bhyve/slirp: Use a dgram socket instead of a seqpacket socket

FreeBSD main has a number of improvements to SEQPACKET sockets which are not yet
backported.  The main reason to use a SEQPACKET socket is so that the slirp
helper is reliably notified when the other end hangs up.

Rather than backporting the upstream improvements, switch to a DGRAM socket and
use an extra control pipe to find out when the parent bhyve process has exited.
No data is written to the pipe, we just use it to poll for POLLHUP.

This commit can be reverted once CheriBSD has been synced up to commit
69f61ce or later.

Author: markjdb

usr.sbin/bhyve/net_backend_slirp.c

@@ -68,6 +68,7 @@
 #define	SLIRP_MTU	2048
 
 struct slirp_priv {
+	int p;
 	int s;
 	pid_t helper;
 	struct mevent *mevp;
@@ -84,14 +85,19 @@ slirp_init(struct net_backend *be, const char *devname __unused,
 	posix_spawn_file_actions_t fa;
 	pid_t child;
 	const char **argv;
-	char sockname[32];
-	int error, s[2];
+	char pipename[32], sockname[32];
+	int error, p[2], s[2];
 
-	if (socketpair(PF_LOCAL, SOCK_SEQPACKET | SOCK_NONBLOCK, 0, s) != 0) {
+	if (socketpair(PF_LOCAL, SOCK_DGRAM | SOCK_NONBLOCK, 0, s) != 0) {
 		EPRINTLN("socketpair");
 		return (-1);
 	}
 
+	if (pipe(p) != 0) {
+		EPRINTLN("pipe");
+		goto err;
+	}
+
 	/*
 	 * The child will exit once its connection goes away, so make sure only
 	 * one end is inherited by the child.
@@ -107,8 +113,10 @@ slirp_init(struct net_backend *be, const char *devname __unused,
 	}
 
 	(void)snprintf(sockname, sizeof(sockname), "%d", s[1]);
+	(void)snprintf(pipename, sizeof(pipename), "%d", p[1]);
 	argv = (const char *[]){
-	    "/usr/libexec/bhyve-slirp-helper", "-S", sockname, NULL
+	    "/usr/libexec/bhyve-slirp-helper", "-S", sockname, "-P", pipename,
+	    NULL
 	};
 	error = posix_spawn(&child, "/usr/libexec/bhyve-slirp-helper",
 	    &fa, NULL, __DECONST(char **, argv), environ);
@@ -140,12 +148,16 @@ slirp_init(struct net_backend *be, const char *devname __unused,
 	}
 
 	priv->helper = child;
+	priv->p = p[0];
+	(void)close(p[1]);
 	priv->s = s[0];
 	(void)close(s[1]);
 
 	return (0);
 
 err:
+	(void)close(p[0]);
+	(void)close(p[1]);
 	(void)close(s[0]);
 	(void)close(s[1]);
 	return (-1);

usr.sbin/bhyve/slirp/slirp-helper.c

@@ -43,6 +43,7 @@
 struct slirp_priv {
 	Slirp *slirp;		/* libslirp handle */
 	int sock;		/* data and control socket */
+	int pipe;		/* used to signal termination */
 	int wakeup[2];		/* used to wake up the pollfd thread */
 	struct pollfd *pollfds;
 	size_t npollfds;
@@ -240,7 +241,7 @@ slirp_pollfd_loop(struct slirp_priv *priv)
 	int error;
 
 	for (;;) {
-		int input, wakeup;
+		int control, input, wakeup;
 
 		for (size_t i = 0; i < priv->npollfds; i++)
 			priv->pollfds[i].fd = -1;
@@ -250,6 +251,8 @@ slirp_pollfd_loop(struct slirp_priv *priv)
 		wakeup = slirp_addpoll(priv, priv->wakeup[0], POLLIN);
 		/* Register for input from our parent process. */
 		input = slirp_addpoll(priv, priv->sock, POLLIN | POLLRDHUP);
+		/* Make sure we get woken up if the parent exits. */
+		control = slirp_addpoll(priv, priv->pipe, POLLIN | POLLHUP);
 
 		timeout = UINT32_MAX;
 		slirp_pollfds_fill_p(priv->slirp, &timeout, slirp_addpoll_cb,
@@ -283,7 +286,8 @@ slirp_pollfd_loop(struct slirp_priv *priv)
 		 * If new packets arrived from our parent, feed them to
 		 * libslirp.
 		 */
-		if ((pollfds[input].revents & (POLLHUP | POLLRDHUP)) != 0)
+		if ((pollfds[input].revents & (POLLHUP | POLLRDHUP)) != 0 ||
+		    (pollfds[control].revents & POLLHUP) != 0)
 			errx(1, "parent process closed connection");
 		if ((pollfds[input].revents & POLLIN) != 0) {
 			ssize_t n;
@@ -462,12 +466,17 @@ main(int argc, char **argv)
 	Slirp *slirp;
 	nvlist_t *config;
 	const char *hostfwd, *vmname;
-	int ch, fd, sd;
+	int ch, fd, pipe, sd;
 	bool restricted;
 
 	sd = -1;
-	while ((ch = getopt(argc, argv, "S:")) != -1) {
+	while ((ch = getopt(argc, argv, "P:S:")) != -1) {
 		switch (ch) {
+		case 'P':
+			pipe = atoi(optarg);
+			if (fcntl(pipe, F_GETFD) == -1)
+				err(1, "invalid pipe %s", optarg);
+			break;
 		case 'S':
 			sd = atoi(optarg);
 			if (fcntl(sd, F_GETFD) == -1)
@@ -498,11 +507,15 @@ main(int argc, char **argv)
 		err(1, "dup2(stderr)");
 	if (dup2(sd, 3) == -1)
 		err(1, "dup2(slirp socket)");
+	if (dup2(pipe, 4) == -1)
+		err(1, "dup2(control pipe)");
 	sd = 3;
-	closefrom(sd + 1);
+	pipe = 4;
+	closefrom(pipe + 1);
 
 	memset(&priv, 0, sizeof(priv));
 	priv.sock = sd;
+	priv.pipe = pipe;
 	if (pipe2(priv.wakeup, O_CLOEXEC | O_NONBLOCK) != 0)
 		err(1, "pipe2");