From 4cedd1c9acac0fba598db2eaf43278dfe8e53ef0 Mon Sep 17 00:00:00 2001
From: Steve Langasek <steve.langasek@ubuntu.com>
Date: Thu, 1 Sep 2022 16:03:37 +0100
Subject: Support systemd socket activation

Unlike inetd socket activation, with systemd socket activation the
supervisor passes the listened-on socket to the child process and lets
the child process handle the accept().  This lets us do delayed start
of the sshd daemon without becoming incompatible with config options
like ClientAliveCountMax.

Last-Update: 2022-09-01

Patch-Name: systemd-socket-activation.patch
---
 sshd.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 75 insertions(+), 14 deletions(-)

diff --git a/sshd.c b/sshd.c
index 0a4eefe01..fc22fcb62 100644
--- a/sshd.c
+++ b/sshd.c
@@ -141,10 +141,16 @@ int deny_severity;
 #endif /* LIBWRAP */
 
 /* Re-exec fds */
-#define REEXEC_DEVCRYPTO_RESERVED_FD	(STDERR_FILENO + 1)
-#define REEXEC_STARTUP_PIPE_FD		(STDERR_FILENO + 2)
-#define REEXEC_CONFIG_PASS_FD		(STDERR_FILENO + 3)
-#define REEXEC_MIN_FREE_FD		(STDERR_FILENO + 4)
+#ifdef HAVE_SYSTEMD
+#define SYSTEMD_OFFSET sd_listen_fds(0)
+#else
+#define SYSTEMD_OFFSET 0
+#endif
+
+#define REEXEC_DEVCRYPTO_RESERVED_FD	(STDERR_FILENO + 1 + SYSTEMD_OFFSET)
+#define REEXEC_STARTUP_PIPE_FD		(STDERR_FILENO + 2 + SYSTEMD_OFFSET)
+#define REEXEC_CONFIG_PASS_FD		(STDERR_FILENO + 3 + SYSTEMD_OFFSET)
+#define REEXEC_MIN_FREE_FD		(STDERR_FILENO + 4 + SYSTEMD_OFFSET)
 
 extern char *__progname;
 
@@ -1025,6 +1031,48 @@ server_accept_inetd(int *sock_in, int *sock_out)
 	debug("inetd sockets after dupping: %d, %d", *sock_in, *sock_out);
 }
 
+#ifdef HAVE_SYSTEMD
+/*
+ * Configure our socket fds that were passed from systemd
+ */
+static void
+setup_systemd_socket(int listen_sock)
+{
+	int ret;
+	struct sockaddr_storage addr;
+	socklen_t len = sizeof(addr);
+	char ntop[NI_MAXHOST], strport[NI_MAXSERV];
+
+	if (getsockname(listen_sock, (struct sockaddr *)&addr, &len) != 0)
+		return;
+
+	if (((struct sockaddr *)&addr)->sa_family != AF_INET
+	    && ((struct sockaddr *)&addr)->sa_family != AF_INET6)
+		return;
+	if (num_listen_socks >= MAX_LISTEN_SOCKS)
+		fatal("Too many listen sockets. "
+		    "Enlarge MAX_LISTEN_SOCKS");
+	if ((ret = getnameinfo((struct sockaddr *)&addr, len, ntop,
+	                       sizeof(ntop), strport, sizeof(strport),
+	                       NI_NUMERICHOST|NI_NUMERICSERV)) != 0) {
+		error("getnameinfo failed: %.100s",
+		    ssh_gai_strerror(ret));
+		return;
+	}
+	if (set_nonblock(listen_sock) == -1) {
+		close(listen_sock);
+		return;
+	}
+	/* Socket options */
+	set_reuseaddr(listen_sock);
+
+	listen_socks[num_listen_socks] = listen_sock;
+	num_listen_socks++;
+
+	logit("Server listening on %s port %s.", ntop, strport);
+}
+#endif
+
 /*
  * Listen for TCP connections
  */
@@ -1104,22 +1152,35 @@ static void
 server_listen(void)
 {
 	u_int i;
+#ifdef HAVE_SYSTEMD
+	int systemd_socket_count;
+#endif
 
 	/* Initialise per-source limit tracking. */
 	srclimit_init(options.max_startups, options.per_source_max_startups,
 	    options.per_source_masklen_ipv4, options.per_source_masklen_ipv6);
 
-	for (i = 0; i < options.num_listen_addrs; i++) {
-		listen_on_addrs(&options.listen_addrs[i]);
-		freeaddrinfo(options.listen_addrs[i].addrs);
-		free(options.listen_addrs[i].rdomain);
-		memset(&options.listen_addrs[i], 0,
-		    sizeof(options.listen_addrs[i]));
+#ifdef HAVE_SYSTEMD
+	systemd_socket_count = sd_listen_fds(0);
+	if (systemd_socket_count > 0)
+	{
+		int i;
+		for (i = 0; i < systemd_socket_count; i++)
+			setup_systemd_socket(SD_LISTEN_FDS_START + i);
+	} else
+#endif
+	{
+		for (i = 0; i < options.num_listen_addrs; i++) {
+			listen_on_addrs(&options.listen_addrs[i]);
+			freeaddrinfo(options.listen_addrs[i].addrs);
+			free(options.listen_addrs[i].rdomain);
+			memset(&options.listen_addrs[i], 0,
+			    sizeof(options.listen_addrs[i]));
+		}
+		free(options.listen_addrs);
+		options.listen_addrs = NULL;
+		options.num_listen_addrs = 0;
 	}
-	free(options.listen_addrs);
-	options.listen_addrs = NULL;
-	options.num_listen_addrs = 0;
-
 	if (!num_listen_socks)
 		fatal("Cannot bind any address.");
 }