net/packet: Add getsockopt support for PACKET_COPY_THRESH

Currently getsockopt does not support PACKET_COPY_THRESH,
and we are unable to get the value of PACKET_COPY_THRESH
socket option through getsockopt.

This patch adds getsockopt support for PACKET_COPY_THRESH.

In addition, this patch converts access to copy_thresh to
READ_ONCE/WRITE_ONCE.

Signed-off-by: Juntong Deng <[e-mail]>
Reviewed-by: Willem de Bruijn <[e-mail]>
Reviewed-by: Jason Xing <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/packet/af_packet.c

···
 	}
 	if (po->tp_version <= TPACKET_V2) {
 		if (macoff + snaplen > po->rx_ring.frame_size) {
-			if (po->copy_thresh &&
+			if (READ_ONCE(po->copy_thresh) &&
 			    atomic_read(&sk->sk_rmem_alloc) < sk->sk_rcvbuf) {
 				if (skb_shared(skb)) {
 					copy_skb = skb_clone(skb, GFP_ATOMIC);
···
 		if (copy_from_sockptr(&val, optval, sizeof(val)))
 			return -EFAULT;
 
-		pkt_sk(sk)->copy_thresh = val;
+		WRITE_ONCE(pkt_sk(sk)->copy_thresh, val);
 		return 0;
 	}
 	case PACKET_VERSION:
···
 		break;
 	case PACKET_VNET_HDR_SZ:
 		val = READ_ONCE(po->vnet_hdr_sz);
+		break;
+	case PACKET_COPY_THRESH:
+		val = READ_ONCE(pkt_sk(sk)->copy_thresh);
 		break;
 	case PACKET_VERSION:
 		val = po->tp_version;

net/packet/diag.c

···
 	pinfo.pdi_index = po->ifindex;
 	pinfo.pdi_version = po->tp_version;
 	pinfo.pdi_reserve = po->tp_reserve;
-	pinfo.pdi_copy_thresh = po->copy_thresh;
+	pinfo.pdi_copy_thresh = READ_ONCE(po->copy_thresh);
 	pinfo.pdi_tstamp = READ_ONCE(po->tp_tstamp);
 
 	pinfo.pdi_flags = 0;