ipv4: nexthop: Correctly update nexthop group when replacing a nexthop

Each nexthop group contains an indication if it has IPv4 nexthops
('has_v4'). Its purpose is to prevent IPv6 routes from using groups with
IPv4 nexthops.

However, the indication is not updated when a nexthop is replaced. This
results in the kernel wrongly rejecting IPv6 routes from pointing to
groups that only contain IPv6 nexthops. Example:

# ip nexthop replace id 1 via 192.0.2.2 dev dummy10
# ip nexthop replace id 10 group 1
# ip nexthop replace id 1 via 2001:db8:1::2 dev dummy10
# ip route replace 2001:db8:10::/64 nhid 10
Error: IPv6 routes can not use an IPv4 nexthop.

Solve this by iterating over all the nexthop groups that the replaced
nexthop is a member of and potentially update their IPv4 indication
according to the new set of member nexthops.

Avoid wasting cycles by only performing the update in case an IPv4
nexthop is replaced by an IPv6 nexthop.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/nexthop.c

@@ -964,6 +964,23 @@ static int replace_nexthop_grp(struct net *net, struct nexthop *old,
 	return 0;
 }
 
+static void nh_group_v4_update(struct nh_group *nhg)
+{
+	struct nh_grp_entry *nhges;
+	bool has_v4 = false;
+	int i;
+
+	nhges = nhg->nh_entries;
+	for (i = 0; i < nhg->num_nh; i++) {
+		struct nh_info *nhi;
+
+		nhi = rtnl_dereference(nhges[i].nh->nh_info);
+		if (nhi->family == AF_INET)
+			has_v4 = true;
+	}
+	nhg->has_v4 = has_v4;
+}
+
 static int replace_nexthop_single(struct net *net, struct nexthop *old,
 				  struct nexthop *new,
 				  struct netlink_ext_ack *extack)
@@ -987,6 +1004,21 @@ static int replace_nexthop_single(struct net *net, struct nexthop *old,
 	rcu_assign_pointer(old->nh_info, newi);
 	rcu_assign_pointer(new->nh_info, oldi);
 
+	/* When replacing an IPv4 nexthop with an IPv6 nexthop, potentially
+	 * update IPv4 indication in all the groups using the nexthop.
+	 */
+	if (oldi->family == AF_INET && newi->family == AF_INET6) {
+		struct nh_grp_entry *nhge;
+
+		list_for_each_entry(nhge, &old->grp_list, nh_list) {
+			struct nexthop *nhp = nhge->nh_parent;
+			struct nh_group *nhg;
+
+			nhg = rtnl_dereference(nhp->nh_grp);
+			nh_group_v4_update(nhg);
+		}
+	}
+
 	return 0;
 }