vlan: Add function to retrieve EtherType from vlan packets.

Depending on how a packet is vlan tagged (i.e. hardware accelerated or
not), the encapsulated protocol is stored in different locations. This
provides a consistent method of accessing that protocol, which is needed
by drivers, security checks, etc.

Signed-off-by: Hao Zheng <[e-mail]>
Signed-off-by: Jesse Gross <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/if_vlan.h

···
 	}
 }
 
+/**
+ * vlan_get_protocol - get protocol EtherType.
+ * @skb: skbuff to query
+ *
+ * Returns the EtherType of the packet, regardless of whether it is
+ * vlan encapsulated (normal or hardware accelerated) or not.
+ */
+static inline __be16 vlan_get_protocol(const struct sk_buff *skb)
+{
+	__be16 protocol = 0;
+
+	if (vlan_tx_tag_present(skb) ||
+	     skb->protocol != cpu_to_be16(ETH_P_8021Q))
+		protocol = skb->protocol;
+	else {
+		__be16 proto, *protop;
+		protop = skb_header_pointer(skb, offsetof(struct vlan_ethhdr,
+						h_vlan_encapsulated_proto),
+						sizeof(proto), &proto);
+		if (likely(protop))
+			protocol = *protop;
+	}
+
+	return protocol;
+}
 #endif /* __KERNEL__ */
 
 /* VLAN IOCTLs are found in sockios.h */