Re: [PATCH 3/5] xfs: silence sparse warning when checking version number

Darrick J. Wong
2024-04-03T03:57:28Z

On Wed, Apr 03, 2024 at 08:28:30AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Scrub checks the superblock version number against the known good
> feature bits that can be set in the version mask. It calculates
> the version mask to compare like so:
>
> vernum_mask = cpu_to_be16(~XFS_SB_VERSION_OKBITS |
> XFS_SB_VERSION_NUMBITS |
> XFS_SB_VERSION_ALIGNBIT |
> XFS_SB_VERSION_DALIGNBIT |
> XFS_SB_VERSION_SHAREDBIT |
> XFS_SB_VERSION_LOGV2BIT |
> XFS_SB_VERSION_SECTORBIT |
> XFS_SB_VERSION_EXTFLGBIT |
> XFS_SB_VERSION_DIRV2BIT);
>
> This generates a sparse warning:
>
> fs/xfs/scrub/agheader.c:168:23: warning: cast truncates bits from constant value (ffff3f8f becomes 3f8f)
>
> This is because '~XFS_SB_VERSION_OKBITS' is considered a 32 bit
> constant, even though it's value is always under 16 bits.
>
> This is a kinda silly thing to do, because:
>
> /*
> * Supported feature bit list is just all bits in the versionnum field because
> * we've used them all up and understand them all. Except, of course, for the
> * shared superblock bit, which nobody knows what it does and so is unsupported.
> */
> #define XFS_SB_VERSION_OKBITS \
> ((XFS_SB_VERSION_NUMBITS | XFS_SB_VERSION_ALLFBITS) & \
> ~XFS_SB_VERSION_SHAREDBIT)
>
> #define XFS_SB_VERSION_NUMBITS 0x000f
> #define XFS_SB_VERSION_ALLFBITS 0xfff0
> #define XFS_SB_VERSION_SHAREDBIT 0x0200
>
>
> XFS_SB_VERSION_OKBITS has a value of 0xfdff, and so
> ~XFS_SB_VERSION_OKBITS == XFS_SB_VERSION_SHAREDBIT. The calculated
> mask already sets XFS_SB_VERSION_SHAREDBIT, so starting with
> ~XFS_SB_VERSION_OKBITS is completely redundant....
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

What a tongue twister!
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
> fs/xfs/scrub/agheader.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/fs/xfs/scrub/agheader.c b/fs/xfs/scrub/agheader.c
> index e954f07679dd..d6a1a9fc63c9 100644
> --- a/fs/xfs/scrub/agheader.c
> +++ b/fs/xfs/scrub/agheader.c
> @@ -165,8 +165,7 @@ xchk_superblock(
> xchk_block_set_corrupt(sc, bp);
>
> /* Check sb_versionnum bits that are set at mkfs time. */
> - vernum_mask = cpu_to_be16(~XFS_SB_VERSION_OKBITS |
> - XFS_SB_VERSION_NUMBITS |
> + vernum_mask = cpu_to_be16(XFS_SB_VERSION_NUMBITS |
> XFS_SB_VERSION_ALIGNBIT |
> XFS_SB_VERSION_DALIGNBIT |
> XFS_SB_VERSION_SHAREDBIT |
> --
> 2.43.0
>
>