Set socket protocol only after packet ring configured

When using the memory mapped packet receive path on Linux, we find
that if the buffer is "large" we get a number of dropped packets.
This number of packets is roughly proportional to the buffer size.
It turns out that these packets are dropped by the kernel in the time
period between the socket being opened and the ring parameter setup
completing (this is where the kernel has to allocate the buffer memory).

To prevent this, we can open the socket on protocol 0, which the kernel
interprets to mean that we don't want to receive any packets at all (this
is described when opening a memory mapped packet socket for TX only).  We
then set the correct protocol once everything is configured.

Signed-off-by: Mark Marshall <mark.marshall@omicronenergy.com>

pcap-linux.c

@@ -1323,17 +1323,21 @@ static void pcap_breakloop_linux(pcap_t *handle)
 static int
 open_pf_packet_socket(pcap_t *handle, int cooked)
 {
-	int	protocol = pcap_protocol(handle);
 	int	sock_fd, ret;
 
 	/*
 	 * Open a socket with protocol family packet. If cooked is true,
 	 * we open a SOCK_DGRAM socket for the cooked interface, otherwise
 	 * we open a SOCK_RAW socket for the raw interface.
+	 *
+	 * The protocol is set to 0.  This means we will receive no
+	 * packets until we "bind" the socket with a non-zero
+	 * protocol.  This allows us to setup the ring buffers without
+	 * dropping any packets.
 	 */
 	sock_fd = cooked ?
-		socket(PF_PACKET, SOCK_DGRAM, protocol) :
-		socket(PF_PACKET, SOCK_RAW, protocol);
+		socket(PF_PACKET, SOCK_DGRAM, 0) :
+		socket(PF_PACKET, SOCK_RAW, 0);
 
 	if (sock_fd == -1) {
 		if (errno == EPERM || errno == EACCES) {
@@ -1371,6 +1375,7 @@ pcap_activate_linux(pcap_t *handle)
 	int		is_any_device;
 	struct ifreq	ifr;
 	int		status = 0;
+	int		status2 = 0;
 	int		ret;
 
 	device = handle->opt.device;
@@ -1469,42 +1474,60 @@ pcap_activate_linux(pcap_t *handle)
 		 * Success.
 		 * Try to use memory-mapped access.
 		 */
-		switch (activate_mmap(handle, &status)) {
+		ret = activate_mmap(handle, &status);
+		if (ret == -1) {
+			/*
+			 * We failed to set up to use it, or the
+			 * kernel supports it, but we failed to
+			 * enable it.  status has been set to the
+			 * error status to return and, if it's
+			 * PCAP_ERROR, handle->errbuf contains
+			 * the error message.
+			 */
+			goto fail;
+		}
 
-		case 1:
+
+		if (ret == 1) {
 			/*
 			 * We succeeded.  status has been
 			 * set to the status to return,
 			 * which might be 0, or might be
 			 * a PCAP_WARNING_ value.
-			 *
-			 * Set the timeout to use in poll() before
-			 * returning.
 			 */
-			set_poll_timeout(handlep);
-			return status;
-
-		case 0:
 			/*
-			 * Kernel doesn't support it - just continue
-			 * with non-memory-mapped access.
+			 * Now that we have activated the mmap ring, we can
+			 * set the correct protocol.
 			 */
-			break;
-
-		case -1:
+			if ((status2 = iface_bind(handle->fd, handlep->ifindex,
+			     handle->errbuf, pcap_protocol(handle))) != 0) {
+				status = status2;
+				goto fail;
+			}
 			/*
-			 * We failed to set up to use it, or the
-			 * kernel supports it, but we failed to
-			 * enable it.  status has been set to the
-			 * error status to return and, if it's
-			 * PCAP_ERROR, handle->errbuf contains
-			 * the error message.
+			 * Set the timeout to use in poll() before
+			 * returning.
 			 */
-			goto fail;
+			set_poll_timeout(handlep);
+			return status;
 		}
+
+		/*
+		 * Kernel doesn't support it (mmap) - just continue
+		 * with non-memory-mapped access.
+		 */
 #endif /* HAVE_PACKET_RING */
 	}
 
+	/*
+	 * We need to set the correct protocol.
+	 */
+	if ((status2 = iface_bind(handle->fd, handlep->ifindex,
+	    handle->errbuf, pcap_protocol(handle))) != 0) {
+		status = status2;
+		goto fail;
+	}
+
 	/*
 	 * We set up the socket, but not with memory-mapped access.
 	 */
@@ -3358,7 +3381,7 @@ activate_sock(pcap_t *handle, int is_any_device)
 		}
 
 		if ((err = iface_bind(sock_fd, handlep->ifindex,
-		    handle->errbuf, pcap_protocol(handle))) != 0) {
+		    handle->errbuf, 0)) != 0) {
 			close(sock_fd);
 			return err;
 		}
@@ -5279,7 +5302,7 @@ iface_bind(int fd, int ifindex, char *ebuf, int protocol)
 
 	memset(&sll, 0, sizeof(sll));
 	sll.sll_family		= AF_PACKET;
-	sll.sll_ifindex		= ifindex;
+	sll.sll_ifindex		= ifindex < 0 ? 0 : ifindex;
 	sll.sll_protocol	= protocol;
 
 	if (bind(fd, (struct sockaddr *) &sll, sizeof(sll)) == -1) {