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From:   Alan Jenkins <alan.christopher.jenkins@gmail.com>
Subject: Re: [PATCH 03/33] teach move_mount(2) to work with OPEN_TREE_CLONE
 [ver #11]
To:     David Howells <dhowells@redhat.com>, viro@zeniv.linux.org.uk
Cc:     torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <153313703562.13253.5766498657900728120.stgit@warthog.procyon.org.uk>
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Date:   Fri, 12 Oct 2018 15:25:42 +0100
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On 01/08/2018 16:24, David Howells wrote:
> From: Al Viro<viro@zeniv.linux.org.uk>
>
> Allow a detached tree created by open_tree(..., OPEN_TREE_CLONE) to be
> attached by move_mount(2).
>
> If by the time of final fput() of OPEN_TREE_CLONE-opened file its tree is
> not detached anymore, it won't be dissolved.  move_mount(2) is adjusted
> to handle detached source.
>
> That gives us equivalents of mount --bind and mount --rbind.
>
> Signed-off-by: Al Viro<viro@zeniv.linux.org.uk>
> Signed-off-by: David Howells<dhowells@redhat.com>
> ---
>
>   fs/namespace.c |   26 ++++++++++++++++++++------
>   1 file changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index e2934a4f342b..3981fd7b13f5 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c

> @@ -2464,10 +2467,19 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
>   	p = real_mount(new_path->mnt);
>   
>   	err = -EINVAL;
> -	if (!check_mnt(p) || !check_mnt(old))
> +	/* The mountpoint must be in our namespace. */
> +	if (!check_mnt(p))
> +		goto out1;
> +	/* The thing moved should be either ours or completely unattached. */
> +	if (old->mnt_ns && !check_mnt(old))
>   		goto out1;
>   
> -	if (!mnt_has_parent(old))
> +	attached = mnt_has_parent(old);
> +	/*
> +	 * We need to allow open_tree(OPEN_TREE_CLONE) followed by
> +	 * move_mount(), but mustn't allow "/" to be moved.
> +	 */
> +	if (old->mnt_ns && !attached)
>   		goto out1;


Technically, I think the comment should say "mustn't allow rootfs to be 
moved". "rootfs", as in 
Documentation/filesystems/ramfs-rootfs-initramfs.txt. Moving "/" is allowed.

# unshare -m
# cd /mnt
# mount -ttmpfs none /
# mount --move --no-canonicalize /.. /mnt
#

(Or if you want to quibble about "/.." v.s. "/" -

# unshare -m
# cd /mnt
# mount --rbind / /
# chroot --skip-chdir /..
# mount --move --no-canonicalize / .