net: Add function for parsing the header length out of linear ethernet frames

This patch updates some of the flow_dissector api so that it can be used to
parse the length of ethernet buffers stored in fragments.  Most of the
changes needed were to __skb_get_poff as it needed to be updated to support
sending a linear buffer instead of a skb.

I have split __skb_get_poff into two functions, the first is skb_get_poff
and it retains the functionality of the original __skb_get_poff.  The other
function is __skb_get_poff which now works much like __skb_flow_dissect in
relation to skb_flow_dissect in that it provides the same functionality but
works with just a data buffer and hlen instead of needing an skb.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ethernet/eth.c

@@ -145,6 +145,33 @@ int eth_rebuild_header(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(eth_rebuild_header);
 
+/**
+ * eth_get_headlen - determine the the length of header for an ethernet frame
+ * @data: pointer to start of frame
+ * @len: total length of frame
+ *
+ * Make a best effort attempt to pull the length for all of the headers for
+ * a given frame in a linear buffer.
+ */
+u32 eth_get_headlen(void *data, unsigned int len)
+{
+	const struct ethhdr *eth = (const struct ethhdr *)data;
+	struct flow_keys keys;
+
+	/* this should never happen, but better safe than sorry */
+	if (len < sizeof(*eth))
+		return len;
+
+	/* parse any remaining L2/L3 headers, check for L4 */
+	if (!__skb_flow_dissect(NULL, &keys, data,
+				eth->h_proto, sizeof(*eth), len))
+		return max_t(u32, keys.thoff, sizeof(*eth));
+
+	/* parse for any L4 headers */
+	return min_t(u32, __skb_get_poff(NULL, data, &keys, len), len);
+}
+EXPORT_SYMBOL(eth_get_headlen);
+
 /**
  * eth_type_trans - determine the packet's protocol ID.
  * @skb: received socket data