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Date: Tue, 22 Aug 2017 21:08:49 +1000 From: Anton Blanchard <anton@...abs.org> To: Arnd Bergmann <arnd@...db.de> Cc: "Theodore Ts'o" <tytso@....edu>, Andreas Dilger <adilger.kernel@...ger.ca>, Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>, Chandan Rajendra <chandan@...ux.vnet.ibm.com>, linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH 1/2] ext4: fix warning about stack corruption 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
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