umount(8): Add -d option to detach vn(4) devices

The '-d' option tells umount(8) to detach the underlying vn(4) device if
the filesystem was mounted from it.  Note that vn(4) is a virtual disk
and can provides multiple filesystems, so the vn(4) detaching can only
succeed when all the filesystems are umounted.

For example:
$ vnconfig -c vn dfly.img
vn4
$ mount_msdos /dev/vn4s1 /mnt/dfly/boot
$ mount_ufs /dev/vn4s2a /mnt/dfly/root
$ umount -d /mnt/dfly/boot
umount: VNIOCDETACH: /dev/vn4: Device busy
umount: detach of /dev/vn4s1 failed
$ umount -d /mnt/dfly/root
(now vn4 is detached)

A similar option exists in Linux's and FreeBSD's umount(8).

GitHub PR: #24
See Also: freebsd/freebsd-src#972

sbin/umount/umount.8

@@ -28,20 +28,20 @@
 .\"     @(#)umount.8	8.2 (Berkeley) 5/8/95
 .\" $FreeBSD: src/sbin/umount/umount.8,v 1.7.2.3 2001/12/14 15:17:57 ru Exp $
 .\"
-.Dd September 29, 2016
+.Dd March 10, 2024
 .Dt UMOUNT 8
 .Os
 .Sh NAME
 .Nm umount
 .Nd unmount filesystems
 .Sh SYNOPSIS
 .Nm
-.Op Fl fv
+.Op Fl dfv
 .Ar special \&| node
 .Nm
 .Fl a | A
 .Op Fl F Ar fstab
-.Op Fl fv
+.Op Fl dfv
 .Op Fl h Ar host
 .Op Fl t Ar type
 .Sh DESCRIPTION
@@ -71,6 +71,10 @@ are unmounted.
 .It Fl A
 All the currently mounted filesystems except
 the root are unmounted.
+.It Fl d
+If the filesystem was mounted from a
+.Xr vn 4
+vnode disk, detach the underlying disk as well.
 .It Fl F Ar fstab
 Specify the
 .Pa fstab
@@ -132,7 +136,8 @@ filesystem table
 .Sh SEE ALSO
 .Xr unmount 2 ,
 .Xr fstab 5 ,
-.Xr mount 8
+.Xr mount 8 ,
+.Xr vnconfig 8
 .Sh HISTORY
 A
 .Nm

sbin/umount/umount.c

@@ -32,14 +32,18 @@
  */
 
 #include <sys/param.h>
+#include <sys/ioctl.h>
 #include <sys/mount.h>
 #include <sys/socket.h>
+#include <sys/vnioctl.h>
 
 #include <netdb.h>
 #include <rpc/rpc.h>
 #include <vfs/nfs/rpcv2.h>
 
 #include <err.h>
+#include <errno.h>
+#include <fcntl.h>
 #include <fstab.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -55,7 +59,7 @@ typedef enum { MNTON, MNTFROM, NOTHING } mntwhat;
 typedef enum { MARK, UNMARK, NAME, COUNT, FREE } dowhat;
 
 struct  addrinfo *nfshost_ai = NULL;
-int	fflag, vflag;
+int	all, dflag, fflag, vflag;
 char   *nfshost;
 
 void	 checkmntlist (char *, char **, char **, char **);
@@ -71,26 +75,30 @@ int	 umountall (char **);
 int	 checkname (char *, char **);
 int	 umountfs (char *, char *, char *);
 void	 usage (void) __dead2;
+int	 vn_detach (const char *);
 int	 xdr_dir (XDR *, char *);
 
 int
 main(int argc, char *argv[])
 {
-	int all, errs, ch, mntsize, error;
+	int errs, ch, mntsize, error;
 	char **typelist = NULL, *mntonname, *mntfromname;
 	char *type, *mntfromnamerev, *mntonnamerev;
 	struct statfs *mntbuf;
 	struct addrinfo hints;
 
 	all = errs = 0;
-	while ((ch = getopt(argc, argv, "AaF:fh:t:v")) != -1) {
+	while ((ch = getopt(argc, argv, "AadF:fh:t:v")) != -1) {
 		switch (ch) {
 		case 'A':
 			all = 2;
 			break;
 		case 'a':
 			all = 1;
 			break;
+		case 'd':
+			dflag = 1;
+			break;
 		case 'F':
 			setfstab(optarg);
 			break;
@@ -443,6 +451,13 @@ umountfs(char *mntfromname, char *mntonname, char *type)
 	}
 	if (vflag)
 		printf("%s: unmount from %s\n", mntfromname, mntonname);
+	if (dflag) {
+		if (vn_detach(mntfromname) != 0 && !all) {
+			warnx("detach of %s failed", mntfromname);
+			return (1);
+		}
+	}
+
 	/*
 	 * Report to mountd-server which nfsname
 	 * has been unmounted.
@@ -478,6 +493,47 @@ umountfs(char *mntfromname, char *mntonname, char *type)
 		auth_destroy(clp->cl_auth);
 		clnt_destroy(clp);
 	}
+
+	return (0);
+}
+
+int
+vn_detach(const char *fromname)
+{
+	struct vn_ioctl vnio;
+	char *p, device[MAXPATHLEN];
+	int fd;
+
+	if (strncmp(fromname, "/dev/vn", sizeof("/dev/vn") - 1) != 0) {
+		if (!all)
+			warnx("invalid vn device: %s", fromname);
+		return (-1);
+	}
+
+	/* Strip the slice/partition part (e.g., /dev/vn4s1a -> /dev/vn4) */
+	snprintf(device, sizeof(device), "%s", fromname);
+	p = device + sizeof("/dev/vn");
+	while (*p >= '0' && *p <= '9')
+		p++;
+	*p = '\0';
+
+	fd = open(device, O_RDONLY);
+	if (fd < 0) {
+		warn("open: %s", device);
+		return (-1);
+	}
+
+	memset(&vnio, 0, sizeof(vnio));
+	if (ioctl(fd, VNIOCDETACH, &vnio) < 0) {
+		warn("VNIOCDETACH: %s", device);
+		close(fd);
+		return (-1);
+	}
+
+	close(fd);
+	if (vflag)
+		printf("%s: detached\n", device);
+
 	return (0);
 }
 
@@ -744,7 +800,7 @@ usage(void)
 {
 
 	fprintf(stderr, "%s\n%s\n",
-	    "usage: umount [-fv] special | node",
-	    "       umount -a | -A [-F fstab] [-fv] [-h host] [-t type]");
+	    "usage: umount [-dfv] special | node",
+	    "       umount -a | -A [-F fstab] [-dfv] [-h host] [-t type]");
 	exit(1);
 }