filter: add SKF_AD_RXHASH and SKF_AD_CPU

Add SKF_AD_RXHASH and SKF_AD_CPU to filter ancillary mechanism,
to be able to build advanced filters.

This can help spreading packets on several sockets with a fast
selection, after RPS dispatch to N cpus for example, or to catch a
percentage of flows in one queue.

tcpdump -s 500 "cpu = 1" :

[0] ld CPU
[1] jeq #1  jt 2  jf 3
[2] ret #500
[3] ret #0

# take 12.5 % of flows (average)
tcpdump -s 1000 "rxhash & 7 = 2" :

[0] ld RXHASH
[1] and #7
[2] jeq #2  jt 3  jf 4
[3] ret #1000
[4] ret #0

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Rui <wirelesser@gmail.com>
Acked-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/filter.h

@@ -124,7 +124,9 @@ struct sock_fprog {	/* Required for SO_ATTACH_FILTER. */
 #define SKF_AD_MARK 	20
 #define SKF_AD_QUEUE	24
 #define SKF_AD_HATYPE	28
-#define SKF_AD_MAX	32
+#define SKF_AD_RXHASH	32
+#define SKF_AD_CPU	36
+#define SKF_AD_MAX	40
 #define SKF_NET_OFF   (-0x100000)
 #define SKF_LL_OFF    (-0x200000)
 

net/core/filter.c

@@ -375,6 +375,12 @@ load_b:
 				return 0;
 			A = skb->dev->type;
 			continue;
+		case SKF_AD_RXHASH:
+			A = skb->rxhash;
+			continue;
+		case SKF_AD_CPU:
+			A = raw_smp_processor_id();
+			continue;
 		case SKF_AD_NLATTR: {
 			struct nlattr *nla;