Re: [PATCH] btrfs-progs: Fix xxhash on big endian machines

[David Sterba <dsterba@suse.cz>]

On Mon, Mar 16, 2020 at 11:05:12AM +0200, Nikolay Borisov wrote:
> xxhash's state and results are always in little, but in progs after the
> hash was calculated it was copied to the final buffer via memcpy,
> meaning it'd be parsed as a big endian number on big endian machines.
> This is incompatible with the kernel implementation of xxhash which
> results in erroneous "checksum didn't match" errors on mount.
> 
> Fix it by using put_unaligned_le64 which always ensures the resulting
> checksum will be copied in little endian format as the kernel expects
> it.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206835
> Fixes: f070ece2e98f ("btrfs-progs: add xxhash64 to mkfs")
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  crypto/hash.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/crypto/hash.c b/crypto/hash.c
> index 48623c798739..4009e84e8b2c 100644
> --- a/crypto/hash.c
> +++ b/crypto/hash.c
> @@ -19,12 +19,7 @@ int hash_xxhash(const u8 *buf, size_t length, u8 *out)
>  	XXH64_hash_t hash;
> 
>  	hash = XXH64(buf, length, 0);
> -	/*
> -	 * NOTE: we're not taking the canonical form here but the plain hash to
> -	 * be compatible with the kernel implementation!
> -	 */
> -	memcpy(out, &hash, 8);
> -
> +	put_unaligned_le64(&hash, out);

This does not work, the test mkfs/019 fails.