From 81ece63e3e63fc43b496beafdd3b46c4cc52839b Mon Sep 17 00:00:00 2001
From: Donatas Abraitis <donatas@opensourcerouting.org>
Date: Mon, 18 Sep 2023 22:34:45 +0300
Subject: [PATCH] bgpd: Set TCP min MSS per listener

Set only if at least one peer is in passive mode.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>
---
 bgpd/bgp_network.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++
 bgpd/bgp_network.h |  1 +
 bgpd/bgpd.c        |  2 ++
 3 files changed, 50 insertions(+)

diff --git a/bgpd/bgp_network.c b/bgpd/bgp_network.c
index 3bfdeb1771..f322de7683 100644
--- a/bgpd/bgp_network.c
+++ b/bgpd/bgp_network.c
@@ -334,6 +334,53 @@ static int bgp_get_instance_for_inc_conn(int sock, struct bgp **bgp_inst)
 #endif
 }
 
+int bgp_tcp_mss_set(struct peer *peer)
+{
+	struct listnode *node;
+	int ret = 0;
+	struct bgp_listener *listener;
+	uint32_t min_mss = 0;
+	struct peer *p;
+
+	for (ALL_LIST_ELEMENTS_RO(peer->bgp->peer, node, p)) {
+		if (!CHECK_FLAG(p->flags, PEER_FLAG_TCP_MSS))
+			continue;
+
+		if (!p->tcp_mss)
+			continue;
+
+		if (!min_mss)
+			min_mss = p->tcp_mss;
+
+		min_mss = MIN(min_mss, p->tcp_mss);
+	}
+
+	frr_with_privs(&bgpd_privs) {
+		for (ALL_LIST_ELEMENTS_RO(bm->listen_sockets, node, listener)) {
+			if (listener->su.sa.sa_family !=
+			    peer->connection->su.sa.sa_family)
+				continue;
+
+			if (!listener->bgp) {
+				if (peer->bgp->vrf_id != VRF_DEFAULT)
+					continue;
+			} else if (listener->bgp != peer->bgp)
+				continue;
+
+			/* Set TCP MSS per listener only if there is at least
+			 * one peer that is in passive mode. Otherwise, TCP MSS
+			 * is set per socket via bgp_connect().
+			 */
+			if (CHECK_FLAG(peer->flags, PEER_FLAG_PASSIVE))
+				sockopt_tcp_mss_set(listener->fd, min_mss);
+
+			break;
+		}
+	}
+
+	return ret;
+}
+
 static void bgp_socket_set_buffer_size(const int fd)
 {
 	if (getsockopt_so_sendbuf(fd) < (int)bm->socket_buffer)
diff --git a/bgpd/bgp_network.h b/bgpd/bgp_network.h
index f26d64f1f8..7a0b3cc67d 100644
--- a/bgpd/bgp_network.h
+++ b/bgpd/bgp_network.h
@@ -30,6 +30,7 @@ extern int bgp_md5_unset_prefix(struct bgp *bgp, struct prefix *p);
 extern int bgp_md5_set(struct peer_connection *connection);
 extern int bgp_md5_unset(struct peer_connection *connection);
 extern int bgp_set_socket_ttl(struct peer_connection *connection);
+extern int bgp_tcp_mss_set(struct peer *peer);
 extern int bgp_update_address(struct interface *ifp, const union sockunion *dst,
 			      union sockunion *addr);
 
diff --git a/bgpd/bgpd.c b/bgpd/bgpd.c
index 895f36f992..585863954c 100644
--- a/bgpd/bgpd.c
+++ b/bgpd/bgpd.c
@@ -5779,6 +5779,7 @@ void peer_tcp_mss_set(struct peer *peer, uint32_t tcp_mss)
 {
 	peer->tcp_mss = tcp_mss;
 	SET_FLAG(peer->flags, PEER_FLAG_TCP_MSS);
+	bgp_tcp_mss_set(peer);
 }
 
 /* Reset the TCP-MSS value in the peer structure,
@@ -5789,6 +5790,7 @@ void peer_tcp_mss_unset(struct peer *peer)
 {
 	UNSET_FLAG(peer->flags, PEER_FLAG_TCP_MSS);
 	peer->tcp_mss = 0;
+	bgp_tcp_mss_set(peer);
 }
 
 /*