add support for the newline-fd readiness protocol

sshd.c

@@ -158,6 +158,12 @@ int debug_flag = 0;
  */
 static int test_flag = 0;
 
+/*
+ * The file descriptor where a newline should be written after listening on
+ * the socket.
+ */
+static int notify_fd = 0;
+
 /* Flag indicating that the daemon is being started from inetd. */
 static int inetd_flag = 0;
 
@@ -899,6 +905,7 @@ usage(void)
 "usage: sshd [-46DdeGiqTtV] [-C connection_spec] [-c host_cert_file]\n"
 "            [-E log_file] [-f config_file] [-g login_grace_time]\n"
 "            [-h host_key_file] [-o option] [-p port] [-u len]\n"
+"            [-3 newline-fd]\n"
 	);
 	exit(1);
 }
@@ -1591,7 +1598,7 @@ main(int ac, char **av)
 
 	/* Parse command-line arguments. */
 	while ((opt = getopt(ac, av,
-	    "C:E:b:c:f:g:h:k:o:p:u:46DGQRTdeiqrtV")) != -1) {
+	    "C:E:b:c:f:g:h:k:o:p:u:46DGQRTdeiqrtV3:")) != -1) {
 		switch (opt) {
 		case '4':
 			options.address_family = AF_INET;
@@ -1699,6 +1706,15 @@ main(int ac, char **av)
 			fprintf(stderr, "%s, %s\n",
 			    SSH_RELEASE, SSH_OPENSSL_VERSION);
 			exit(0);
+		case '3':
+			/* implies process will be supervised */
+			no_daemon_flag = 1;
+			notify_fd = (int)strtonum(optarg, 3, INT_MAX, NULL);
+			if (notify_fd == 0) {
+				fprintf(stderr, "Invalid notification file-descriptor.\n");
+				exit(1);
+			}
+			break;
 		default:
 			usage();
 			break;
@@ -1708,9 +1724,12 @@ main(int ac, char **av)
 		rexec_flag = 0;
 	if (!test_flag && !do_dump_cfg && rexec_flag && !path_absolute(av[0]))
 		fatal("sshd re-exec requires execution with an absolute path");
-	if (rexeced_flag)
+	if (rexeced_flag) {
 		closefrom(REEXEC_MIN_FREE_FD);
-	else
+		/* avoid writing newline */
+		notify_fd = 0;
+	} else if (notify_fd == 0)
+		/* must inherit fd from supervisor parent */
 		closefrom(REEXEC_DEVCRYPTO_RESERVED_FD);
 
 	seed_rng();
@@ -2077,6 +2096,10 @@ main(int ac, char **av)
 		ssh_signal(SIGTERM, sigterm_handler);
 		ssh_signal(SIGQUIT, sigterm_handler);
 
+		if (notify_fd >= 3) {
+			write(notify_fd, "\n", 1);
+			close(notify_fd);
+		}
 		platform_post_listen();
 
 		/*