Btrfs: catch invalid free space trees

There are two separate issues that can lead to corrupted free space
trees.

1. The free space tree bitmaps had an endianness issue on big-endian
   systems which is fixed by an earlier patch in this series.
2. btrfs-progs before v4.7.3 modified filesystems without updating the
   free space tree.

To catch both of these issues at once, we need to force the free space
tree to be rebuilt. To do so, add a FREE_SPACE_TREE_VALID compat_ro bit.
If the bit isn't set, we know that it was either produced by a broken
big-endian kernel or may have been corrupted by btrfs-progs.

This also provides us with a way to add rudimentary read-write support
for the free space tree to btrfs-progs: it can just clear this bit and
have the kernel rebuild the free space tree.

Cc: stable@vger.kernel.org # 4.5+
Tested-by: NHolger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: NChandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -251,7 +251,8 @@ struct btrfs_super_block {
 #define BTRFS_FEATURE_COMPAT_SAFE_CLEAR		0ULL
 
 #define BTRFS_FEATURE_COMPAT_RO_SUPP			\
-	(BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE)
+	(BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE |	\
+	 BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE_VALID)
 
 #define BTRFS_FEATURE_COMPAT_RO_SAFE_SET	0ULL
 #define BTRFS_FEATURE_COMPAT_RO_SAFE_CLEAR	0ULL

fs/btrfs/disk-io.c

@@ -2566,6 +2566,7 @@ int open_ctree(struct super_block *sb,
 	int num_backups_tried = 0;
 	int backup_index = 0;
 	int max_active;
+	int clear_free_space_tree = 0;
 
 	tree_root = fs_info->tree_root = btrfs_alloc_root(fs_info, GFP_KERNEL);
 	chunk_root = fs_info->chunk_root = btrfs_alloc_root(fs_info, GFP_KERNEL);
@@ -3131,6 +3132,14 @@ int open_ctree(struct super_block *sb,
 
 	if (btrfs_test_opt(fs_info, CLEAR_CACHE) &&
 	    btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE)) {
+		clear_free_space_tree = 1;
+	} else if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE) &&
+		   !btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID)) {
+		btrfs_warn(fs_info, "free space tree is invalid");
+		clear_free_space_tree = 1;
+	}
+
+	if (clear_free_space_tree) {
 		btrfs_info(fs_info, "clearing free space tree");
 		ret = btrfs_clear_free_space_tree(fs_info);
 		if (ret) {

fs/btrfs/free-space-tree.c

@@ -1182,6 +1182,7 @@ int btrfs_create_free_space_tree(struct btrfs_fs_info *fs_info)
 	}
 
 	btrfs_set_fs_compat_ro(fs_info, FREE_SPACE_TREE);
+	btrfs_set_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID);
 	fs_info->creating_free_space_tree = 0;
 
 	ret = btrfs_commit_transaction(trans, tree_root);
@@ -1250,6 +1251,7 @@ int btrfs_clear_free_space_tree(struct btrfs_fs_info *fs_info)
 		return PTR_ERR(trans);
 
 	btrfs_clear_fs_compat_ro(fs_info, FREE_SPACE_TREE);
+	btrfs_clear_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID);
 	fs_info->free_space_root = NULL;
 
 	ret = clear_free_space_tree(trans, free_space_root);

include/uapi/linux/btrfs.h

@@ -240,6 +240,16 @@ struct btrfs_ioctl_fs_info_args {
  * struct btrfs_ioctl_feature_flags
  */
 #define BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE		(1ULL << 0)
+/*
+ * Older kernels (< 4.9) on big-endian systems produced broken free space tree
+ * bitmaps, and btrfs-progs also used to corrupt the free space tree (versions
+ * < 4.7.3).  If this bit is clear, then the free space tree cannot be trusted.
+ * btrfs-progs can also intentionally clear this bit to ask the kernel to
+ * rebuild the free space tree, however this might not work on older kernels
+ * that do not know about this bit. If not sure, clear the cache manually on
+ * first mount when booting older kernel versions.
+ */
+#define BTRFS_FEATURE_COMPAT_RO_FREE_SPACE_TREE_VALID	(1ULL << 1)
 
 #define BTRFS_FEATURE_INCOMPAT_MIXED_BACKREF	(1ULL << 0)
 #define BTRFS_FEATURE_INCOMPAT_DEFAULT_SUBVOL	(1ULL << 1)