From 3c6d24dd3021bb294a7aa182a95a9cb868ca6cb4 Mon Sep 17 00:00:00 2001
From: Stefano Brivio <sbrivio@redhat.com>
Date: Thu, 14 Oct 2021 13:05:56 +0200
Subject: [PATCH] netlink, pasta: Configure MTU of tap interface on
 --config-net

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 conf.c    |  2 +-
 netlink.c | 44 ++++++++++++++++++++++++++++----------------
 netlink.h |  2 +-
 pasta.c   |  6 +++---
 4 files changed, 33 insertions(+), 21 deletions(-)

diff --git a/conf.c b/conf.c
index db9c350..483daeb 100644
--- a/conf.c
+++ b/conf.c
@@ -484,7 +484,7 @@ static void conf_ip(struct ctx *c)
 		memcpy(&c->addr4_seen, &c->addr4, sizeof(c->addr4_seen));
 
 		if (!memcmp(c->mac, MAC_ZERO, ETH_ALEN))
-			nl_link(0, c->ifi, c->mac, 0);
+			nl_link(0, c->ifi, c->mac, 0, 0);
 	}
 
 	if (c->mode == MODE_PASST)
diff --git a/netlink.c b/netlink.c
index ca2e77f..a26078b 100644
--- a/netlink.c
+++ b/netlink.c
@@ -454,27 +454,28 @@ next:
  * @ifi:	Interface index
  * @mac:	MAC address to fill, if passed as zero, to set otherwise
  * @up:		If set, bring up the link
+ * @mtu:	If non-zero, set interface MTU
  */
-void nl_link(int ns, unsigned int ifi, void *mac, int up)
+void nl_link(int ns, unsigned int ifi, void *mac, int up, int mtu)
 {
-	int change = !MAC_IS_ZERO(mac) || up;
+	int change = !MAC_IS_ZERO(mac) || up || mtu;
 	struct {
 		struct nlmsghdr nlh;
 		struct ifinfomsg ifm;
 		struct rtattr rta;
-		unsigned char mac[ETH_ALEN];
+		union {
+			unsigned char mac[ETH_ALEN];
+			unsigned int mtu;
+		};
 	} req = {
-		.nlh.nlmsg_type  = change ? RTM_NEWLINK : RTM_GETLINK,
-		.nlh.nlmsg_len   = NLMSG_LENGTH(sizeof(struct ifinfomsg)),
-		.nlh.nlmsg_flags = NLM_F_REQUEST | (change ? NLM_F_ACK : 0),
-		.nlh.nlmsg_seq	 = nl_seq++,
-		.ifm.ifi_family	 = AF_UNSPEC,
-		.ifm.ifi_index	 = ifi,
-		.ifm.ifi_flags	 = up ? IFF_UP : 0,
-		.ifm.ifi_change	 = up ? IFF_UP : 0,
-
-		.rta.rta_type	 = IFLA_ADDRESS,
-		.rta.rta_len	 = RTA_LENGTH(ETH_ALEN),
+		.nlh.nlmsg_type   = change ? RTM_NEWLINK : RTM_GETLINK,
+		.nlh.nlmsg_len    = NLMSG_LENGTH(sizeof(struct ifinfomsg)),
+		.nlh.nlmsg_flags  = NLM_F_REQUEST | (change ? NLM_F_ACK : 0),
+		.nlh.nlmsg_seq	  = nl_seq++,
+		.ifm.ifi_family	  = AF_UNSPEC,
+		.ifm.ifi_index	  = ifi,
+		.ifm.ifi_flags	  = up ? IFF_UP : 0,
+		.ifm.ifi_change	  = up ? IFF_UP : 0,
 	};
 	struct ifinfomsg *ifm;
 	struct nlmsghdr *nh;
@@ -485,13 +486,24 @@ void nl_link(int ns, unsigned int ifi, void *mac, int up)
 	if (!MAC_IS_ZERO(mac)) {
 		req.nlh.nlmsg_len = sizeof(req);
 		memcpy(req.mac, mac, ETH_ALEN);
+		req.rta.rta_type = IFLA_ADDRESS;
+		req.rta.rta_len = RTA_LENGTH(ETH_ALEN);
+		nl_req(ns, buf, &req, req.nlh.nlmsg_len);
 	}
 
-	n = nl_req(ns, buf, &req, req.nlh.nlmsg_len);
+	if (mtu) {
+		req.nlh.nlmsg_len = sizeof(req);
+		req.mtu = mtu;
+		req.rta.rta_type = IFLA_MTU;
+		req.rta.rta_len = RTA_LENGTH(sizeof(unsigned int));
+		nl_req(ns, buf, &req, req.nlh.nlmsg_len);
+	}
 
-	if (!MAC_IS_ZERO(mac) || up)
+	if (change)
 		return;
 
+	n = nl_req(ns, buf, &req, req.nlh.nlmsg_len);
+
 	nh = (struct nlmsghdr *)buf;
 	for ( ; NLMSG_OK(nh, n); nh = NLMSG_NEXT(nh, n)) {
 		if (nh->nlmsg_type != RTM_NEWLINK)
diff --git a/netlink.h b/netlink.h
index 654e17e..460c2f1 100644
--- a/netlink.h
+++ b/netlink.h
@@ -3,4 +3,4 @@ unsigned int nl_get_ext_if(int *v4, int *v6);
 void nl_route(int ns, unsigned int ifi, sa_family_t af, void *gw);
 void nl_addr(int ns, unsigned int ifi, sa_family_t af,
 	     void *addr, int prefix_len, void *addr_l);
-void nl_link(int ns, unsigned int ifi, void *mac, int up);
+void nl_link(int ns, unsigned int ifi, void *mac, int up, int mtu);
diff --git a/pasta.c b/pasta.c
index 7c53c13..b8f556e 100644
--- a/pasta.c
+++ b/pasta.c
@@ -230,10 +230,10 @@ void pasta_start_ns(struct ctx *c)
  */
 void pasta_ns_conf(struct ctx *c)
 {
-	nl_link(1, 1 /* lo */, MAC_ZERO, 1);
+	nl_link(1, 1 /* lo */, MAC_ZERO, 1, 0);
 
 	if (c->pasta_conf_ns) {
-		nl_link(1, c->pasta_ifi, c->mac_guest, 1);
+		nl_link(1, c->pasta_ifi, c->mac_guest, 1, c->mtu);
 
 		if (c->v4) {
 			nl_addr(1, c->pasta_ifi, AF_INET, &c->addr4,
@@ -246,7 +246,7 @@ void pasta_ns_conf(struct ctx *c)
 			nl_route(1, c->pasta_ifi, AF_INET6, &c->gw6);
 		}
 	} else {
-		nl_link(1, c->pasta_ifi, c->mac_guest, 0);
+		nl_link(1, c->pasta_ifi, c->mac_guest, 0, 0);
 	}
 
 	proto_update_l2_buf(c->mac_guest, NULL, NULL);