network: clean up link_may_have_ipv6ll() and allow to run RADV on Tun interface (#38175)

Closes #38170.

src/network/networkd-address.c

@@ -1345,7 +1345,7 @@ int link_drop_ipv6ll_addresses(Link *link) {
         /* IPv6LL address may be in the tentative state, and in that case networkd has not received it.
          * So, we need to dump all IPv6 addresses. */
 
-        if (link_may_have_ipv6ll(link, /* check_multicast = */ false))
+        if (link_ipv6ll_enabled_harder(link))
                 return 0;
 
         r = sd_rtnl_message_new_addr(link->manager->rtnl, &req, RTM_GETADDR, link->ifindex, AF_INET6);

src/network/networkd-ipv6ll.c

@@ -43,41 +43,26 @@ bool link_ipv6ll_enabled(Link *link) {
         return link->network->link_local & ADDRESS_FAMILY_IPV6;
 }
 
-bool link_may_have_ipv6ll(Link *link, bool check_multicast) {
+bool link_ipv6ll_enabled_harder(Link *link) {
         assert(link);
 
-        /*
-         * This is equivalent to link_ipv6ll_enabled() for non-WireGuard interfaces.
-         *
-         * For WireGuard interface, the kernel does not assign any IPv6LL addresses, but we can assign
-         * it manually. It is necessary to set an IPv6LL address manually to run NDisc or RADV on
-         * WireGuard interface. Note, also Multicast=yes must be set. See #17380.
-         *
-         * TODO: May be better to introduce GenerateIPv6LinkLocalAddress= setting, and use algorithms
-         *       used in networkd-address-generation.c
-         */
+        /* This is mostly equivalent to link_ipv6ll_enabled(), but also checks if an IPv6LL address is
+         * manually configured. */
 
         if (link_ipv6ll_enabled(link))
                 return true;
 
-        /* IPv6LL address can be manually assigned on WireGuard interface. */
-        if (streq_ptr(link->kind, "wireguard")) {
-                Address *a;
+        if (!link->network)
+                return false;
 
-                if (!link->network)
-                        return false;
-
-                if (check_multicast && !FLAGS_SET(link->flags, IFF_MULTICAST) && link->network->multicast <= 0)
-                        return false;
-
-                ORDERED_HASHMAP_FOREACH(a, link->network->addresses_by_section) {
-                        if (a->family != AF_INET6)
-                                continue;
-                        if (in6_addr_is_set(&a->in_addr_peer.in6))
-                                continue;
-                        if (in6_addr_is_link_local(&a->in_addr.in6))
-                                return true;
-                }
+        Address *a;
+        ORDERED_HASHMAP_FOREACH(a, link->network->addresses_by_section) {
+                if (a->family != AF_INET6)
+                        continue;
+                if (in6_addr_is_set(&a->in_addr_peer.in6))
+                        continue;
+                if (in6_addr_is_link_local(&a->in_addr.in6))
+                        return true;
         }
 
         return false;

src/network/networkd-ipv6ll.h

@@ -15,7 +15,7 @@ typedef enum IPv6LinkLocalAddressGenMode {
 } IPv6LinkLocalAddressGenMode;
 
 bool link_ipv6ll_enabled(Link *link);
-bool link_may_have_ipv6ll(Link *link, bool check_multicast);
+bool link_ipv6ll_enabled_harder(Link *link);
 
 IPv6LinkLocalAddressGenMode link_get_ipv6ll_addrgen_mode(Link *link);
 int ipv6ll_addrgen_mode_fill_message(sd_netlink_message *message, IPv6LinkLocalAddressGenMode mode);

src/network/networkd-link.c

@@ -133,7 +133,7 @@ bool link_ipv6_enabled(Link *link) {
         if (link->network->bond)
                 return false;
 
-        if (link_may_have_ipv6ll(link, /* check_multicast = */ false))
+        if (link_ipv6ll_enabled(link))
                 return true;
 
         if (network_has_static_ipv6_configurations(link->network))
@@ -2126,6 +2126,17 @@ bool link_has_carrier(Link *link) {
         return netif_has_carrier(link->kernel_operstate, link->flags);
 }
 
+bool link_multicast_enabled(Link *link) {
+        assert(link);
+
+        /* If Multicast= is specified, use the value. */
+        if (link->network && link->network->multicast >= 0)
+                return link->network->multicast;
+
+        /* Otherwise, return the current state. */
+        return FLAGS_SET(link->flags, IFF_MULTICAST);
+}
+
 #define FLAG_STRING(string, flag, old, new)                      \
         (((old ^ new) & flag)                                    \
          ? ((old & flag) ? (" -" string) : (" +" string))        \

src/network/networkd-link.h

@@ -229,6 +229,7 @@ void link_check_ready(Link *link);
 void link_update_operstate(Link *link, bool also_update_bond_master);
 
 bool link_has_carrier(Link *link);
+bool link_multicast_enabled(Link *link);
 
 bool link_ipv6_enabled(Link *link);
 int link_ipv6ll_gained(Link *link);

src/network/networkd-ndisc.c

@@ -65,7 +65,10 @@ bool link_ndisc_enabled(Link *link) {
         if (!link->network)
                 return false;
 
-        if (!link_may_have_ipv6ll(link, /* check_multicast = */ true))
+        if (!link_multicast_enabled(link))
+                return false;
+
+        if (!link_ipv6ll_enabled_harder(link))
                 return false;
 
         /* Honor explicitly specified value. */

src/network/networkd-radv.c

@@ -31,10 +31,10 @@
 bool link_radv_enabled(Link *link) {
         assert(link);
 
-        if (!link_may_have_ipv6ll(link, /* check_multicast = */ true))
+        if (!link_multicast_enabled(link))
                 return false;
 
-        if (link->hw_addr.length != ETH_ALEN)
+        if (!link_ipv6ll_enabled_harder(link))
                 return false;
 
         return link->network->router_prefix_delegation;