[PATCH] xfs: growfs: don't read garbage for new secondary superblocks

[Dave Chinner]

From: Dave Chinner <dchinner at redhat.com>

When updating new secondary superblocks in a growfs operation, the
sueprblock buffer is read from the newly grown region of the
underlying device. This is not guaranteed to be zero, so violates
the underlying assumption that the unused parts of superblocks are
zero filled. Get a new buffer for these secondary superblocks to
ensure that the unused regions are zero filled correctly.

Signed-off-by: Dave Chinner <dchinner at redhat.com>
---
 fs/xfs/xfs_fsops.c |   21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
index c25b094..4beaede 100644
--- a/fs/xfs/xfs_fsops.c
+++ b/fs/xfs/xfs_fsops.c
@@ -399,9 +399,26 @@ xfs_growfs_data_private(
 
 	/* update secondary superblocks. */
 	for (agno = 1; agno < nagcount; agno++) {
-		error = xfs_trans_read_buf(mp, NULL, mp->m_ddev_targp,
+		error = 0;
+		/*
+		 * new secondary superblocks need to be zeroed, not read from
+		 * disk as the contents of the new area we are growing into is
+		 * completely unknown.
+		 */
+		if (agno < oagcount) {
+			error = xfs_trans_read_buf(mp, NULL, mp->m_ddev_targp,
 				  XFS_AGB_TO_DADDR(mp, agno, XFS_SB_BLOCK(mp)),
 				  XFS_FSS_TO_BB(mp, 1), 0, &bp);
+		} else {
+			bp = xfs_trans_get_buf(NULL, mp->m_ddev_targp,
+				  XFS_AGB_TO_DADDR(mp, agno, XFS_SB_BLOCK(mp)),
+				  XFS_FSS_TO_BB(mp, 1), 0);
+			if (bp)
+				xfs_buf_zero(bp, 0, BBTOB(bp->b_length));
+			else
+				error = ENOMEM;
+		}
+
 		if (error) {
 			xfs_warn(mp,
 		"error %d reading secondary superblock for ag %d",
@@ -423,7 +440,7 @@ xfs_growfs_data_private(
 			break; /* no point in continuing */
 		}
 	}
-	return 0;
+	return error;
 
  error0:
 	xfs_trans_cancel(tp, XFS_TRANS_ABORT);
-- 
1.7.10