c/r: use /proc/self/tid/children instead of pidfile

All we really needed a unique temp file for was passing the pid. Since CRIU
opened this with O_EXCL | O_CREAT, this was "safe" (users could still
overwrite it afterwards, but the monitor would immediately die since the
only valid number in there was the init process).

In any case, we can just read /proc/self/tid/children, which lists the
child process.

Closes #1150

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>

src/lxc/criu.c

@@ -69,7 +69,6 @@ struct criu_opts {
 	char tty_id[32]; /* the criu tty id for /dev/console, i.e. "tty[${rdev}:${dev}]" */
 
 	/* restore: the file to write the init process' pid into */
-	char *pidfile;
 	const char *cgroup_path;
 	int console_fd;
 	/* The path that is bind mounted from /dev/console, if any. We don't
@@ -176,10 +175,10 @@ static void exec_criu(struct criu_opts *opts)
 			static_args += 2;
 	} else if (strcmp(opts->action, "restore") == 0) {
 		/* --root $(lxc_mount_point) --restore-detached
-		 * --restore-sibling --pidfile $foo --cgroup-root $foo
+		 * --restore-sibling --cgroup-root $foo
 		 * --lsm-profile apparmor:whatever
 		 */
-		static_args += 10;
+		static_args += 8;
 
 		tty_info[0] = 0;
 		if (load_tty_major_minor(opts->user->directory, tty_info, sizeof(tty_info)))
@@ -330,8 +329,6 @@ static void exec_criu(struct criu_opts *opts)
 		DECLARE_ARG(opts->c->lxc_conf->rootfs.mount);
 		DECLARE_ARG("--restore-detached");
 		DECLARE_ARG("--restore-sibling");
-		DECLARE_ARG("--pidfile");
-		DECLARE_ARG(opts->pidfile);
 		DECLARE_ARG("--cgroup-root");
 		DECLARE_ARG(opts->cgroup_path);
 
@@ -604,13 +601,8 @@ static void do_restore(struct lxc_container *c, int status_pipe, struct migrate_
 {
 	pid_t pid;
 	struct lxc_handler *handler;
-	int fd, status;
+	int status;
 	int pipes[2] = {-1, -1};
-	char pidfile[] = "criu_restore_XXXXXX";
-
-	fd = mkstemp(pidfile);
-	if (fd < 0)
-		goto out;
 
 	handler = lxc_init(c->name, c->lxc_conf, c->config_path);
 	if (!handler)
@@ -690,7 +682,6 @@ static void do_restore(struct lxc_container *c, int status_pipe, struct migrate_
 		os.action = "restore";
 		os.user = opts;
 		os.c = c;
-		os.pidfile = pidfile;
 		os.cgroup_path = cgroup_canonical_path(handler);
 		os.console_fd = c->lxc_conf->console.slave;
 		os.criu_version = criu_version;
@@ -742,8 +733,9 @@ static void do_restore(struct lxc_container *c, int status_pipe, struct migrate_
 		}
 
 		if (WIFEXITED(status)) {
+			char buf[4096];
+
 			if (WEXITSTATUS(status)) {
-				char buf[4096];
 				int n;
 
 				n = read(pipes[0], buf, sizeof(buf));
@@ -758,18 +750,21 @@ static void do_restore(struct lxc_container *c, int status_pipe, struct migrate_
 				goto out_fini_handler;
 			} else {
 				int ret;
-				FILE *f = fdopen(fd, "r");
+
+				ret = snprintf(buf, sizeof(buf), "/proc/self/task/%" PRId64 "/children", syscall(__NR_gettid));
+				if (ret < 0 || ret >= sizeof(buf)) {
+					ERROR("snprintf'd too many characters: %d", ret);
+					goto out_fini_handler;
+				}
+
+				FILE *f = fopen(buf, "r");
 				if (!f) {
-					SYSERROR("couldn't read restore's init pidfile %s\n", pidfile);
+					SYSERROR("couldn't read restore's children file %s\n", buf);
 					goto out_fini_handler;
 				}
-				fd = -1;
 
 				ret = fscanf(f, "%d", (int*) &handler->pid);
 				fclose(f);
-				if (unlink(pidfile) < 0 && errno != ENOENT)
-					SYSERROR("unlinking pidfile failed");
-
 				if (ret != 1) {
 					ERROR("reading restore pid failed");
 					goto out_fini_handler;
@@ -809,8 +804,6 @@ static void do_restore(struct lxc_container *c, int status_pipe, struct migrate_
 		close(pipes[1]);
 
 	lxc_fini(c->name, handler);
-	if (unlink(pidfile) < 0 && errno != ENOENT)
-		SYSERROR("unlinking pidfile failed");
 
 out:
 	if (status_pipe >= 0) {
@@ -821,9 +814,6 @@ static void do_restore(struct lxc_container *c, int status_pipe, struct migrate_
 		close(status_pipe);
 	}
 
-	if (fd > 0)
-		close(fd);
-
 	exit(1);
 }