restore: add an option for changing a root file system (v2)

The option is -r|--pivot-root and an argument is a path to new root.
A root task will make pivot_root. LXC CT does that, so we need that
for restoring.

v2: s/pivot-root/root/
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

crtools.c

@@ -59,7 +59,7 @@ int main(int argc, char *argv[])
 	int log_inited = 0;
 	int log_level = 0;
 
-	static const char short_opts[] = "dsf:t:hcD:o:n:vxV";
+	static const char short_opts[] = "dsf:t:hcD:o:n:vxVr:";
 
 	BUILD_BUG_ON(PAGE_SIZE != PAGE_IMAGE_SIZE);
 
@@ -79,6 +79,7 @@ int main(int argc, char *argv[])
 			{ "images-dir", required_argument, 0, 'D' },
 			{ "log-file", required_argument, 0, 'o' },
 			{ "namespaces", required_argument, 0, 'n' },
+			{ "root", required_argument, 0, 'r' },
 			{ "ext-unix-sk", no_argument, 0, 'x' },
 			{ "help", no_argument, 0, 'h' },
 			{ SK_EST_PARAM, no_argument, 0, 42 },
@@ -108,6 +109,9 @@ int main(int argc, char *argv[])
 		case 'f':
 			opts.show_dump_file = optarg;
 			break;
+		case 'r':
+			opts.root = optarg;
+			break;
 		case 'd':
 			opts.restore_detach = true;
 			break;
@@ -245,6 +249,7 @@ usage:
 	pr_msg("                        supported: uts, ipc, pid\n");
 	pr_msg("  -x|--ext-unix-sk      allow external unix connections\n");
 	pr_msg("     --%s  checkpoint/restore established TCP connections\n", SK_EST_PARAM);
+	pr_msg("  -r|--root [PATH]	change the root filesystem (when run in mount namespace)\n");
 
 	pr_msg("\n* Logging:\n");
 	pr_msg("  -o|--log-file [NAME]  log file name (relative path is relative to --images-dir)\n");

include/crtools.h

@@ -87,6 +87,7 @@ struct cr_options {
 	unsigned int		namespaces_flags;
 	bool			log_file_per_pid;
 	char			*output;
+	char			*root;
 };
 
 extern struct cr_options opts;

include/mount.h

@@ -15,4 +15,5 @@ struct cr_options;
 extern void show_mountpoints(int fd, struct cr_options *);
 int prepare_mnt_ns(int pid);
 
+extern int pivot_root(const char *new_root, const char *put_old);
 #endif /* MOUNT_H__ */

mount.c

@@ -378,6 +378,13 @@ static int do_umount_one(struct mount_info *mi)
 	if (!mi->parent)
 		return 0;
 
+	/*
+	 * Don't umount the future root. It can be a mountpoint only,
+	 * otherwise pivot_root() fails.
+	 */
+	if (opts.root && !strcmp(opts.root, mi->mountpoint))
+		return 0;
+
 	if (umount(mi->mountpoint)) {
 		pr_perror("Can't umount at %s", mi->mountpoint);
 		return -1;
@@ -421,6 +428,33 @@ static int populate_mnt_ns(int ns_pid)
 
 	pr_info("Populating mount namespace\n");
 
+	if (opts.root) {
+		char put_root[PATH_MAX] = "crtools-put-root.XXXXXX";
+
+		if (chdir(opts.root)) {
+			pr_perror("chdir(%s) failed", opts.root);
+			return -1;
+		}
+		if (mkdtemp(put_root) == NULL) {
+			pr_perror("Can't create a temparary directory");
+			return -1;
+		}
+		if (pivot_root(".", put_root)) {
+			pr_perror("pivot_root(., %s) failed", put_root);
+			if (rmdir(put_root))
+				pr_perror("Can't remove the directory %s", put_root);
+			return -1;
+		}
+		if (umount2(put_root, MNT_DETACH)) {
+			pr_perror("Can't umount %s", put_root);
+			return -1;
+		}
+		if (rmdir(put_root)) {
+			pr_perror("Can't remove the directory %s", put_root);
+			return -1;
+		}
+	}
+
 	img = open_image_ro(CR_FD_MOUNTPOINTS, ns_pid);
 	if (img < 0)
 		return -1;