xfs: fix dir3 freespace block corruption

When the directory freespace index grows to a second block (2017
4k data blocks in the directory), the initialisation of the second
new block header goes wrong. The write verifier fires a corruption
error indicating that the block number in the header is zero. This
was being tripped by xfs/110.

The problem is that the initialisation of the new block is done just
fine in xfs_dir3_free_get_buf(), but the caller then users a dirv2
structure to zero on-disk header fields that xfs_dir3_free_get_buf()
has already zeroed. These lined up with the block number in the dir
v3 header format.

While looking at this, I noticed that the struct xfs_dir3_free_hdr()
had 4 bytes of padding in it that wasn't defined as padding or being
zeroed by the initialisation. Add a pad field declaration and fully
zero the on disk and in-core headers in xfs_dir3_free_get_buf() so
that this is never an issue in the future. Note that this doesn't
change the on-disk layout, just makes the 32 bits of padding in the
layout explicit.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

(cherry picked from commit 5ae6e6a401957698f2bd8c9f4a86d86d02199fea)

fs/xfs/xfs_dir2_format.h

@@ -715,6 +715,7 @@ struct xfs_dir3_free_hdr {
 	__be32			firstdb;	/* db of first entry */
 	__be32			nvalid;		/* count of valid entries */
 	__be32			nused;		/* count of used entries */
+	__be32			pad;		/* 64 bit alignment. */
 };
 
 struct xfs_dir3_free {

fs/xfs/xfs_dir2_node.c

@@ -263,18 +263,19 @@ xfs_dir3_free_get_buf(
 	 * Initialize the new block to be empty, and remember
 	 * its first slot as our empty slot.
 	 */
-	hdr.magic = XFS_DIR2_FREE_MAGIC;
-	hdr.firstdb = 0;
-	hdr.nused = 0;
-	hdr.nvalid = 0;
+	memset(bp->b_addr, 0, sizeof(struct xfs_dir3_free_hdr));
+	memset(&hdr, 0, sizeof(hdr));
+
 	if (xfs_sb_version_hascrc(&mp->m_sb)) {
 		struct xfs_dir3_free_hdr *hdr3 = bp->b_addr;
 
 		hdr.magic = XFS_DIR3_FREE_MAGIC;
+
 		hdr3->hdr.blkno = cpu_to_be64(bp->b_bn);
 		hdr3->hdr.owner = cpu_to_be64(dp->i_ino);
 		uuid_copy(&hdr3->hdr.uuid, &mp->m_sb.sb_uuid);
-	}
+	} else
+		hdr.magic = XFS_DIR2_FREE_MAGIC;
 	xfs_dir3_free_hdr_to_disk(bp->b_addr, &hdr);
 	*bpp = bp;
 	return 0;
@@ -1921,8 +1922,6 @@ xfs_dir2_node_addname_int(
 			 */
 			freehdr.firstdb = (fbno - XFS_DIR2_FREE_FIRSTDB(mp)) *
 					xfs_dir3_free_max_bests(mp);
-			free->hdr.nvalid = 0;
-			free->hdr.nused = 0;
 		} else {
 			free = fbp->b_addr;
 			bests = xfs_dir3_free_bests_p(mp, free);