On Tue, Oct 30, 2012 at 02:40:04PM -0400, Behan Webster wrote:
> From: Mark Charlebois <charlebm@xxxxxxxxx>
>
> The use of variable length arrays in structs (VLAIS) in the Linux Kernel code
> precludes the use of compilers which don't implement VLAIS (for instance the
> Clang compiler). Since ctx is always a 32-bit CRC, hard coding a size of 4
> bytes accomplishes the same thing without the use of VLAIS. This is the same
> technique already employed in fs/ext4/ext4.h
>
> Signed-off-by: Mark Charlebois <charlebm@xxxxxxxxx>
> Signed-off-by: Behan Webster <behanw@xxxxxxxxxxxxxxxxxx>
> ---
> include/linux/jbd2.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> index 3efc43f..efcbdfc 100644
> --- a/include/linux/jbd2.h
> +++ b/include/linux/jbd2.h
> @@ -1308,7 +1308,7 @@ static inline u32 jbd2_chksum(journal_t *journal, u32 crc,
> {
> struct {
> struct shash_desc shash;
> - char ctx[crypto_shash_descsize(journal->j_chksum_driver)];
> + char ctx[4];
I wonder if this code ought to have a defensive programming check such as
BUG_ON(crypto_shash_descsize(journal->j_chksum_driver) > 4) here just in case
we ever decide to support a checksum function that is bigger than 32 bits?
At this stage of the game I doubt we'll be adding larger checksums to
ext4/jbd2, but I wouldn't want to rely on remembering this detail if we ever
do want to change it. I guess we could hide the check behind CONFIG_JBD_DEBUG
if we're concerned about slowing down the hot path.
The same comment applies to Ted's patch ("ext4: remove dynamic array size in
ext4_chksum()") back in July.
<shrug> Otherwise,
Acked-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
--D
> } desc;
> int err;
>
> --
> 1.7.9.5
>
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