[CRIU] [PATCH] mount: rework perparation for pivot_root

[Tycho Andersen]

Hi Andrey,

On Fri, Aug 08, 2014 at 04:18:10PM +0400, Andrey Vagin wrote:
> We can't bind-mount the required root into itself instead of
> resolving a parent mount.
> 
> This patch is required to support userns, because if we want to make
> pivot_root, the parent mount can't be locked. When we create userns
> and mntns, all inherited mounts are marked as locked.
> 
> Cc: Tycho Andersen <tycho.andersen at canonical.com>
> Signed-off-by: Andrey Vagin <avagin at openvz.org>
> ---
>  mount.c | 48 +++++++++++++++++++++---------------------------
>  1 file changed, 21 insertions(+), 27 deletions(-)
> 
> diff --git a/mount.c b/mount.c
> index 3287282..15902c7 100644
> --- a/mount.c
> +++ b/mount.c
> @@ -1910,20 +1910,13 @@ int fini_mnt_ns(void)
>  int prepare_mnt_ns(void)
>  {
>  	int ret = -1;
> -	struct mount_info *mis, *old;
> -	struct ns_id ns = { .pid = PROC_SELF, .nd = &mnt_ns_desc };
> +	struct mount_info *mis;
>  
>  	if (!(root_ns_mask & CLONE_NEWNS))
>  		return rst_collect_local_mntns();
>  
>  	pr_info("Restoring mount namespace\n");
>  
> -	old = collect_mntinfo(&ns);
> -	if (old == NULL)
> -		return -1;
> -
> -	close_proc();
> -
>  	mis = read_mnt_ns_img();
>  	if (!mis)
>  		goto out;
> @@ -1939,36 +1932,37 @@ int prepare_mnt_ns(void)
>  	 * prior to recreating new ones.
>  	 */
>  	if (!opts.root) {
> -		if (clean_mnt_ns(ns.mnt.mntinfo_tree))
> +		struct ns_id ns = { .pid = PROC_SELF, .nd = &mnt_ns_desc };
> +		struct mount_info *old;
> +		int err;
> +
> +		old = collect_mntinfo(&ns);
> +		if (old == NULL)
>  			return -1;
> -	} else {
> -		struct mount_info *mi;
>  
> -		/* moving a mount residing under a shared mount is invalid. */
> -		mi = mount_resolve_path(ns.mnt.mntinfo_tree, opts.root);
> -		if (mi == NULL) {
> -			pr_err("Unable to find mount point for %s\n", opts.root);
> +		close_proc();
> +
> +		err = clean_mnt_ns(ns.mnt.mntinfo_tree);
> +		free_mntinfo(old);
> +		if (err)
>  			return -1;
> -		}
> -		if (mi->parent == NULL) {
> -			pr_err("New root and old root are the same\n");
> +	} else {
> +		close_proc();
> +
> +		if (mount("none", opts.root, "none", MS_SLAVE, NULL)) {
> +			pr_perror("Can't remount the parent of the new root with MS_SLAVE");
>  			return -1;
>  		}

I get this error:

Error (mount.c:1953): Can't remount the parent of the new root with MS_SLAVE: Invalid argument

when I try with this patch.

Tycho

> -
> -		/* Our root is mounted over the parent (in the same directory) */
> -		if (!strcmp(mi->parent->mountpoint, mi->mountpoint)) {
> -			pr_err("The parent of the new root is unreachable\n");
> +		if (mount(opts.root, opts.root, NULL, MS_BIND, NULL)) {
> +			pr_perror("Unable to bind-mount %s", opts.root);
>  			return -1;
>  		}
> -
> -		if (mount("none", mi->parent->mountpoint + 1, "none", MS_SLAVE, NULL)) {
> -			pr_perror("Can't remount the parent of the new root with MS_SLAVE");
> +		if (chdir(opts.root)) {
> +			pr_perror("chdir(%s) failed", opts.root);
>  			return -1;
>  		}
>  	}
>  
> -	free_mntinfo(old);
> -
>  	ret = populate_mnt_ns(mis);
>  	if (ret)
>  		goto out;
> -- 
> 1.9.3
>