start: only use host's /dev/null when absolutely necessary

src/lxc/start.c

@@ -710,7 +710,8 @@ static int do_start(void *data)
 {
 	struct lxc_list *iterator;
 	struct lxc_handler *handler = data;
-	int devnull_fd = -1;
+	int devnull_fd = -1, ret;
+	char path[PATH_MAX];
 
 	if (sigprocmask(SIG_SETMASK, &handler->oldmask, NULL)) {
 		SYSERROR("failed to set sigprocmask");
@@ -789,11 +790,28 @@ static int do_start(void *data)
 	}
 	#endif
 
-	if (handler->backgrounded) {
+	ret = sprintf(path, "%s/dev/null", handler->conf->rootfs.mount);
+	if (ret < 0 || ret >= sizeof(path)) {
+		SYSERROR("sprintf'd too many chars");
+		goto out_warn_father;
+	}
+
+	/* In order to checkpoint restore, we need to have everything in the
+	 * same mount namespace. However, some containers may not have a
+	 * reasonable /dev (in particular, they may not have /dev/null), so we
+	 * can't set init's std fds to /dev/null by opening it from inside the
+	 * container.
+	 *
+	 * If that's the case, fall back to using the host's /dev/null. This
+	 * means that migration won't work, but at least we won't spew output
+	 * where it isn't wanted.
+	 */
+	if (handler->backgrounded && !handler->conf->autodev && access(path, F_OK) < 0) {
 		devnull_fd = open_devnull();
 
 		if (devnull_fd < 0)
 			goto out_warn_father;
+		WARN("using host's /dev/null for container init's std fds, migraiton won't work");
 	}
 
 	/* Setup the container, ip, names, utsname, ... */
@@ -861,6 +879,13 @@ static int do_start(void *data)
 
 	close(handler->sigfd);
 
+	if (devnull_fd < 0) {
+		devnull_fd = open_devnull();
+
+		if (devnull_fd < 0)
+			goto out_warn_father;
+	}
+
 	if (handler->backgrounded && set_stdfds(devnull_fd))
 		goto out_warn_father;