Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]

[Al Viro]

On Tue, Oct 23, 2018 at 12:19:35PM +0100, Alan Jenkins wrote:

> I think there's another small hole.  It is possible to move a sub-mount from
> a detached tree (instead of moving the root of the tree).  Then
> do_move_mount() calls attach_recursive_mnt() with a non-NULL parent_path.
> 
> This causes it to skip count_mounts().  So, it doesn't count the number of
> attached mounts, and it allows you to exceed sysctl_mount_max.

That's trivial to repair, fortunately - we just need to check source_mnt->mnt_ns
instead of parent_path.