net: core: add MPLS update core helper and use in OvS

Open vSwitch allows the updating of an existing MPLS header on a packet.
In preparation for supporting similar functionality in TC, move this to a
common skb helper function.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -5531,6 +5531,39 @@ int skb_mpls_pop(struct sk_buff *skb, __be16 next_proto)
 }
 EXPORT_SYMBOL_GPL(skb_mpls_pop);
 
+/**
+ * skb_mpls_update_lse() - modify outermost MPLS header and update csum
+ *
+ * @skb: buffer
+ * @mpls_lse: new MPLS label stack entry to update to
+ *
+ * Expects skb->data at mac header.
+ *
+ * Returns 0 on success, -errno otherwise.
+ */
+int skb_mpls_update_lse(struct sk_buff *skb, __be32 mpls_lse)
+{
+	int err;
+
+	if (unlikely(!eth_p_mpls(skb->protocol)))
+		return -EINVAL;
+
+	err = skb_ensure_writable(skb, skb->mac_len + MPLS_HLEN);
+	if (unlikely(err))
+		return err;
+
+	if (skb->ip_summed == CHECKSUM_COMPLETE) {
+		__be32 diff[] = { ~mpls_hdr(skb)->label_stack_entry, mpls_lse };
+
+		skb->csum = csum_partial((char *)diff, sizeof(diff), skb->csum);
+	}
+
+	mpls_hdr(skb)->label_stack_entry = mpls_lse;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(skb_mpls_update_lse);
+
 /**
  * alloc_skb_with_frags - allocate skb with page frags
  *

net/openvswitch/actions.c

@@ -193,19 +193,12 @@ static int set_mpls(struct sk_buff *skb, struct sw_flow_key *flow_key,
 	__be32 lse;
 	int err;
 
-	err = skb_ensure_writable(skb, skb->mac_len + MPLS_HLEN);
-	if (unlikely(err))
-		return err;
-
 	stack = mpls_hdr(skb);
 	lse = OVS_MASKED(stack->label_stack_entry, *mpls_lse, *mask);
-	if (skb->ip_summed == CHECKSUM_COMPLETE) {
-		__be32 diff[] = { ~(stack->label_stack_entry), lse };
+	err = skb_mpls_update_lse(skb, lse);
+	if (err)
+		return err;
 
-		skb->csum = csum_partial((char *)diff, sizeof(diff), skb->csum);
-	}
-
-	stack->label_stack_entry = lse;
 	flow_key->mpls.top_lse = lse;
 	return 0;
 }