btrfs: export: handle invalid inode or root reference in btrfs_get_parent()

BugLink: https://bugs.launchpad.net/bugs/2065899

[ Upstream commit 26b66d1d366a375745755ca7365f67110bbf6bd5 ]

The get_parent handler looks up a parent of a given dentry, this can be
either a subvolume or a directory. The search is set up with offset -1
but it's never expected to find such item, as it would break allowed
range of inode number or a root id. This means it's a corruption (ext4
also returns this error code).

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Manuel Diewald <manuel.diewald@canonical.com>
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>

Author: kdave

fs/btrfs/export.c

@@ -174,8 +174,15 @@ struct dentry *btrfs_get_parent(struct dentry *child)
 	ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
 	if (ret < 0)
 		goto fail;
+	if (ret == 0) {
+		/*
+		 * Key with offset of -1 found, there would have to exist an
+		 * inode with such number or a root with such id.
+		 */
+		ret = -EUCLEAN;
+		goto fail;
+	}
 
-	BUG_ON(ret == 0); /* Key with offset of -1 found */
 	if (path->slots[0] == 0) {
 		ret = -ENOENT;
 		goto fail;