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Date:   Sat, 13 Oct 2018 07:06:19 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     David Howells <dhowells@redhat.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Alan Jenkins <alan.christopher.jenkins@gmail.com>,
        torvalds@linux-foundation.org, 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 Thu, Oct 11, 2018 at 09:17:54PM +0100, David Howells wrote:
> +/*
> + * Object if there are any nsfs mounts in the specified subtree.  These can act
> + * as pins for mount namespaces that aren't checked by the mount-cycle checking
> + * code, thereby allowing cycles to be made.
> + */
> +static bool check_for_nsfs_mounts(struct mount *subtree)
> +{
> +	struct mount *p;
> +	bool ret = false;
> +
> +	lock_mount_hash();
> +	for (p = subtree; p; p = next_mnt(p, subtree))
> +		if (mnt_ns_loop(p->mnt.mnt_root))
> +			goto out;
> +
> +	ret = true;
> +out:
> +	unlock_mount_hash();
> +	return ret;
> +}

Umm...  The comment doesn't match the behaviour - you are
accepting references to later namespaces.  Behaviour is
not a problem, the comment is.