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From:   David Howells <dhowells@redhat.com>
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Cc:     dhowells@redhat.com,
        Alan Jenkins <alan.christopher.jenkins@gmail.com>,
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Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]
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Date:   Thu, 11 Oct 2018 13:14:56 +0100
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David Howells <dhowells@redhat.com> wrote:

> The reason that you can do this with open_tree()/move_mount() is that it
> allows you to create a mount tree (OPEN_TREE_CLONE) that has no namespace
> assignment, pass it through the namespace switch and then attach it inside the
> child namespace.  The cross-namespace checks in do_move_mount() are bypassed
> because the root of the newly-cloned mount tree doesn't have one.

It's worse than that.  The apparently disconnected tree given you by
open_tree(OPEN_TREE_CLONE) is still subject to modification by outside
forces.  All it takes is one shared object within that tree.

So I do wonder if it's possible to form a ring, even in an upstream kernel, by
using the propagation mechanism to push through an nsfs mount into itself,
possibly with a layer of indirection (ie. having two mutually-referential
namespaces).

David