ext4: fix potential negative array index in do_split()

If for any reason a directory passed to do_split() does not have enough
active entries to exceed half the size of the block, we can end up
iterating over all "count" entries without finding a split point.

In this case, count == move, and split will be zero, and we will
attempt a negative index into map[].

Guard against this by detecting this case, and falling back to
split-to-half-of-count instead; in this case we will still have
plenty of space (> half blocksize) in each split block.

Fixes: ef2b02d3e617 ("ext34: ensure do_split leaves enough free space in both blocks")
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/f53e246b-647c-64bb-16ec-135383c70ad7@redhat.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 56738b538ddf4c066df33a59a06020352313c749..ef606301a1065d6a03ce14cde3e9002f1707609f 100644 (file)
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -1858,7 +1858,7 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
                             blocksize, hinfo, map);
        map -= count;
        dx_sort_map(map, count);
-       /* Split the existing block in the middle, size-wise */
+       /* Ensure that neither split block is over half full */
        size = 0;
        move = 0;
        for (i = count-1; i >= 0; i--) {
@@ -1868,8 +1868,18 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
                size += map[i].size;
                move++;
        }
-       /* map index at which we will split */
-       split = count - move;
+       /*
+        * map index at which we will split
+        *
+        * If the sum of active entries didn't exceed half the block size, just
+        * split it in half by count; each resulting block will have at least
+        * half the space free.
+        */
+       if (i > 0)
+               split = count - move;
+       else
+               split = count/2;
+
        hash2 = map[split].hash;
        continued = hash2 == map[split - 1].hash;
        dxtrace(printk(KERN_INFO "Split block %lu at %x, %i/%i\n",