Add --fd to pass existing FDs to read/write packets

This allows for ex qemu management layer to setup a socketpair and
connect an interface with slirpnetstack thanks to -netdev socket,fd=.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

Author: elmarco

main.go

@@ -16,6 +16,7 @@ import (
 )
 
 var (
+	fd             int
 	netNsPath      string
 	ifName         string
 	remoteFwd      FwdAddrSlice
@@ -25,6 +26,7 @@ var (
 )
 
 func init() {
+	flag.IntVar(&fd, "fd", -1, "Unix datagram socket file descriptor")
 	flag.StringVar(&netNsPath, "netns", "", "path to network namespace")
 	flag.StringVar(&ifName, "interface", "tun0", "interface name within netns")
 	flag.Var(&remoteFwd, "R", "Connections to remote side forwarded local")

qemu-run.py

@@ -0,0 +1,15 @@
+import subprocess
+import socket
+import os
+import sys
+from shlex import split, quote
+
+a, b = socket.socketpair(socket.AF_UNIX, socket.SOCK_DGRAM)
+
+slirp_cmd = 'bin/slirpnetstack --pcap /tmp/slirpnetstack-%d.pcap --fd %d' % (os.getpid(), a.fileno())
+
+qemu_cmd = 'qemu-system-x86_64 -enable-kvm -smp 4 -m 4G -cpu host -device virtio-net-pci,netdev=net0 -netdev socket,fd=%d,id=net0 -snapshot %s' % (b.fileno(), quote(sys.argv[1]))
+
+slirp = subprocess.Popen(split(slirp_cmd), pass_fds=[a.fileno()])
+qemu = subprocess.Popen(split(qemu_cmd), pass_fds=[b.fileno()])
+qemu.wait()