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

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

When searching the extent tree to gather the needed extent info,
btrfs_search_old_slot() 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_search_old_slot().

Found by code review.

Cc: stable@vger.kernel.org
Fixes: 0b246afa62b0 ("btrfs: root->fs_info cleanup, add fs_info convenience variables")
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 3dc5a35dd19b..4e2e1c38d33a 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2232,7 +2232,7 @@ ALLOW_ERROR_INJECTION(btrfs_search_slot, ERRNO);
 int btrfs_search_old_slot(struct btrfs_root *root, const struct btrfs_key *key,
 			  struct btrfs_path *p, u64 time_seq)
 {
-	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_fs_info *fs_info;
 	struct extent_buffer *b;
 	int slot;
 	int ret;
@@ -2241,6 +2241,10 @@ int btrfs_search_old_slot(struct btrfs_root *root, const struct btrfs_key *key,
 	int lowest_unlock = 1;
 	u8 lowest_level = 0;
 
+	if (!root)
+		return -EINVAL;
+
+	fs_info = root->fs_info;
 	lowest_level = p->lowest_level;
 	WARN_ON(p->nodes[0] != NULL);
 	ASSERT(!p->nowait);
-- 
2.25.1