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

[Eric Sandeen <sandeen@xxxxxxxxxx>]

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 significant
corruption.

There are known prior bugs which leave junk at the end
of the 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 is not V5, don't treat this as a serious
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.

Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
---

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 2b0ba35..5ce25ae 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -769,8 +769,12 @@ 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 here for a real 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);