[XFS] Prevent xfs_bmap_check_leaf_extents() referencing unmapped memory.

While investigating the extent corruption bug I ran into this bug in debug
only code. xfs_bmap_check_leaf_extents() loops through the leaf blocks of
the extent btree checking that every extent is entirely before the next
extent. It also compares the last extent in the previous block to the
first extent in the current block when the previous block has been
released and potentially unmapped. So take a copy of the last extent
instead of a pointer. Also move the last extent check out of the loop
because we only need to do it once.

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30718a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>

1 files changed, 6 insertions, 9 deletions

diff --git a/fs/xfs/xfs_bmap.c b/fs/xfs/xfs_bmap.c
index bce8e3bd8ad1..7d683e0b8ef7 100644
--- a/fs/xfs/xfs_bmap.c
+++ b/fs/xfs/xfs_bmap.c
@@ -6194,7 +6194,7 @@ xfs_bmap_check_leaf_extents(
         xfs_mount_t             *mp;    /* file system mount structure */
         __be64                  *pp;    /* pointer to block address */
         xfs_bmbt_rec_t          *ep;    /* pointer to current extent */
-        xfs_bmbt_rec_t          *lastp; /* pointer to previous extent */
+        xfs_bmbt_rec_t          last = {0, 0}; /* last extent in prev block */
         xfs_bmbt_rec_t          *nextp; /* pointer to next extent */
         int                     bp_release = 0;
 
@@ -6264,7 +6264,6 @@ xfs_bmap_check_leaf_extents(
         /*
          * Loop over all leaf nodes checking that all extents are in the right order.
          */
-        lastp = NULL;
         for (;;) {
                 xfs_fsblock_t   nextbno;
                 xfs_extnum_t    num_recs;
@@ -6285,18 +6284,16 @@ xfs_bmap_check_leaf_extents(
                  */
 
                 ep = XFS_BTREE_REC_ADDR(xfs_bmbt, block, 1);
+                if (i) {
+                        xfs_btree_check_rec(XFS_BTNUM_BMAP, &last, ep);
+                }
                 for (j = 1; j < num_recs; j++) {
                         nextp = XFS_BTREE_REC_ADDR(xfs_bmbt, block, j + 1);
-                        if (lastp) {
-                                xfs_btree_check_rec(XFS_BTNUM_BMAP,
-                                        (void *)lastp, (void *)ep);
-                        }
-                        xfs_btree_check_rec(XFS_BTNUM_BMAP, (void *)ep,
-                                (void *)(nextp));
-                        lastp = ep;
+                        xfs_btree_check_rec(XFS_BTNUM_BMAP, ep, nextp);
                         ep = nextp;
                 }
 
+                last = *ep;
                 i += num_recs;
                 if (bp_release) {
                         bp_release = 0;