ipv6: Simplify link-local address generation for IPv6 GRE.

Since commit 3e6a024 ("gre: Fix again IPv6 link-local address
generation."), addrconf_gre_config() has stopped handling IP6GRE
devices specially and just calls the regular addrconf_addr_gen()
function to create their link-local IPv6 addresses.

We can thus avoid using addrconf_gre_config() for IP6GRE devices and
use the normal IPv6 initialisation path instead (that is, jump directly
to addrconf_dev_config() in addrconf_init_auto_addrs()).

See commit 3e6a024 ("gre: Fix again IPv6 link-local address
generation.") for a deeper explanation on how and why GRE devices
started handling their IPv6 link-local address generation specially,
why it was a problem, and why this is not even necessary in most cases
(especially for GRE over IPv6).

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/a9144be9c7ec3cf09f25becae5e8fdf141fde9f6.1750075076.git.gnault@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Guillaume Nault

net/ipv6/addrconf.c

@@ -3208,7 +3208,7 @@ static void add_addr(struct inet6_dev *idev, const struct in6_addr *addr,
 	}
 }
 
-#if IS_ENABLED(CONFIG_IPV6_SIT) || IS_ENABLED(CONFIG_NET_IPGRE) || IS_ENABLED(CONFIG_IPV6_GRE)
+#if IS_ENABLED(CONFIG_IPV6_SIT) || IS_ENABLED(CONFIG_NET_IPGRE)
 static void add_v4_addrs(struct inet6_dev *idev)
 {
 	struct in6_addr addr;
@@ -3463,6 +3463,7 @@ static void addrconf_dev_config(struct net_device *dev)
 	    (dev->type != ARPHRD_IEEE1394) &&
 	    (dev->type != ARPHRD_TUNNEL6) &&
 	    (dev->type != ARPHRD_6LOWPAN) &&
+	    (dev->type != ARPHRD_IP6GRE) &&
 	    (dev->type != ARPHRD_TUNNEL) &&
 	    (dev->type != ARPHRD_NONE) &&
 	    (dev->type != ARPHRD_RAWIP)) {
@@ -3518,7 +3519,7 @@ static void addrconf_sit_config(struct net_device *dev)
 }
 #endif
 
-#if IS_ENABLED(CONFIG_NET_IPGRE) || IS_ENABLED(CONFIG_IPV6_GRE)
+#if IS_ENABLED(CONFIG_NET_IPGRE)
 static void addrconf_gre_config(struct net_device *dev)
 {
 	struct inet6_dev *idev;
@@ -3536,7 +3537,7 @@ static void addrconf_gre_config(struct net_device *dev)
 	 * which is in EUI64 mode (as __ipv6_isatap_ifid() would fail in this
 	 * case). Such devices fall back to add_v4_addrs() instead.
 	 */
-	if (!(dev->type == ARPHRD_IPGRE && *(__be32 *)dev->dev_addr == 0 &&
+	if (!(*(__be32 *)dev->dev_addr == 0 &&
 	      idev->cnf.addr_gen_mode == IN6_ADDR_GEN_MODE_EUI64)) {
 		addrconf_addr_gen(idev, true);
 		return;
@@ -3557,8 +3558,7 @@ static void addrconf_init_auto_addrs(struct net_device *dev)
 		addrconf_sit_config(dev);
 		break;
 #endif
-#if IS_ENABLED(CONFIG_NET_IPGRE) || IS_ENABLED(CONFIG_IPV6_GRE)
-	case ARPHRD_IP6GRE:
+#if IS_ENABLED(CONFIG_NET_IPGRE)
 	case ARPHRD_IPGRE:
 		addrconf_gre_config(dev);
 		break;