Bump the AF_PACKET socket rcv buf size to 4MB by default.

Packet socket receive buffers default to the sysctl value of
net.core.rmem_default and are capped by net.core.rmem_max both
which are usually set to 208KB on most systems.

Since we can't expect every gVisor user to bump these we use
SO_RCVBUFFORCE to exceed the limit. This is possible as runsc runs
with CAP_NET_ADMIN outside the sandbox and can do this before
the FD is passed to the sentry inside the sandbox.

Updates #211

iperf output w/ 4MB buffer.

 iperf3 -c 172.17.0.2 -t 100
 Connecting to host 172.17.0.2, port 5201
 [  4] local 172.17.0.1 port 40378 connected to 172.17.0.2 port 5201
 [ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
 [  4]   0.00-1.00   sec  1.15 GBytes  9.89 Gbits/sec    0   1.02 MBytes
 [  4]   1.00-2.00   sec  1.18 GBytes  10.2 Gbits/sec    0   1.02 MBytes
 [  4]   2.00-3.00   sec   965 MBytes  8.09 Gbits/sec    0   1.02 MBytes
 [  4]   3.00-4.00   sec   942 MBytes  7.90 Gbits/sec    0   1.02 MBytes
 [  4]   4.00-5.00   sec   952 MBytes  7.99 Gbits/sec    0   1.02 MBytes
 [  4]   5.00-6.00   sec  1.14 GBytes  9.81 Gbits/sec    0   1.02 MBytes
 [  4]   6.00-7.00   sec  1.13 GBytes  9.68 Gbits/sec    0   1.02 MBytes
 [  4]   7.00-8.00   sec   930 MBytes  7.80 Gbits/sec    0   1.02 MBytes
 [  4]   8.00-9.00   sec  1.15 GBytes  9.91 Gbits/sec    0   1.02 MBytes
 [  4]   9.00-10.00  sec   938 MBytes  7.87 Gbits/sec    0   1.02 MBytes
 [  4]  10.00-11.00  sec   737 MBytes  6.18 Gbits/sec    0   1.02 MBytes
 [  4]  11.00-12.00  sec  1.16 GBytes  9.93 Gbits/sec    0   1.02 MBytes
 [  4]  12.00-13.00  sec   917 MBytes  7.69 Gbits/sec    0   1.02 MBytes
 [  4]  13.00-14.00  sec  1.19 GBytes  10.2 Gbits/sec    0   1.02 MBytes
 [  4]  14.00-15.00  sec  1.01 GBytes  8.70 Gbits/sec    0   1.02 MBytes
 [  4]  15.00-16.00  sec  1.20 GBytes  10.3 Gbits/sec    0   1.02 MBytes
 [  4]  16.00-17.00  sec  1.14 GBytes  9.80 Gbits/sec    0   1.02 MBytes
 ^C[  4]  17.00-17.60  sec   718 MBytes  10.1 Gbits/sec    0   1.02 MBytes
 - - - - - - - - - - - - - - - - - - - - - - - - -
 [ ID] Interval           Transfer     Bandwidth       Retr
 [  4]   0.00-17.60  sec  18.4 GBytes  8.98 Gbits/sec    0             sender
 [  4]   0.00-17.60  sec  0.00 Bytes  0.00 bits/sec                  receiver

PiperOrigin-RevId: 245470590
Change-Id: I1c08c5ee8345de6ac070513656a4703312dc3c00

pkg/tcpip/link/fdbased/endpoint.go

@@ -197,8 +197,9 @@ func New(opts *Options) (tcpip.LinkEndpointID, error) {
 			return stack.RegisterLinkEndpoint(e), nil
 
 		case RecvMMsg:
-			// If the provided FD is a socket then we optimize packet reads by
-			// using recvmmsg() instead of read() to read packets in a batch.
+			// If the provided FD is a socket then we optimize
+			// packet reads by using recvmmsg() instead of read() to
+			// read packets in a batch.
 			e.inboundDispatcher = e.recvMMsgDispatch
 			msgsPerRecv = MaxMsgsPerRecv
 		}

runsc/sandbox/network.go

@@ -262,6 +262,17 @@ func createInterfacesAndRoutesFromNS(conn *urpc.Client, nsPath string, enableGSO
 			}
 		}
 
+		// Use SO_RCVBUFFORCE because on linux the receive buffer for an
+		// AF_PACKET socket is capped by "net.core.rmem_max". rmem_max
+		// defaults to a unusually low value of 208KB. This is too low
+		// for gVisor to be able to receive packets at high throughputs
+		// without incurring packet drops.
+		const rcvBufSize = 4 << 20 // 4MB.
+
+		if err := syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_RCVBUFFORCE, rcvBufSize); err != nil {
+			return fmt.Errorf("failed to increase socket rcv buffer to %d: %v", rcvBufSize, err)
+		}
+
 		// Collect the addresses for the interface, enable forwarding,
 		// and remove them from the host.
 		for _, addr := range ip4addrs {