device: properly handle MTU for devices with @iface != @ip_iface

When the device has an IP interface different from the main one, we
previously took the MTU saved in priv->mtu (which is the MTU initially
set on the underlying interface) and applied it to the IP interface.

This is wrong as it forces the two MTUs to be equal and breaks
connectivity for devices with encapsulation (as PPP). Instead, track
the two MTUs in different variables.

https://bugzilla.redhat.com/show_bug.cgi?id=1385198

src/devices/nm-device.c

@@ -298,6 +298,7 @@ typedef struct _NMDevicePrivate {
 	bool            ignore_carrier;
 	gulong          ignore_carrier_id;
 	guint32         mtu;
+	guint32         ip_mtu;
 	bool            up;   /* IFF_UP */
 
 	/* Generic DHCP stuff */
@@ -815,12 +816,16 @@ nm_device_set_ip_iface (NMDevice *self, const char *iface)
 		}
 	}
 
+
 	/* We don't care about any saved values from the old iface */
 	g_hash_table_remove_all (priv->ip6_saved_properties);
 
 	/* Emit change notification */
-	if (g_strcmp0 (old_ip_iface, priv->ip_iface))
+	if (g_strcmp0 (old_ip_iface, priv->ip_iface)) {
+		if (priv->ip_ifindex)
+			priv->ip_mtu = nm_platform_link_get_mtu (NM_PLATFORM_GET, priv->ip_ifindex);
 		_notify (self, PROP_IP_IFACE);
+	}
 	g_free (old_ip_iface);
 }
 
@@ -2067,6 +2072,9 @@ device_ip_link_changed (NMDevice *self)
 
 	_stats_update_counters_from_pllink (self, pllink);
 
+	if (priv->ip_mtu != pllink->mtu)
+		priv->ip_mtu = pllink->mtu;
+
 	if (pllink->name[0] && g_strcmp0 (priv->ip_iface, pllink->name)) {
 		_LOGI (LOGD_DEVICE, "interface index %d renamed ip_iface (%d) from '%s' to '%s'",
 		       priv->ifindex, nm_device_get_ip_ifindex (self),
@@ -6366,36 +6374,61 @@ linklocal6_start (NMDevice *self)
 
 static void nm_device_ipv6_set_mtu (NMDevice *self, guint32 mtu);
 
+static void
+set_ip_mtu (NMDevice *self, guint32 mtu)
+{
+	NMDevicePrivate *priv = NM_DEVICE_GET_PRIVATE (self);
+
+	if (priv->ip_ifindex)
+		priv->ip_mtu = mtu;
+	else
+		priv->mtu = mtu;
+}
+
+static guint32
+get_ip_mtu (NMDevice *self)
+{
+	NMDevicePrivate *priv = NM_DEVICE_GET_PRIVATE (self);
+
+	if (priv->ip_ifindex)
+		return priv->ip_mtu;
+	else
+		return priv->mtu;
+}
+
 static void
 nm_device_set_mtu (NMDevice *self, guint32 mtu)
 {
 	NMDevicePrivate *priv = NM_DEVICE_GET_PRIVATE (self);
 	int ifindex = nm_device_get_ip_ifindex (self);
+	guint32 ip_mtu;
 
 	if (mtu)
-		priv->mtu = mtu;
+		set_ip_mtu (self, mtu);
 
 	/* Ensure the IPv6 MTU is still alright. */
 	if (priv->ip6_mtu)
 		nm_device_ipv6_set_mtu (self, priv->ip6_mtu);
 
-	if (priv->mtu && priv->mtu != nm_platform_link_get_mtu (NM_PLATFORM_GET, ifindex))
+	ip_mtu = get_ip_mtu (self);
-		nm_platform_link_set_mtu (NM_PLATFORM_GET, ifindex, priv->mtu);
+	if (ip_mtu && ip_mtu != nm_platform_link_get_mtu (NM_PLATFORM_GET, ifindex))
+		nm_platform_link_set_mtu (NM_PLATFORM_GET, ifindex, ip_mtu);
 }
 
 static void
 nm_device_ipv6_set_mtu (NMDevice *self, guint32 mtu)
 {
 	NMDevicePrivate *priv = NM_DEVICE_GET_PRIVATE (self);
-	guint32 plat_mtu = nm_device_ipv6_sysctl_get_int32 (self, "mtu", priv->mtu);
+	guint32 ip_mtu = get_ip_mtu (self);
+	guint32 plat_mtu = nm_device_ipv6_sysctl_get_int32 (self, "mtu", ip_mtu);
 	char val[16];
 
 	priv->ip6_mtu = mtu ?: plat_mtu;
 
-	if (priv->ip6_mtu && priv->mtu && priv->mtu < priv->ip6_mtu) {
+	if (priv->ip6_mtu && ip_mtu && ip_mtu < priv->ip6_mtu) {
 		_LOGI (LOGD_DEVICE | LOGD_IP6, "Lowering IPv6 MTU (%d) to match device MTU (%d)",
-		       priv->ip6_mtu, priv->mtu);
+		       priv->ip6_mtu, ip_mtu);
-		priv->ip6_mtu = priv->mtu;
+		priv->ip6_mtu = ip_mtu;
 	}
 
 	if (priv->ip6_mtu && priv->ip6_mtu < 1280) {
@@ -6404,9 +6437,9 @@ nm_device_ipv6_set_mtu (NMDevice *self, guint32 mtu)
 		priv->ip6_mtu = 1280;
 	}
 
-	if (priv->ip6_mtu && priv->mtu && priv->mtu < priv->ip6_mtu) {
+	if (priv->ip6_mtu && ip_mtu && ip_mtu < priv->ip6_mtu) {
 		_LOGI (LOGD_DEVICE | LOGD_IP6, "Raising device MTU (%d) to match IPv6 MTU (%d)",
-		       priv->mtu, priv->ip6_mtu);
+		       ip_mtu, priv->ip6_mtu);
 		nm_device_set_mtu (self, priv->ip6_mtu);
 	}