Aug 2017 21:08:49 +1000 Message-ID: <20170822210849.0edb91cc@kryten> References: <20170726185219.GA57833@beast> <20170801120438.1582336-1-arnd@arndb.de> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: "Theodore Ts'o" , Andreas Dilger , Kees Cook , Andrew Morton , Jan Kara , Chandan Rajendra , linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org To: Arnd Bergmann Return-path: Received: from ozlabs.org ([103.22.144.67]:36147 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932350AbdHVLIx (ORCPT ); Tue, 22 Aug 2017 07:08:53 -0400 In-Reply-To: <20170801120438.1582336-1-arnd@arndb.de> Sender: linux-ext4-owner@vger.kernel.org List-ID: Hi Arnd, > After commit 62d1034f53e3 ("fortify: use WARN instead of BUG for > now"), we get a warning about possible stack overflow from a memcpy > that was not strictly bounded to the size of the local variable: > > inlined from 'ext4_mb_seq_groups_show' at > fs/ext4/mballoc.c:2322:2: include/linux/string.h:309:9: error: > '__builtin_memcpy': writing between 161 and 1116 bytes into a region > of size 160 overflows the destination [-Werror=stringop-overflow=] > > We actually had a bug here that would have been found by the warning, > but it was already fixed last year in commit 30a9d7afe70e ("ext4: fix > stack memory corruption with 64k block size"). > > This replaces the fixed-length structure on the stack with a > variable-length structure, using the correct upper bound that tells > the compiler that everything is really fine here. I also change the > loop count to check for the same upper bound for consistency, but the > existing code is already correct here. > > Note that while clang won't allow certain kinds of variable-length > arrays in structures, this particular instance is fine, as the array > is at the end of the structure, and the size is strictly bounded. Unfortunately it doesn't appear to work, at least with ppc64le clang: fs/ext4/mballoc.c:2303:17: error: fields must have a constant size: 'variable length array in structure' extension will never be supported ext4_grpblk_t counters[blocksize_bits + 2]; Anton