Re: [PATCH 1/2] ext4: fix warning about stack corruption

[Anton Blanchard <anton@ozlabs.org>]

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