btrfs: fix dereference before check

The superblock's ->s_fs_info is properly set in btrfs_fill_super, after
a call to open_ctree, which derefereces it before check. Although
tree_root is set via btrfs_set_super, let's be defensive and leave the
check in place.

Signed-off-by: David Sterba <dsterba@suse.cz>

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 990afa8656a27efc3bbe378f82de7dbb12fc16ec..25e4b8f1d0ef7767dc5ba04f83b8b6ef839c0dec 100644 (file)
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1611,7 +1611,7 @@ struct btrfs_root *open_ctree(struct super_block *sb,
        struct btrfs_root *csum_root = kzalloc(sizeof(struct btrfs_root),
                                                 GFP_NOFS);
        struct btrfs_root *tree_root = btrfs_sb(sb);
-       struct btrfs_fs_info *fs_info = tree_root->fs_info;
+       struct btrfs_fs_info *fs_info = NULL;
        struct btrfs_root *chunk_root = kzalloc(sizeof(struct btrfs_root),
                                                GFP_NOFS);
        struct btrfs_root *dev_root = kzalloc(sizeof(struct btrfs_root),
@@ -1623,11 +1623,12 @@ struct btrfs_root *open_ctree(struct super_block *sb,
 
        struct btrfs_super_block *disk_super;
 
-       if (!extent_root || !tree_root || !fs_info ||
+       if (!extent_root || !tree_root || !tree_root->fs_info ||
            !chunk_root || !dev_root || !csum_root) {
                err = -ENOMEM;
                goto fail;
        }
+       fs_info = tree_root->fs_info;
 
        ret = init_srcu_struct(&fs_info->subvol_srcu);
        if (ret) {