[PATCH] btrfs: fix null pointer dereference in write_pinned_extent_entries

[Anand Jain <anand.jain@oracle.com>]

btrfs/049 fails with null pointer dereference

 kernel: BUG: unable to handle kernel NULL pointer dereference at 0000000000000030
::
 kernel: RIP: 0010:write_pinned_extent_entries+0x27/0x100 [btrfs]
::
 kernel: Call Trace:
 kernel:  __btrfs_write_out_cache+0x273/0x450 [btrfs]
 kernel:  btrfs_write_out_ino_cache+0x84/0xd0 [btrfs]
 kernel:  btrfs_save_ino_cache+0x414/0x500 [btrfs]
 kernel:  commit_fs_roots+0xc6/0x190 [btrfs]
 kernel:  btrfs_commit_transaction+0x4de/0xac0 [btrfs]
 kernel:  btrfs_sync_file+0x35b/0x3d0 [btrfs]
 kernel:  do_fsync+0x38/0x60
 kernel:  __x64_sys_fsync+0x10/0x20

 %block_group::fs_info should be accessed only when block_group is not null.

(Fixes: de965ddc6f29 btrfs: get fs_info from block group in
 write_pinned_extent_entries)

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 fs/btrfs/free-space-cache.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index ea7fecf63fbb..a065041e7c80 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -1044,7 +1044,7 @@ static noinline_for_stack int write_pinned_extent_entries(
 			    struct btrfs_io_ctl *io_ctl,
 			    int *entries)
 {
-	struct btrfs_fs_info *fs_info = block_group->fs_info;
+	struct btrfs_fs_info *fs_info;
 	u64 start, extent_start, extent_end, len;
 	struct extent_io_tree *unpin = NULL;
 	int ret;
@@ -1052,6 +1052,7 @@ static noinline_for_stack int write_pinned_extent_entries(
 	if (!block_group)
 		return 0;
 
+	fs_info = block_group->fs_info;
 	/*
 	 * We want to add any pinned extents to our free space cache
 	 * so we don't leak the space
-- 
2.20.1 (Apple Git-117)