mount: add plain mountpoints

This is a preparation of mounts-v2 new algorithm for mount restore, we
add an alternative mountpoints to each mount, so that if we mount mounts
in these mountpoints they will be "plain": each mount in separate
sub-directory of root_yard, mounts will be mounted without tree. Tree
reconstruction will be done in separate step.

Cherry-picked from Virtuozzo criu:
https://src.openvz.org/projects/OVZ/repos/criu/commits/5e6de171a

Changes: improve get_plain_mountpoint().

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

criu/include/mount.h

@@ -48,6 +48,7 @@ struct mount_info {
 	 */
 	char *mountpoint;
 	char *ns_mountpoint;
+	char *plain_mountpoint;
 	int fd;
 	unsigned flags;
 	unsigned sb_flags;
@@ -168,6 +169,8 @@ extern int read_mnt_ns_img(void);
 extern void cleanup_mnt_ns(void);
 extern void clean_cr_time_mounts(void);
 
+extern char *get_plain_mountpoint(int mnt_id, char *name);
+
 extern bool add_skip_mount(const char *mountpoint);
 struct ns_id;
 extern int get_sdev_from_fd(int fd, unsigned int *sdev, bool parse_mountinfo);

criu/mount.c

@@ -49,6 +49,10 @@ static LIST_HEAD(delayed_unbindable);
 
 char *service_mountpoint(const struct mount_info *mi)
 {
+	if (!opts.mntns_compat_mode && opts.mode == CR_RESTORE) {
+		BUG_ON(!mi->plain_mountpoint);
+		return mi->plain_mountpoint;
+	}
 	return mi->mountpoint;
 }
 
@@ -1627,6 +1631,28 @@ err:
 	return -1;
 }
 
+/*
+ * Helper for getting a path to mount's plain mountpoint
+ */
+char *get_plain_mountpoint(int mnt_id, char *name)
+{
+	static char tmp[PATH_MAX];
+	int ret;
+
+	if (!mnt_roots)
+		return NULL;
+
+	if (name)
+		ret = snprintf(tmp, sizeof(tmp), "%s/mnt-%s", mnt_roots, name);
+	else
+		ret = snprintf(tmp, sizeof(tmp), "%s/mnt-%010d", mnt_roots, mnt_id);
+
+	if (ret >= sizeof(tmp))
+		return NULL;
+
+	return xstrdup(tmp);
+}
+
 static __maybe_unused struct mount_info *add_cr_time_mount(struct mount_info *root, char *fsname, const char *path,
 							   unsigned int s_dev, bool rst)
 {
@@ -1654,6 +1680,11 @@ static __maybe_unused struct mount_info *add_cr_time_mount(struct mount_info *ro
 		sprintf(mi->mountpoint, "%s%s", root->mountpoint, path);
 	else
 		sprintf(mi->mountpoint, "%s/%s", root->mountpoint, path);
+	if (rst) {
+		mi->plain_mountpoint = get_plain_mountpoint(-1, "crtime");
+		if (!mi->plain_mountpoint)
+			goto err;
+	}
 	mi->mnt_id = HELPER_MNT_ID;
 	mi->flags = mi->sb_flags = 0;
 	mi->root = xstrdup("/");
@@ -2995,6 +3026,7 @@ void mnt_entry_free(struct mount_info *mi)
 	if (mi) {
 		xfree(mi->root);
 		xfree(mi->mountpoint);
+		xfree(mi->plain_mountpoint);
 		xfree(mi->source);
 		xfree(mi->options);
 		xfree(mi->fsname);
@@ -3120,6 +3152,10 @@ static int get_mp_mountpoint(char *mountpoint, struct mount_info *mi, char *root
 
 	mi->ns_mountpoint = mi->mountpoint + root_len;
 
+	mi->plain_mountpoint = get_plain_mountpoint(mi->mnt_id, NULL);
+	if (!mi->plain_mountpoint)
+		return -1;
+
 	pr_debug("\t\tWill mount %d @ %s %s\n", mi->mnt_id, service_mountpoint(mi), mi->ns_mountpoint);
 	return 0;
 }
@@ -3293,6 +3329,9 @@ static int merge_mount_trees(void)
 		return -1;
 
 	root_yard_mp->mountpoint = mnt_roots;
+	root_yard_mp->plain_mountpoint = xstrdup(mnt_roots);
+	if (!root_yard_mp->plain_mountpoint)
+		return -1;
 	root_yard_mp->mounted = true;
 	root_yard_mp->mnt_bind_is_populated = true;
 	root_yard_mp->is_overmounted = false;