[RFC PATCH 08/27] containers, vfs: Honour CONTAINER_NEW_EMPTY_FS_NS

[Al Viro]

On Fri, Feb 15, 2019 at 04:08:29PM +0000, David Howells wrote:

> +	mnt_ns = alloc_mnt_ns(container->cred->user_ns, false);
> +	if (IS_ERR(mnt_ns)) {
> +		ret = PTR_ERR(mnt_ns);
> +		goto out_fd;
> +	}
> +
> +	mnt = real_mount(path->mnt);
> +	mnt_add_count(mnt, 1);
> +	mnt->mnt_ns = mnt_ns;
> +	mnt_ns->root = mnt;
> +	mnt_ns->mounts++;
> +	list_add(&mnt->mnt_list, &mnt_ns->list);
> +
> +	ret = -EBUSY;
> +	spin_lock(&container->lock);
> +	if (!container->ns->mnt_ns) {
> +		container->ns->mnt_ns = mnt_ns;
> +		write_seqcount_begin(&container->seq);
> +		container->root.mnt = path->mnt;
> +		container->root.dentry = path->dentry;
> +		write_seqcount_end(&container->seq);
> +		path_get(&container->root);
> +		mnt_ns = NULL;
> +		ret = 0;
> +	}

Almost certainly buggered.  Assumptions that we _won't_ get
to absolute root of namespace (it's overmounted and we are
chrooted into it, basically) had been made in quite a few
places.  The thing you are creating is *not* like normal
namespaces in that respect.