[PATCH v2] xfs: don't crash when read verifier reports errors with null b_ops

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

From: Darrick J. Wong <darrick.wong@oracle.com>

In xrep_findroot_block, we work out the btree type and correctness of a
given block by calling different btree verifiers on root block
candidates.  However, we leave the NULL b_ops while ->verify_read
validates the block, which means that if the verifier calls
xfs_buf_verifier_error it'll crash on the null b_ops.  Fix it to avoid
this crash.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
v2: leave a comment about what we're doing above the function
---
 fs/xfs/xfs_error.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_error.c b/fs/xfs/xfs_error.c
index 9866f542e77b..6e80e438b27a 100644
--- a/fs/xfs/xfs_error.c
+++ b/fs/xfs/xfs_error.c
@@ -340,6 +340,9 @@ xfs_corruption_error(
 /*
  * Warnings specifically for verifier errors.  Differentiate CRC vs. invalid
  * values, and omit the stack trace unless the error level is tuned high.
+ *
+ * NOTE: Some callers might be calling the verifiers directly (rather than
+ * through the b_ops structure) so bp->b_ops may be NULL.
  */
 void
 xfs_buf_verifier_error(
@@ -359,7 +362,7 @@ xfs_buf_verifier_error(
 
 	xfs_alert(mp, "Metadata %s detected at %pS, %s block 0x%llx %s",
 		  bp->b_error == -EFSBADCRC ? "CRC error" : "corruption",
-		  fa, bp->b_ops->name, bp->b_bn, name);
+		  fa, bp->b_ops ? bp->b_ops->name : "unknown", bp->b_bn, name);
 
 	xfs_alert(mp, "Unmount and run xfs_repair");