net: Add flow_keys digest

Some users of flow keys (well just sch_choke now) need to pass
flow_keys in skbuff cb, and use them for exact comparisons of flows
so that skb->hash is not sufficient. In order to increase size of
the flow_keys structure, we introduce another structure for
the purpose of passing flow keys in skbuff cb. We limit this structure
to sixteen bytes, and we will technically treat this as a digest of
flow_keys struct hence its name flow_keys_digest. In the first
incaranation we just copy the flow_keys structure up to 16 bytes--
this is the same information previously passed in the cb. In the
future, we'll adapt this for larger flow_keys and could use something
like SHA-1 over the whole flow_keys to improve the quality of the
digest.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/flow_keys.h

@@ -42,4 +42,20 @@ static inline __be32 skb_flow_get_ports(const struct sk_buff *skb, int thoff, u8
 u32 flow_hash_from_keys(struct flow_keys *keys);
 unsigned int flow_get_hlen(const unsigned char *data, unsigned int max_len,
 			   __be16 protocol);
+
+/* struct flow_keys_digest:
+ *
+ * This structure is used to hold a digest of the full flow keys. This is a
+ * larger "hash" of a flow to allow definitively matching specific flows where
+ * the 32 bit skb->hash is not large enough. The size is limited to 16 bytes so
+ * that it can by used in CB of skb (see sch_choke for an example).
+ */
+#define FLOW_KEYS_DIGEST_LEN	16
+struct flow_keys_digest {
+	u8	data[FLOW_KEYS_DIGEST_LEN];
+};
+
+void make_flow_keys_digest(struct flow_keys_digest *digest,
+			   const struct flow_keys *flow);
+
 #endif

net/core/flow_dissector.c

@@ -310,6 +310,33 @@ static inline u32 ___skb_get_hash(const struct sk_buff *skb,
 	return __flow_hash_from_keys(keys, keyval);
 }
 
+struct _flow_keys_digest_data {
+	__be16	n_proto;
+	u8	ip_proto;
+	u8	padding;
+	__be32	ports;
+	__be32	src;
+	__be32	dst;
+};
+
+void make_flow_keys_digest(struct flow_keys_digest *digest,
+			   const struct flow_keys *flow)
+{
+	struct _flow_keys_digest_data *data =
+	    (struct _flow_keys_digest_data *)digest;
+
+	BUILD_BUG_ON(sizeof(*data) > sizeof(*digest));
+
+	memset(digest, 0, sizeof(*digest));
+
+	data->n_proto = flow->n_proto;
+	data->ip_proto = flow->ip_proto;
+	data->ports = flow->ports;
+	data->src = flow->src;
+	data->dst = flow->dst;
+}
+EXPORT_SYMBOL(make_flow_keys_digest);
+
 /*
  * __skb_get_hash: calculate a flow hash based on src/dst addresses
  * and src/dst port numbers.  Sets hash in skb to non-zero hash value