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Date:   Tue, 23 Oct 2018 17:22:03 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Alan Jenkins <alan.christopher.jenkins@gmail.com>
Cc:     David Howells <dhowells@redhat.com>, torvalds@linux-foundation.org,
        ebiederm@xmission.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, mszeredi@redhat.com
Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE
 [ver #12]
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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.