net: Separate out SKB validation logic from transmit path.

dev_hard_start_xmit() does two things, it first validates and
canonicalizes the SKB, then it actually sends it.

Make a set of helper functions for doing the first part.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2644,32 +2644,35 @@ static struct sk_buff *xmit_list(struct sk_buff *first, struct net_device *dev,
 	return skb;
 }
 
-int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
-			struct netdev_queue *txq)
+struct sk_buff *validate_xmit_vlan(struct sk_buff *skb, netdev_features_t features)
 {
-	int rc = NETDEV_TX_OK;
+	if (vlan_tx_tag_present(skb) &&
+	    !vlan_hw_offload_capable(features, skb->vlan_proto)) {
+		skb = __vlan_put_tag(skb, skb->vlan_proto,
+				     vlan_tx_tag_get(skb));
+		if (skb)
+			skb->vlan_tci = 0;
+	}
+	return skb;
+}
 
-	if (likely(!skb->next)) {
+static struct sk_buff *validate_xmit_skb(struct sk_buff *skb, struct net_device *dev)
+{
 	netdev_features_t features;
 
-		/*
-		 * If device doesn't need skb->dst, release it right now while
+	if (skb->next)
+		return skb;
+
+	/* If device doesn't need skb->dst, release it right now while
 	 * its hot in this cpu cache
 	 */
 	if (dev->priv_flags & IFF_XMIT_DST_RELEASE)
 		skb_dst_drop(skb);
 
 	features = netif_skb_features(skb);
-
-		if (vlan_tx_tag_present(skb) &&
-		    !vlan_hw_offload_capable(features, skb->vlan_proto)) {
-			skb = __vlan_put_tag(skb, skb->vlan_proto,
-					     vlan_tx_tag_get(skb));
+	skb = validate_xmit_vlan(skb, features);
 	if (unlikely(!skb))
-				goto out;
-
-			skb->vlan_tci = 0;
-		}
+		goto out_null;
 
 	/* If encapsulation offload request, verify we are testing
 	 * hardware encapsulation features instead of standard
@@ -2681,8 +2684,6 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 	if (netif_needs_gso(skb, features)) {
 		if (unlikely(dev_gso_segment(skb, features)))
 			goto out_kfree_skb;
-			if (skb->next)
-				goto gso;
 	} else {
 		if (skb_needs_linearize(skb, features) &&
 		    __skb_linearize(skb))
@@ -2705,19 +2706,35 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 		}
 	}
 
+	return skb;
+
+out_kfree_skb:
+	kfree_skb(skb);
+out_null:
+	return NULL;
+}
+
+int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
+			struct netdev_queue *txq)
+{
+	int rc = NETDEV_TX_OK;
+
+	skb = validate_xmit_skb(skb, dev);
+	if (!skb)
+		return rc;
+
+	if (likely(!skb->next))
 		return xmit_one(skb, dev, txq, false);
-	}
 
-gso:
 	skb->next = xmit_list(skb->next, dev, txq, &rc);
 	if (likely(skb->next == NULL)) {
 		skb->destructor = DEV_GSO_CB(skb)->destructor;
 		consume_skb(skb);
 		return rc;
 	}
-out_kfree_skb:
+
 	kfree_skb(skb);
-out:
+
 	return rc;
 }
 EXPORT_SYMBOL_GPL(dev_hard_start_xmit);