Btrfs: deny sys_link across subvolumes.

I rebased Christian Parpart's patch to deny hard link across
subvolumes. Original patch modifies also btrfs_rename, but
I excluded it because we can move across subvolumes now and
it make no problem.
-----------------

Hard link across subvolumes should not allowed in Btrfs.
btrfs_link checks root of 'to' directory is same as root
of 'from' file. If not same, btrfs_link returns -EPERM.

Signed-off-by: TARUISI Hiroaki <taruishi.hiroak@jp.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 168e8c040aab..da76cad92ecf 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4462,6 +4462,10 @@ static int btrfs_link(struct dentry *old_dentry, struct inode *dir,
         if (inode->i_nlink == 0)
                 return -ENOENT;
 
+        /* do not allow sys_link's with other subvols of the same device */
+        if (root->objectid != BTRFS_I(inode)->root->objectid)
+                return -EPERM;
+
         /*
          * 1 item for inode ref
          * 2 items for dir items