[CRIU] [PATCH 2/2] mnt: cut shared root path from bind path

[Andrew Vagin]

On Tue, Apr 14, 2015 at 11:23:26PM +0000, Tycho Andersen wrote:
> If a mount like:
> 
> 96 95 0:21 /cgmanager /sys/fs/cgroup/cgmanager rw master:9 - tmpfs tmpfs rw,mode=755
> 
> is present in the container and the host has a similar bind mount, e.g.
> 
> 46 27 0:21 /cgmanager /sys/fs/cgroup/cgmanager rw shared:9 - tmpfs tmpfs rw,mode=755
> 
> then the best match mount's root path /and/ the target mountpoint have part of
> the path in them; we should cut the shared piece of the path and just
> concatenate the non-duplicate pieces.
> 
> Reported-by: Andrew Vagin <avagin at odin.com>
> CC: Andrew Vagin <avagin at odin.com>
> Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>

Acked-by: Andrew Vagin <avagin at odin.com>

I think we need to fix find_best_external_match() to compare info->root
and it->root. it->root must be a subpath of info->root, otherwise we
will not able to restore info from it.

Thanks,
Andrey


> ---
>  mount.c | 47 +++++++++++++++++++++++++++++------------------
>  1 file changed, 29 insertions(+), 18 deletions(-)
> 
> diff --git a/mount.c b/mount.c
> index 381c483..1bb2ebc 100644
> --- a/mount.c
> +++ b/mount.c
> @@ -647,6 +647,25 @@ static int validate_mounts(struct mount_info *info, bool for_dump)
>  	return 0;
>  }
>  
> +static char *cut_root_for_bind(char *target_root, char *source_root)
> +{
> +	int tok = 0;
> +	/*
> +	 * Cut common part of root.
> +	 * For non-root binds the source is always "/" (checked)
> +	 * so this will result in this slash removal only.
> +	 */
> +	while (target_root[tok] == source_root[tok]) {
> +		tok++;
> +		if (source_root[tok] == '\0')
> +			break;
> +		BUG_ON(target_root[tok] == '\0');
> +	}
> +
> +	return target_root + tok;
> +
> +}
> +
>  static struct mount_info *find_best_external_match(struct mount_info *list, struct mount_info *info)
>  {
>  	struct mount_info *it, *candidate = NULL;
> @@ -722,7 +741,7 @@ static int resolve_external_mounts(struct mount_info *info)
>  
>  	for (m = info; m; m = m->next) {
>  		int ret, size;
> -		char *p;
> +		char *p, *cut_root;
>  		struct ext_mount *em;
>  		struct mount_info *match;
>  
> @@ -764,12 +783,16 @@ static int resolve_external_mounts(struct mount_info *info)
>  				continue;
>  		}
>  
> -		size = strlen(match->mountpoint + 1) + strlen(m->root) + 1;
> +		cut_root = cut_root_for_bind(m->root, match->root);
> +
> +		/* +2 for the NULL byte and the extra / in the sprintf below,
> +		 * which we cut off in cut_root_for_bind(). */
> +		size = strlen(match->mountpoint + 1) + strlen(cut_root) + 2;
>  		p = xmalloc(sizeof(char) * size);
>  		if (!p)
>  			return -1;
>  
> -		ret = snprintf(p, size, "%s%s", match->mountpoint + 1, m->root);
> +		ret = snprintf(p, size, "%s/%s", match->mountpoint + 1, cut_root);
>  		if (ret < 0 || ret >= size) {
>  			free(p);
>  			return -1;
> @@ -1817,8 +1840,7 @@ static int do_bind_mount(struct mount_info *mi)
>  	bool force_private_remount = false;
>  
>  	if (!mi->need_plugin) {
> -		char *root, rpath[PATH_MAX];
> -		int tok = 0;
> +		char *root, *cut_root, rpath[PATH_MAX];
>  
>  		if (mi->external) {
>  			/*
> @@ -1833,21 +1855,10 @@ static int do_bind_mount(struct mount_info *mi)
>  		}
>  
>  		shared = mi->shared_id && mi->shared_id == mi->bind->shared_id;
> -
> -		/*
> -		 * Cut common part of root.
> -		 * For non-root binds the source is always "/" (checked)
> -		 * so this will result in this slash removal only.
> -		 */
> -		while (mi->root[tok] == mi->bind->root[tok]) {
> -			tok++;
> -			if (mi->bind->root[tok] == '\0')
> -				break;
> -			BUG_ON(mi->root[tok] == '\0');
> -		}
> +		cut_root = cut_root_for_bind(mi->root, mi->bind->root);
>  
>  		snprintf(rpath, sizeof(rpath), "%s/%s",
> -				mi->bind->mountpoint, mi->root + tok);
> +				mi->bind->mountpoint, cut_root);
>  		root = rpath;
>  do_bind:
>  		pr_info("\tBind %s to %s\n", root, mi->mountpoint);
> -- 
> 2.1.4
>