Re: [RFC PATCH v3 13/13] btrfs: add sha256 as another checksum algorithm

[David Sterba <dsterba@suse.cz>]

On Wed, May 22, 2019 at 10:19:10AM +0200, Johannes Thumshirn wrote:
> Now that we everything in place, we can add SHA-256 as another checksum
> algorithm.
> 
> SHA-256 was taken as it was the cryptographically strongest algorithm that
> can fit into the 32 Bytes we have left.
> 
> Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
> 
> ---
> changes to v2:
> - Add pre dependency on sha256
> Changes to v1:
> - Select SHA-256 in KConfig
> - Minimalize switch() in btrfs_supported_super_csum() (Nikolay)
> - Use enum for new on-disk checksum type (Nikolay/David)
> - Format SHA256 using sprintf()'s hexdump mode
> ---
>  fs/btrfs/Kconfig                | 1 +
>  fs/btrfs/btrfs_inode.h          | 3 +++
>  fs/btrfs/ctree.h                | 4 ++--
>  fs/btrfs/disk-io.c              | 1 +
>  fs/btrfs/super.c                | 1 +
>  include/uapi/linux/btrfs_tree.h | 6 ++++--
>  6 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/btrfs/Kconfig b/fs/btrfs/Kconfig
> index 212b4a854f2c..2521a24f74be 100644
> --- a/fs/btrfs/Kconfig
> +++ b/fs/btrfs/Kconfig
> @@ -4,6 +4,7 @@ config BTRFS_FS
>  	tristate "Btrfs filesystem support"
>  	select CRYPTO
>  	select CRYPTO_CRC32C
> +	select CRYPTO_SHA256
>  	select ZLIB_INFLATE
>  	select ZLIB_DEFLATE
>  	select LZO_COMPRESS
> diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
> index e79fd9129075..125bc7f3b871 100644
> --- a/fs/btrfs/btrfs_inode.h
> +++ b/fs/btrfs/btrfs_inode.h
> @@ -346,6 +346,9 @@ static inline void btrfs_csum_format(struct btrfs_super_block *sb,
>  	case BTRFS_CSUM_TYPE_CRC32:
>  		snprintf(cbuf, size, "0x%08x", *(u32 *)csum);
>  		break;
> +	case BTRFS_CSUM_TYPE_SHA256:
> +		snprintf(cbuf, size, "%*phN", (int)size / 8, csum);

This seems to print the buffer in the host order ('h', other formats use
that). I wonder if there needs to be some caution about endianity here.
Ideally we won't need to care and just use the right format that will
printk handle transparently for us.