[PATCH V2] xfs: be more forgiving of a v4 secondary sb w/ junk in v5 fields

[Mark Tinguely]

On 09/09/13 15:33, Eric Sandeen wrote:
> Today, if xfs_sb_read_verify encounters a v4 superblock
> with junk past v4 fields which includes data in sb_crc,
> it will be treated as a failing checksum and a significant
> corruption.
>
> There are known prior bugs which leave junk at the end
> of the V4 superblock; we don't need to actually fail the
> verification in this case if other checks pan out ok.
>
> So if this is a secondary superblock, and the primary
> superblock doesn't indicate that this is a V5 filesystem,
> don't treat this as an actual checksum failure.
>
> We should probably check the garbage condition as
> we do in xfs_repair, and possibly warn about it
> or self-heal, but that's a different scope of work.
>
> Stable folks: This can go back to v3.10, which is what
> introduced the sb CRC checking that is tripped up by old,
> stale, incorrect V4 superblocks w/ unzeroed bits.
>
> Cc: stable at vger.kernel.org
> Signed-off-by: Eric Sandeen<sandeen at redhat.com>
> ---
>
> V2: Comment changes: More!  (No code changes)
>
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index 2b0ba35..b2deab1 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -749,6 +749,11 @@ xfs_sb_verify(
>    * single bit error could clear the feature bit and unused parts of the
>    * superblock are supposed to be zero. Hence a non-null crc field indicates that
>    * we've potentially lost a feature bit and we should check it anyway.
> + *
> + * However, past bugs (i.e. in growfs) left non-zeroed regions beyond the
> + * last field in V4 secondary superblocks.  So for secondary superblocks,
> + * we are more forgiving, and ignore CRC failures if the primary doesn't
> + * indicate that the fs version is V5.
>    */
>   static void
>   xfs_sb_read_verify(
> @@ -769,8 +774,12 @@ xfs_sb_read_verify(
>
>   		if (!xfs_verify_cksum(bp->b_addr, be16_to_cpu(dsb->sb_sectsize),
>   				      offsetof(struct xfs_sb, sb_crc))) {
> -			error = EFSCORRUPTED;
> -			goto out_error;
> +			/* Only fail bad secondaries on a known V5 filesystem */
> +			if (bp->b_bn != XFS_SB_DADDR&&
> +			    xfs_sb_version_hascrc(&mp->m_sb)) {
> +				error = EFSCORRUPTED;
> +				goto out_error;
> +			}
>   		}
>   	}
>   	error = xfs_sb_verify(bp, true);

This moved to fs/xfs/xfs_sb.c in TOT, but the patch looks good to me.

Reviewed-by: Mark Tinguely <tinguely at sgi.com>