packet: add sockopt to ignore outgoing packets

Currently, the only way to ignore outgoing packets on a packet socket is
via the BPF filter.  With MSG_ZEROCOPY, packets that are looped into
AF_PACKET are copied in dev_queue_xmit_nit(), and this copy happens even
if the filter run from packet_rcv() would reject them.  So the presence
of a packet socket on the interface takes away the benefits of
MSG_ZEROCOPY, even if the packet socket is not interested in outgoing
packets.  (Even when MSG_ZEROCOPY is not used, the skb is unnecessarily
cloned, but the cost for that is much lower.)

Add a socket option to allow AF_PACKET sockets to ignore outgoing
packets to solve this.  Note that the *BSDs already have something
similar: BIOCSSEESENT/BIOCSDIRECTION and BIOCSDIRFILT.

The first intended user is lldpd.
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -2343,6 +2343,7 @@ static inline struct sk_buff *call_gro_receive_sk(gro_receive_sk_t cb,
 
 struct packet_type {
 	__be16			type;	/* This is really htons(ether_type). */
+	bool			ignore_outgoing;
 	struct net_device	*dev;	/* NULL is wildcarded here	     */
 	int			(*func) (struct sk_buff *,
 					 struct net_device *,

include/uapi/linux/if_packet.h

@@ -57,6 +57,7 @@ struct sockaddr_ll {
 #define PACKET_QDISC_BYPASS		20
 #define PACKET_ROLLOVER_STATS		21
 #define PACKET_FANOUT_DATA		22
+#define PACKET_IGNORE_OUTGOING		23
 
 #define PACKET_FANOUT_HASH		0
 #define PACKET_FANOUT_LB		1

net/core/dev.c

@@ -1969,6 +1969,9 @@ void dev_queue_xmit_nit(struct sk_buff *skb, struct net_device *dev)
 	rcu_read_lock();
 again:
 	list_for_each_entry_rcu(ptype, ptype_list, list) {
+		if (ptype->ignore_outgoing)
+			continue;
+
 		/* Never send packets back to the socket
 		 * they originated from - MvS (miquels@drinkel.ow.org)
 		 */

net/packet/af_packet.c

@@ -3805,6 +3805,20 @@ packet_setsockopt(struct socket *sock, int level, int optname, char __user *optv
 
 		return fanout_set_data(po, optval, optlen);
 	}
+	case PACKET_IGNORE_OUTGOING:
+	{
+		int val;
+
+		if (optlen != sizeof(val))
+			return -EINVAL;
+		if (copy_from_user(&val, optval, sizeof(val)))
+			return -EFAULT;
+		if (val < 0 || val > 1)
+			return -EINVAL;
+
+		po->prot_hook.ignore_outgoing = !!val;
+		return 0;
+	}
 	case PACKET_TX_HAS_OFF:
 	{
 		unsigned int val;
@@ -3928,6 +3942,9 @@ static int packet_getsockopt(struct socket *sock, int level, int optname,
 			((u32)po->fanout->flags << 24)) :
 		       0);
 		break;
+	case PACKET_IGNORE_OUTGOING:
+		val = po->prot_hook.ignore_outgoing;
+		break;
 	case PACKET_ROLLOVER_STATS:
 		if (!po->rollover)
 			return -EINVAL;