net/packet: add PACKET_FANOUT_FLAG_IGNORE_OUTGOING

Extend packet socket option PACKET_IGNORE_OUTGOING to fanout groups.

The socket option sets ptype.ignore_outgoing, which makes
dev_queue_xmit_nit skip the socket.

When the socket joins a fanout group, the option is not reflected in
the struct ptype of the group. dev_queue_xmit_nit only tests the
fanout ptype, so the flag is ignored once a socket joins a
fanout group.

Inheriting the option from a socket would change established behavior.
Different sockets in the group can set different flags, and can also
change them at runtime.

Testing in packet_rcv_fanout defeats the purpose of the original
patch, which is to avoid skb_clone in dev_queue_xmit_nit (esp. for
MSG_ZEROCOPY packets).

Instead, introduce a new fanout group flag with the same behavior.

Tested with https://github.com/wdebruij/kerneltools/blob/master/tests/test_psock_fanout_ignore_outgoing.cSigned-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20221027211014.3581513-1-willemdebruijn.kernel@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/if_packet.h

@@ -70,6 +70,7 @@ struct sockaddr_ll {
 #define PACKET_FANOUT_EBPF		7
 #define PACKET_FANOUT_FLAG_ROLLOVER	0x1000
 #define PACKET_FANOUT_FLAG_UNIQUEID	0x2000
+#define PACKET_FANOUT_FLAG_IGNORE_OUTGOING     0x4000
 #define PACKET_FANOUT_FLAG_DEFRAG	0x8000
 
 struct tpacket_stats {

net/packet/af_packet.c

@@ -1777,6 +1777,7 @@ static int fanout_add(struct sock *sk, struct fanout_args *args)
 		match->prot_hook.af_packet_net = read_pnet(&match->net);
 		match->prot_hook.id_match = match_fanout_group;
 		match->max_num_members = args->max_num_members;
+		match->prot_hook.ignore_outgoing = type_flags & PACKET_FANOUT_FLAG_IGNORE_OUTGOING;
 		list_add(&match->list, &fanout_list);
 	}
 	err = -EINVAL;