rtnetlink: Move input checking for rtnl_fdb_dump to helper

Move the existing input checking for rtnl_fdb_dump into a helper,
valid_fdb_dump_legacy. This function will retain the current
logic that works around the 2 headers that userspace has been
allowed to send up to this point.
Signed-off-by: NDavid Ahern <dsahern@gmail.com>
Acked-by: NChristian Brauner <christian@brauner.io>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -3799,22 +3799,13 @@ int ndo_dflt_fdb_dump(struct sk_buff *skb,
 }
 EXPORT_SYMBOL(ndo_dflt_fdb_dump);
 
-static int rtnl_fdb_dump(struct sk_buff *skb, struct netlink_callback *cb)
+static int valid_fdb_dump_legacy(const struct nlmsghdr *nlh,
+				 int *br_idx, int *brport_idx,
+				 struct netlink_ext_ack *extack)
 {
-	struct net_device *dev;
+	struct ifinfomsg *ifm = nlmsg_data(nlh);
 	struct nlattr *tb[IFLA_MAX+1];
-	struct net_device *br_dev = NULL;
-	const struct net_device_ops *ops = NULL;
-	const struct net_device_ops *cops = NULL;
-	struct ifinfomsg *ifm = nlmsg_data(cb->nlh);
-	struct net *net = sock_net(skb->sk);
-	struct hlist_head *head;
-	int brport_idx = 0;
-	int br_idx = 0;
-	int h, s_h;
-	int idx = 0, s_idx;
-	int err = 0;
-	int fidx = 0;
+	int err;
 
 	/* A hack to preserve kernel<->userspace interface.
 	 * Before Linux v4.12 this code accepted ndmsg since iproute2 v3.3.0.
@@ -3823,20 +3814,42 @@ static int rtnl_fdb_dump(struct sk_buff *skb, struct netlink_callback *cb)
 	 * Fortunately these sizes don't conflict with the size of ifinfomsg
 	 * with an optional attribute.
 	 */
-	if (nlmsg_len(cb->nlh) != sizeof(struct ndmsg) &&
-	    (nlmsg_len(cb->nlh) != sizeof(struct ndmsg) +
+	if (nlmsg_len(nlh) != sizeof(struct ndmsg) &&
+	    (nlmsg_len(nlh) != sizeof(struct ndmsg) +
 	     nla_attr_size(sizeof(u32)))) {
-		err = nlmsg_parse(cb->nlh, sizeof(struct ifinfomsg), tb,
-				  IFLA_MAX, ifla_policy, cb->extack);
+		err = nlmsg_parse(nlh, sizeof(struct ifinfomsg), tb, IFLA_MAX,
+				  ifla_policy, extack);
 		if (err < 0) {
 			return -EINVAL;
 		} else if (err == 0) {
 			if (tb[IFLA_MASTER])
-				br_idx = nla_get_u32(tb[IFLA_MASTER]);
+				*br_idx = nla_get_u32(tb[IFLA_MASTER]);
 		}
 
-		brport_idx = ifm->ifi_index;
+		*brport_idx = ifm->ifi_index;
 	}
+	return 0;
+}
+
+static int rtnl_fdb_dump(struct sk_buff *skb, struct netlink_callback *cb)
+{
+	struct net_device *dev;
+	struct net_device *br_dev = NULL;
+	const struct net_device_ops *ops = NULL;
+	const struct net_device_ops *cops = NULL;
+	struct net *net = sock_net(skb->sk);
+	struct hlist_head *head;
+	int brport_idx = 0;
+	int br_idx = 0;
+	int h, s_h;
+	int idx = 0, s_idx;
+	int err = 0;
+	int fidx = 0;
+
+	err = valid_fdb_dump_legacy(cb->nlh, &br_idx, &brport_idx,
+				    cb->extack);
+	if (err < 0)
+		return err;
 
 	if (br_idx) {
 		br_dev = __dev_get_by_index(net, br_idx);