The below diff improves the error messages related to opening the "next
available" tap(4) device when using vmd(8). I feel this pops up every
once in awhile and just did again [1] on misc@, so about time we improve
it a bit.

We really need either custom error codes or something sent back to
vmctl and not just piggy backing on errnos as they often don't make
sense. That's a diff for another day as it's going to be pretty
invasive. This one still relies on ENOENT being communicated...not the
best but far better than "Unknown error". The bigger imrovement is in
the vmd logging pointing out the tap we couldn't open.

ok?

before the diff:

 # for i in `jot 5`; do vmctl start -L -b /bsd.rd "test-${i}"; done
 vmctl: started vm 5 successfully, tty /dev/ttyp4
 vmctl: started vm 6 successfully, tty /dev/ttyp5
 vmctl: started vm 7 successfully, tty /dev/ttyp6
 vmctl: started vm 8 successfully, tty /dev/ttyp7
 vmctl: start vm command failed: Unknown error: -1

 # $(which vmd) -dv
 vmd: startup
 vmd: started test-1 (vm 5) successfully, tty /dev/ttyp4
 vmd: started test-2 (vm 6) successfully, tty /dev/ttyp5
 vmd: started test-3 (vm 7) successfully, tty /dev/ttyp6
 vmd: started test-4 (vm 8) successfully, tty /dev/ttyp7
 vmd: config_setvm: can't open tap tap
 vmd: failed to start vm test-5

w/ the diff:

 # for i in `jot 5`; do vmctl start -L -b /bsd.rd "test-${i}"; done
 vmctl: started vm 5 successfully, tty /dev/ttyp4
 vmctl: started vm 6 successfully, tty /dev/ttyp5
 vmctl: started vm 7 successfully, tty /dev/ttyp6
 vmctl: started vm 8 successfully, tty /dev/ttyp7
 vmctl: start vm command failed: No such file or directory

 # $(which vmd) -dv
 vmd: startup
 vmd: started test-1 (vm 5) successfully, tty /dev/ttyp4
 vmd: started test-2 (vm 6) successfully, tty /dev/ttyp5
 vmd: started test-3 (vm 7) successfully, tty /dev/ttyp6
 vmd: started test-4 (vm 8) successfully, tty /dev/ttyp7
 vmd: config_setvm: can't open /dev/tap4
 vmd: failed to start vm test-5

[1] https://marc.info/?l=openbsd-misc&m=170421999315682&w=2

-dv@

diffstat refs/heads/master refs/heads/vmd-tap
 M  usr.sbin/vmd/config.c  |   2+  1-
 M  usr.sbin/vmd/vmm.c     |  23+  8-

2 files changed, 25 insertions(+), 9 deletions(-)

diff refs/heads/master refs/heads/vmd-tap
commit - b98228b267823b1070a646dd33a2951b3fcc0082
commit + eae7a7dce824b151f8307a72f0be9673441175e7
blob - 3bb246a0092cbd4318721794c81ee11964528820
blob + 23afa2ddf617bbfefcc66a4ef46ec34a990fd27d
--- usr.sbin/vmd/config.c
+++ usr.sbin/vmd/config.c
@@ -417,7 +417,8 @@ config_setvm(struct privsep *ps, struct vmd_vm *vm, ui
 			s = ifname;
 		}
 		if (tapfds[i] == -1) {
-			log_warnx("%s: can't open tap %s", __func__, s);
+			ret = errno;
+			log_warnx("%s: can't open /dev/%s", __func__, s);
 			goto fail;
 		}
 		if ((vif->vif_name = strdup(s)) == NULL) {
blob - b6b138a6e3f1cc8660e95fb2b6668be256a36308
blob + 7bc6adf721df80d51b3a0775a3e633d00eb7a2fc
--- usr.sbin/vmd/vmm.c
+++ usr.sbin/vmd/vmm.c
@@ -586,26 +586,41 @@ terminate_vm(struct vm_terminate_params *vtp)
  * Parameters
  *  ifname: a buffer of at least IF_NAMESIZE bytes.
  *
- * Returns a file descriptor to the tap node opened, or -1 if no tap
- * devices were available.
+ * Returns a file descriptor to the tap node opened or -1 if no tap devices were
+ * available, setting errno to the open(2) error.
  */
 int
 opentap(char *ifname)
 {
-	int i, fd;
+	int err = 0, i, fd;
 	char path[PATH_MAX];

 	for (i = 0; i < MAX_TAP; i++) {
 		snprintf(path, PATH_MAX, "/dev/tap%d", i);
+
+		errno = 0;
 		fd = open(path, O_RDWR | O_NONBLOCK);
-		if (fd != -1) {
-			snprintf(ifname, IF_NAMESIZE, "tap%d", i);
-			return (fd);
+		if (fd != -1)
+			break;
+		err = errno;
+		if (err == EBUSY) {
+			/* Busy...try next tap. */
+			continue;
+		} else if (err == ENOENT) {
+			/* Ran out of /dev/tap* special files. */
+			break;
+		} else {
+			log_warn("%s: unexpected error", __func__);
+			break;
 		}
 	}
-	strlcpy(ifname, "tap", IF_NAMESIZE);

-	return (-1);
+	/* Record the last opened tap device. */
+	snprintf(ifname, IF_NAMESIZE, "tap%d", i);
+
+	if (err)
+		errno = err;
+	return (fd);
 }

 /*