merd: Switch to AF_UNIX for qemu tap, provide wrapper

We can bypass a full-fledged network interface between qemu and merd by
connecting the qemu tap file descriptor to a provided UNIX domain
socket: this could be implemented in qemu eventually, qrap covers this
meanwhile.

This also avoids the need for the AF_PACKET socket towards the guest.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

qrap.c

@@ -0,0 +1,79 @@
+/* MERD - MacVTap Egress and Routing Daemon
+ *
+ * qrap.c - qemu wrapper connecting UNIX domain socket to tap file descriptor
+ *
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ * License: GPLv2
+ *
+ * TODO: Implement this functionality directly in qemu: we have TCP and UDP
+ * socket back-ends already.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <errno.h>
+#include <limits.h>
+
+#include "merd.h"
+
+/**
+ * usage() - Print usage and exit
+ * @name:	Executable name
+ */
+void usage(const char *name)
+{
+	fprintf(stderr, "Usage: %s FDNUM QEMU_CMD ...\n", name);
+
+	exit(EXIT_FAILURE);
+}
+
+/**
+ * main() - Entry point and main loop
+ * @argc:	Argument count
+ * @argv:	File descriptor number, then qemu with arguments
+ *
+ * Return: 0 once interrupted, non-zero on failure
+ */
+int main(int argc, char **argv)
+{
+	struct sockaddr_un addr = {
+		.sun_family = AF_UNIX,
+		.sun_path = UNIX_SOCK_PATH,
+	};
+	long fd;
+	int s;
+
+	if (argc < 3)
+		usage(argv[0]);
+
+	fd = strtol(argv[1], NULL, 0);
+	if (fd < 3 || fd > INT_MAX || errno)
+		usage(argv[0]);
+
+	s = socket(AF_UNIX, SOCK_STREAM, 0);
+	if (s < 0) {
+		perror("socket");
+		exit(EXIT_FAILURE);
+	}
+
+	if (connect(s, (const struct sockaddr *)&addr, sizeof(addr)) < 0) {
+		perror("connect");
+		exit(EXIT_FAILURE);
+	}
+
+	if (dup2(s, (int)fd) < 0) {
+		perror("dup");
+		exit(EXIT_FAILURE);
+	}
+
+	close(s);
+
+	execvp(argv[2], argv + 2);
+	perror("execvp");
+
+	return EXIT_FAILURE;
+}