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

["Darrick J. Wong" <djwong@kernel.org>]

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
> 
>