[PATCH] btrfs: add a sanity check for btrfs root in btrfs_next_old_leaf()

[Ma Ke <make24@iscas.ac.cn>]

btrfs_next_old_leaf() doesn't check if the target root is NULL or not,
resulting the null-ptr-deref. Add sanity check for btrfs root before
using it in btrfs_next_old_leaf().

Found by code review.

Cc: stable@vger.kernel.org
Fixes: d96b34248c2f ("btrfs: make send work with concurrent block group relocation")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
---
 fs/btrfs/ctree.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 4e2e1c38d33a..1a3fc3863860 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -4794,13 +4794,17 @@ int btrfs_next_old_leaf(struct btrfs_root *root, struct btrfs_path *path,
 	int level;
 	struct extent_buffer *c;
 	struct extent_buffer *next;
-	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_fs_info *fs_info;
 	struct btrfs_key key;
 	bool need_commit_sem = false;
 	u32 nritems;
 	int ret;
 	int i;
 
+	if (!root)
+		return -EINVAL;
+
+	fs_info = root->fs_info;
 	/*
 	 * The nowait semantics are used only for write paths, where we don't
 	 * use the tree mod log and sequence numbers.
-- 
2.25.1