passt, tap: Add --fd option

This passes a fully connected stream socket to passt.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
[sbrivio: reuse fd_tap instead of adding a new descriptor,
 imply --one-off on --fd, add to optstring and usage()]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

conf.c

@@ -719,6 +719,7 @@ static void usage(const char *name)
 		     UNIX_SOCK_PATH, 1);
 	}
 
+	info(   "  -F, --fd FD		Use FD as pre-opened connected socket");
 	info(   "  -p, --pcap FILE	Log tap-facing traffic to pcap file");
 	info(   "  -P, --pid FILE	Write own PID to the given file");
 	info(   "  -m, --mtu MTU	Assign MTU via DHCP/NDP");
@@ -1079,6 +1080,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		{"log-file",	required_argument,	NULL,		'l' },
 		{"help",	no_argument,		NULL,		'h' },
 		{"socket",	required_argument,	NULL,		's' },
+		{"fd",		required_argument,	NULL,		'F' },
 		{"ns-ifname",	required_argument,	NULL,		'I' },
 		{"pcap",	required_argument,	NULL,		'p' },
 		{"pid",		required_argument,	NULL,		'P' },
@@ -1138,9 +1140,9 @@ void conf(struct ctx *c, int argc, char **argv)
 
 	if (c->mode == MODE_PASTA) {
 		c->no_dhcp_dns = c->no_dhcp_dns_search = 1;
-		optstring = "dqfel:hI:p:P:m:a:n:M:g:i:D:S:46t:u:T:U:";
+		optstring = "dqfel:hF:I:p:P:m:a:n:M:g:i:D:S:46t:u:T:U:";
 	} else {
-		optstring = "dqfel:hs:p:P:m:a:n:M:g:i:D:S:461t:u:";
+		optstring = "dqfel:hs:F:p:P:m:a:n:M:g:i:D:S:461t:u:";
 	}
 
 	c->tcp.fwd_in.mode = c->tcp.fwd_out.mode = 0;
@@ -1355,6 +1357,23 @@ void conf(struct ctx *c, int argc, char **argv)
 				err("Invalid socket path: %s", optarg);
 				usage(argv[0]);
 			}
+			break;
+		case 'F':
+			if (c->fd_tap >= 0) {
+				err("Multiple --fd options given");
+				usage(argv[0]);
+			}
+
+			errno = 0;
+			c->fd_tap = strtol(optarg, NULL, 0);
+
+			if (c->fd_tap < 0 || errno) {
+				err("Invalid --fd: %s", optarg);
+				usage(argv[0]);
+			}
+
+			c->one_off = true;
+
 			break;
 		case 'I':
 			if (*c->pasta_ifn) {
@@ -1590,6 +1609,11 @@ void conf(struct ctx *c, int argc, char **argv)
 		usage(argv[0]);
 	}
 
+	if (*c->sock_path && c->fd_tap >= 0) {
+		err("Options --socket and --fd are mutually exclusive");
+		usage(argv[0]);
+	}
+
 	ret = conf_ugid(runas, &uid, &gid);
 	if (ret)
 		usage(argv[0]);

passt.1

@@ -297,6 +297,16 @@ Path for UNIX domain socket used by \fBqemu\fR(1) or \fBqrap\fR(1) to connect to
 Default is to probe a free socket, not accepting connections, starting from
 \fI/tmp/passt_1.socket\fR to \fI/tmp/passt_64.socket\fR.
 
+.TP
+.BR \-F ", " \-\-fd " " \fIFD
+Pass a pre-opened, connected socket to \fBpasst\fR. Usually the socket is opened
+in the parent process and \fBpasst\fR inherits it when run as a child. This
+allows the parent process to open sockets using another address family or
+requiring special privileges.
+
+This option implies the behaviour described for \-\-one-off, once this socket
+is closed.
+
 .TP
 .BR \-1 ", " \-\-one-off
 Quit after handling a single client connection, that is, once the client closes

passt.c

@@ -255,7 +255,6 @@ int main(int argc, char **argv)
 
 	quit_fd = pasta_netns_quit_init(&c);
 
-	c.fd_tap = c.fd_tap_listen = -1;
 	tap_sock_init(&c);
 
 	clock_gettime(CLOCK_MONOTONIC, &now);

passt.h

@@ -159,7 +159,7 @@ struct ip6_ctx {
  * @proc_net_udp:	Stored handles for /proc/net/udp{,6} in init and ns
  * @epollfd:		File descriptor for epoll instance
  * @fd_tap_listen:	File descriptor for listening AF_UNIX socket, if any
- * @fd_tap:		File descriptor for AF_UNIX socket or tuntap device
+ * @fd_tap:		AF_UNIX socket, tuntap device, or pre-opened socket
  * @mac:		Host MAC address
  * @mac_guest:		MAC address of guest or namespace, seen or configured
  * @ifi4:		Index of routable interface for IPv4, 0 if IPv4 disabled

tap.c

@@ -1069,6 +1069,15 @@ void tap_sock_init(struct ctx *c)
 	}
 
 	if (c->fd_tap != -1) {
+		if (c->one_off) {	/* Passed as --fd */
+			struct epoll_event ev = { 0 };
+
+			ev.data.fd = c->fd_tap;
+			ev.events = EPOLLIN | EPOLLET | EPOLLRDHUP;
+			epoll_ctl(c->epollfd, EPOLL_CTL_ADD, c->fd_tap, &ev);
+			return;
+		}
+
 		epoll_ctl(c->epollfd, EPOLL_CTL_DEL, c->fd_tap, NULL);
 		close(c->fd_tap);
 		c->fd_tap = -1;