Re: [PATCH] xfs: don't fail verifier on empty attr3 leaf block

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, May 13, 2020 at 10:53:43AM -0400, Brian Foster wrote:
> The attr fork can transition from shortform to leaf format while
> empty if the first xattr doesn't fit in shortform. While this empty
> leaf block state is intended to be transient, it is technically not
> due to the transactional implementation of the xattr set operation.
> 
> We historically have a couple of bandaids to work around this
> problem. The first is to hold the buffer after the format conversion
> to prevent premature writeback of the empty leaf buffer and the
> second is to bypass the xattr count check in the verifier during
> recovery. The latter assumes that the xattr set is also in the log
> and will be recovered into the buffer soon after the empty leaf
> buffer is reconstructed. This is not guaranteed, however.
> 
> If the filesystem crashes after the format conversion but before the
> xattr set that induced it, only the format conversion may exist in
> the log. When recovered, this creates a latent corrupted state on
> the inode as any subsequent attempts to read the buffer fail due to
> verifier failure. This includes further attempts to set xattrs on
> the inode or attempts to destroy the attr fork, which prevents the
> inode from ever being removed from the unlinked list.
> 
> To avoid this condition, accept that an empty attr leaf block is a
> valid state and remove the count check from the verifier. This means
> that on rare occasions an attr fork might exist in an unexpected
> state, but is otherwise consistent and functional. Note that we
> retain the logic to avoid racing with metadata writeback to reduce
> the window where this can occur.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
> 
> v1:
> - Remove the verifier check instead of warn.
> rfc: https://lore.kernel.org/linux-xfs/20200511185016.33684-1-bfoster@redhat.com/
> 
>  fs/xfs/libxfs/xfs_attr_leaf.c | 8 --------
>  1 file changed, 8 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_attr_leaf.c b/fs/xfs/libxfs/xfs_attr_leaf.c
> index 863444e2dda7..6b94bb9de378 100644
> --- a/fs/xfs/libxfs/xfs_attr_leaf.c
> +++ b/fs/xfs/libxfs/xfs_attr_leaf.c
> @@ -308,14 +308,6 @@ xfs_attr3_leaf_verify(
>  	if (fa)
>  		return fa;
>  
> -	/*
> -	 * In recovery there is a transient state where count == 0 is valid
> -	 * because we may have transitioned an empty shortform attr to a leaf
> -	 * if the attr didn't fit in shortform.

/me wonders if it would be useful for future spelunkers to retain some
sort of comment here that we once thought count==0 was bad but screwed
it up enough that we now allow it?

Moreso that future me/fuzzrobot won't come along having forgotten
everything and think "Oh, we need to validate hdr.count!" :P

--D

> -	 */
> -	if (!xfs_log_in_recovery(mp) && ichdr.count == 0)
> -		return __this_address;
> -
>  	/*
>  	 * firstused is the block offset of the first name info structure.
>  	 * Make sure it doesn't go off the block or crash into the header.
> -- 
> 2.21.1
>