Support BESS protocol (for UML)

[AkihiroSuda]

BESS protocol transferrs L2 packets as AF_UNIX SOCK_SEQPACKET .
BESS protocol has been used by the vector network interfaces of User Mode Linux (UML).

(terminal 1) $ slirp4netns --target-type=bess /tmp/bess.sock
(terminal 2) $ linux.uml vec0:transport=bess,dst=/tmp/bess.sock,depth=128,gro=1 root=/dev/root rootfstype=hostfs init=/bin/bash mem=2G
(terminal 2: UML)$ ip addr add 10.0.2.100/24 dev vec0
(terminal 2: UML)$ ip link set vec0 up
(terminal 2: UML)$ ip route add default via 10.0.2.2

More docs about the User Mode Linux with BESS socket transport: https://www.kernel.org/doc/html/latest/virt/uml/user_mode_linux_howto_v2.html#bess-socket-transport

[AkihiroSuda]

cc @giuseppe @elmarco FYI

[giuseppe]

I am not familiar with BESS but the code LGTM

Just left two minor comments

[giuseppe]

should this go to stderr?

[AkihiroSuda]

We have been using stdout or printing WARNING messages (why? 😅 )

slirp4netns/main.c

Lines 563 to 604 in f2ae176

	case _DEPRECATED_NO_HOST_LOOPBACK:
	// There was no tagged release with support for --no-host-loopback.
	// So no one will be affected by removal of --no-host-loopback.
	printf("WARNING: --no-host-loopback is deprecated and will be "
	"removed in future releases, please use "
	"--disable-host-loopback instead.\n");
	/* FALLTHROUGH */
	case DISABLE_HOST_LOOPBACK:
	options->disable_host_loopback = true;
	break;
	case _DEPRECATED_CREATE_SANDBOX:
	// There was no tagged release with support for --create-sandbox.
	// So no one will be affected by removal of --create-sandbox.
	printf("WARNING: --create-sandbox is deprecated and will be "
	"removed in future releases, please use "
	"--enable-sandbox instead.\n");
	/* FALLTHROUGH */
	case ENABLE_SANDBOX:
	options->enable_sandbox = true;
	break;
	case ENABLE_SECCOMP:
	printf("WARNING: Support for seccomp is experimental\n");
	options->enable_seccomp = true;
	break;
	case NETNS_TYPE:
	optarg_netns_type = optarg;
	break;
	case USERNS_PATH:
	optarg_userns_path = optarg;
	if (access(optarg_userns_path, F_OK) == -1) {
	fprintf(stderr, "userns path doesn't exist: %s\n",
	optarg_userns_path);
	goto error;
	}
	break;
	case 'a':
	optarg_api_socket = optarg;
	break;
	case '6':
	options->enable_ipv6 = true;
	printf("WARNING: Support for IPv6 is experimental\n");
	break;

[giuseppe]

we are using both stdout and stderr for diagnostic messages. Should we just stick to stderr?

[AkihiroSuda]

fixed

[AkihiroSuda]

Step 9/19 : RUN  git pull && git checkout ${LIBSLIRP_COMMIT} && meson setup build && ninja -C build install
 ---> Running in f5c0822a3dc4
fatal: unable to access 'https://gitlab.freedesktop.org/slirp/libslirp.git/': The requested URL returned error: 504
The command '/bin/sh -c git pull && git checkout ${LIBSLIRP_COMMIT} && meson setup build && ninja -C build install' returned a non-zero code: 1

😢
https://github.com/rootless-containers/slirp4netns/runs/4723883332?check_suite_focus=true