[PATCH] btrfs: check file extent backref offset underflow

["Yan, Zheng" <zheng.z.yan@intel.com>]

Offset field in data extent backref can underflow if clone range ioctl
is used. We can reliably detect the underflow because max file size is
limited to 2^63 and max data extent size is limited by block group size.

Signed-off-by: Zheng Yan  <zheng.z.yan@intel.com>
---
diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 59bb176..107c9cf 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -3323,8 +3323,11 @@ static int find_data_references(struct reloc_control *rc,
 	}
 
 	key.objectid = ref_objectid;
-	key.offset = ref_offset;
 	key.type = BTRFS_EXTENT_DATA_KEY;
+	if (ref_offset > ((u64)-1 << 32))
+		key.offset = 0;
+	else
+		key.offset = ref_offset;
 
 	path->search_commit_root = 1;
 	path->skip_locking = 1;