[PATCH 01/13] libxfs: be more forgiving of a v4 secondary sb w/ junk in v5 fields

[Dave Chinner <david@fromorbit.com>]

From: Dave Chinner <dchinner@redhat.com>

[userspace port]

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.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 libxfs/xfs_sb.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/libxfs/xfs_sb.c b/libxfs/xfs_sb.c
index 48b1a97..16088a6 100644
--- a/libxfs/xfs_sb.c
+++ b/libxfs/xfs_sb.c
@@ -542,6 +542,11 @@ 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(
@@ -562,8 +567,12 @@ 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);
-- 
1.9.0

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