From 9fb09da21a4080e921f9fc7bb2f77baf5085b2cb Mon Sep 17 00:00:00 2001
From: Jukka Rissanen <jukka.rissanen@nordicsemi.no>
Date: Wed, 13 Nov 2024 09:26:53 +0200
Subject: [PATCH] net: Add support for IPV6_MTU IPv6 socket option

Add IPV6_MTU IPv6 socket option and implement getsockopt() and
setsockopt() calls for the option.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>
---
 include/zephyr/net/socket.h           |  7 ++++
 subsys/net/ip/net_context.c           | 52 +++++++++++++++++++++++++++
 subsys/net/lib/sockets/sockets_inet.c | 28 +++++++++++++++
 3 files changed, 87 insertions(+)

diff --git a/include/zephyr/net/socket.h b/include/zephyr/net/socket.h
index 94fc5efda92..06fd0be5ffa 100644
--- a/include/zephyr/net/socket.h
+++ b/include/zephyr/net/socket.h
@@ -1248,6 +1248,13 @@ struct ipv6_mreq {
 	int ipv6mr_ifindex;
 };
 
+/** For getsockopt(), retrieve the current known IPv6 path MTU of the given socket.
+ * Valid only when the socket has been connected.
+ * For setsockopt(), set the MTU to be used for the socket. The MTU is limited by
+ * the device MTU or the path MTU when path MTU discovery is enabled.
+ */
+#define IPV6_MTU 24
+
 /** Don't support IPv4 access */
 #define IPV6_V6ONLY 26
 
diff --git a/subsys/net/ip/net_context.c b/subsys/net/ip/net_context.c
index 173000fad0d..0d304e624eb 100644
--- a/subsys/net/ip/net_context.c
+++ b/subsys/net/ip/net_context.c
@@ -3102,6 +3102,55 @@ static int set_context_reuseport(struct net_context *context,
 #endif
 }
 
+static int set_context_ipv6_mtu(struct net_context *context,
+				const void *value, size_t len)
+{
+#if defined(CONFIG_NET_IPV6)
+	struct net_if *iface;
+	uint16_t mtu;
+
+	if (len != sizeof(int)) {
+		return -EINVAL;
+	}
+
+	mtu = *((int *)value);
+
+	if (IS_ENABLED(CONFIG_NET_IPV6_PMTU)) {
+		int ret;
+
+		ret = net_pmtu_update_mtu(&context->remote, mtu);
+		if (ret < 0) {
+			return ret;
+		}
+
+		return 0;
+	}
+
+	if (net_context_is_bound_to_iface(context)) {
+		iface = net_context_get_iface(context);
+	} else {
+		sa_family_t family = net_context_get_family(context);
+
+		if (IS_ENABLED(CONFIG_NET_IPV6) && family == AF_INET6) {
+			iface = net_if_ipv6_select_src_iface(
+				&net_sin6(&context->remote)->sin6_addr);
+		} else {
+			return -EAFNOSUPPORT;
+		}
+	}
+
+	net_if_set_mtu(iface, (uint16_t)mtu);
+
+	return 0;
+#else
+	ARG_UNUSED(context);
+	ARG_UNUSED(value);
+	ARG_UNUSED(len);
+
+	return -ENOTSUP;
+#endif
+}
+
 static int set_context_ipv6_v6only(struct net_context *context,
 				   const void *value, size_t len)
 {
@@ -3236,6 +3285,9 @@ int net_context_set_option(struct net_context *context,
 		if (IS_ENABLED(CONFIG_NET_IPV4) &&
 		    net_context_get_family(context) == AF_INET) {
 			ret = -EOPNOTSUPP;
+		} else if (IS_ENABLED(CONFIG_NET_IPV6) &&
+			   net_context_get_family(context) == AF_INET6) {
+			ret = set_context_ipv6_mtu(context, value, len);
 		}
 
 		break;
diff --git a/subsys/net/lib/sockets/sockets_inet.c b/subsys/net/lib/sockets/sockets_inet.c
index af57c40e5bc..8e5673208aa 100644
--- a/subsys/net/lib/sockets/sockets_inet.c
+++ b/subsys/net/lib/sockets/sockets_inet.c
@@ -1858,6 +1858,20 @@ int zsock_getsockopt_ctx(struct net_context *ctx, int level, int optname,
 
 	case IPPROTO_IPV6:
 		switch (optname) {
+		case IPV6_MTU:
+			if (IS_ENABLED(CONFIG_NET_IPV6)) {
+				ret = net_context_get_option(ctx, NET_OPT_MTU,
+							     optval, optlen);
+				if (ret < 0) {
+					errno  = -ret;
+					return -1;
+				}
+
+				return 0;
+			}
+
+			break;
+
 		case IPV6_V6ONLY:
 			if (IS_ENABLED(CONFIG_NET_IPV4_MAPPING_TO_IPV6)) {
 				ret = net_context_get_option(ctx,
@@ -2422,6 +2436,20 @@ int zsock_setsockopt_ctx(struct net_context *ctx, int level, int optname,
 
 	case IPPROTO_IPV6:
 		switch (optname) {
+		case IPV6_MTU:
+			if (IS_ENABLED(CONFIG_NET_IPV6)) {
+				ret = net_context_set_option(ctx, NET_OPT_MTU,
+							     optval, optlen);
+				if (ret < 0) {
+					errno  = -ret;
+					return -1;
+				}
+
+				return 0;
+			}
+
+			break;
+
 		case IPV6_V6ONLY:
 			if (IS_ENABLED(CONFIG_NET_IPV4_MAPPING_TO_IPV6)) {
 				ret = net_context_set_option(ctx,