Index: head/sys/fs/nfsclient/nfs_clvfsops.c
===================================================================
--- head/sys/fs/nfsclient/nfs_clvfsops.c
+++ head/sys/fs/nfsclient/nfs_clvfsops.c
@@ -741,6 +741,101 @@
     NULL };
 
 /*
+ * Parse the "from" mountarg, passed by the generic mount(8) program
+ * or the mountroot code.  This is used when rerooting into NFS.
+ *
+ * Note that the "hostname" is actually a "hostname:/share/path" string.
+ */
+static int
+nfs_mount_parse_from(struct vfsoptlist *opts, char **hostnamep,
+    struct sockaddr_in **sinp, char *dirpath, size_t dirpathsize, int *dirlenp)
+{
+	char nam[MNAMELEN + 1];
+	char *delimp, *hostp, *spec;
+	int error, have_bracket = 0, offset, rv, speclen;
+	struct sockaddr_in *sin;
+	size_t len;
+
+	error = vfs_getopt(opts, "from", (void **)&spec, &speclen);
+	if (error != 0)
+		return (error);
+
+	/*
+	 * This part comes from sbin/mount_nfs/mount_nfs.c:getnfsargs().
+	 */
+        if (*spec == '[' && (delimp = strchr(spec + 1, ']')) != NULL &&
+            *(delimp + 1) == ':') {
+                hostp = spec + 1;
+                spec = delimp + 2;
+                have_bracket = 1;
+        } else if ((delimp = strrchr(spec, ':')) != NULL) {
+                hostp = spec;
+                spec = delimp + 1;
+        } else if ((delimp = strrchr(spec, '@')) != NULL) {
+                printf("%s: path@server syntax is deprecated, "
+		    "use server:path\n", __func__);
+                hostp = delimp + 1;
+        } else {
+                printf("%s: no <host>:<dirpath> nfs-name\n", __func__);
+                return (EINVAL);
+        }
+        *delimp = '\0';
+
+        /*
+         * If there has been a trailing slash at mounttime it seems
+         * that some mountd implementations fail to remove the mount
+         * entries from their mountlist while unmounting.
+         */
+        for (speclen = strlen(spec);
+                speclen > 1 && spec[speclen - 1] == '/';
+                speclen--)
+                spec[speclen - 1] = '\0';
+        if (strlen(hostp) + strlen(spec) + 1 > MNAMELEN) {
+                printf("%s: %s:%s: name too long", __func__, hostp, spec);
+                return (EINVAL);
+        }
+	/* Make both '@' and ':' notations equal */
+	if (*hostp != '\0') {
+		len = strlen(hostp);
+		offset = 0;
+		if (have_bracket)
+			nam[offset++] = '[';
+		memmove(nam + offset, hostp, len);
+		if (have_bracket)
+			nam[len + offset++] = ']';
+		nam[len + offset++] = ':';
+		memmove(nam + len + offset, spec, speclen);
+		nam[len + speclen + offset] = '\0';
+	}
+
+	/*
+	 * XXX: IPv6
+	 */
+	sin = malloc(sizeof(*sin), M_SONAME, M_WAITOK);
+	rv = inet_pton(AF_INET, hostp, &sin->sin_addr);
+	if (rv != 1) {
+		printf("%s: cannot parse '%s', inet_pton() returned %d\n",
+		    __func__, hostp, rv);
+		free(sin, M_SONAME);
+		return (EINVAL);
+	}
+
+	sin->sin_len = sizeof(*sin);
+	sin->sin_family = AF_INET;
+	/*
+	 * XXX: hardcoded port number.
+	 */
+	sin->sin_port = htons(2049);
+
+	*hostnamep = strdup(nam, M_NEWNFSMNT);
+	*sinp = sin;
+	strlcpy(dirpath, spec, dirpathsize);
+	*dirlenp = strlen(dirpath);
+
+	return (0);
+}
+
+/*
  * VFS Operations.
  *
  * mount system call
@@ -785,17 +880,20 @@
 	int nametimeo = NFS_DEFAULT_NAMETIMEO;
 	int negnametimeo = NFS_DEFAULT_NEGNAMETIMEO;
 	int minvers = 0;
-	int dirlen, has_nfs_args_opt, krbnamelen, srvkrbnamelen;
+	int dirlen, has_nfs_args_opt, has_nfs_from_opt,
+	    krbnamelen, srvkrbnamelen;
 	size_t hstlen;
 
 	has_nfs_args_opt = 0;
+	has_nfs_from_opt = 0;
 	if (vfs_filteropt(mp->mnt_optnew, nfs_opts)) {
 		error = EINVAL;
 		goto out;
 	}
 
 	td = curthread;
-	if ((mp->mnt_flag & (MNT_ROOTFS | MNT_UPDATE)) == MNT_ROOTFS) {
+	if ((mp->mnt_flag & (MNT_ROOTFS | MNT_UPDATE)) == MNT_ROOTFS &&
+	    nfs_diskless_valid != 0) {
 		error = nfs_mountroot(mp);
 		goto out;
 	}
@@ -1135,6 +1233,19 @@
 		    args.addrlen);
 		if (error != 0)
 			goto out;
+	} else if (nfs_mount_parse_from(mp->mnt_optnew,
+	    &args.hostname, (struct sockaddr_in **)&nam, dirpath,
+	    sizeof(dirpath), &dirlen) == 0) {
+		has_nfs_from_opt = 1;
+		bcopy(args.hostname, hst, MNAMELEN);
+		hst[MNAMELEN - 1] = '\0';
+
+		/*
+		 * This only works with NFSv4 for now.
+		 */
+		args.fhsize = 0;
+		args.flags |= NFSMNT_NFSV4;
+		args.sotype = SOCK_STREAM;
 	} else {
 		if (vfs_getopt(mp->mnt_optnew, "fh", (void **)&args.fh,
 		    &args.fhsize) == 0) {
@@ -1174,13 +1285,16 @@
 		krbname[0] = '\0';
 	krbnamelen = strlen(krbname);
 
-	if (vfs_getopt(mp->mnt_optnew, "dirpath", (void **)&name, NULL) == 0)
-		strlcpy(dirpath, name, sizeof (dirpath));
-	else
-		dirpath[0] = '\0';
-	dirlen = strlen(dirpath);
+	if (has_nfs_from_opt == 0) {
+		if (vfs_getopt(mp->mnt_optnew,
+		    "dirpath", (void **)&name, NULL) == 0)
+			strlcpy(dirpath, name, sizeof (dirpath));
+		else
+			dirpath[0] = '\0';
+		dirlen = strlen(dirpath);
+	}
 
-	if (has_nfs_args_opt == 0) {
+	if (has_nfs_args_opt == 0 && has_nfs_from_opt == 0) {
 		if (vfs_getopt(mp->mnt_optnew, "addr",
 		    (void **)&args.addr, &args.addrlen) == 0) {
 			if (args.addrlen > SOCK_MAXADDRLEN) {