linux-kernel - Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE

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Message-ID: <10422.1539263439@warthog.procyon.org.uk>
Date:   Thu, 11 Oct 2018 14:10:39 +0100
From:   David Howells <dhowells@...hat.com>
To:     Alan Jenkins <alan.christopher.jenkins@...il.com>
Cc:     dhowells@...hat.com, viro@...iv.linux.org.uk,
        torvalds@...ux-foundation.org, ebiederm@...ssion.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        mszeredi@...hat.com
Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]

Alan Jenkins <alan.christopher.jenkins@...il.com> wrote:

> It sounds like you're under-estimating how we can use mnt_ns->seq (as is
> currently used in mnt_ns_loop()).  Or maybe I am over-estimating it :).

I don't see how it helps.  The duplication and attachment of the nsfs object
is already done by open_tree(), but as it's a detached tree, there are no
namespace assignments on the objects therein.  move_mount() is attaching the
subtree as a whole.

I modified my example to put everything under /a, setting up initially on /a/x
and then moving to /a/y within the namespace.  Then I made it print the mount
tree in more places.  So after setup, I see:

	[root@...romeda x]# findmnt -R /a
	TARGET                          SOURCE
	/a                              none
	\_/a/x                          none
	  \_/a/x/private_mnt            xxx
	    \_/a/x/private_mnt/child_ns nsfs[mnt:[4026532272]]

this looks fine.  Then I do:

	~/open_tree 3</a/x/private_mnt 3 \
	    nsenter --mount=/a/x/private_mnt/child_ns \
	    sh -c 'findmnt -R /a; ~/move_mount 4</a/y; findmnt -R /a'

and I see:

	TARGET               SOURCE
	/a                   none
	\_/a/x               none
	  \_/a/x/private_mnt xxx
	TARGET               SOURCE
	/a                   none
	|_/a/x               none
	| \_/a/x/private_mnt xxx
	\_/a/y               xxx
	  \_/a/y/child_ns    nsfs[mnt:[4026532272]]

in which /a/x/private_mnt got cloned and the clone mounted on "/a/y".

So, you're right, it's nothing to do with propagation.  But I'm not sure how I
check this.  Reject it in move_mount() if there's an nsfs?  I'm not sure if
the seq number is actually useful here.

David