diff --git a/sbin/mdmfs/mdmfs.8 b/sbin/mdmfs/mdmfs.8
index 9db5194e4fea..daf66c271f12 100644
--- a/sbin/mdmfs/mdmfs.8
+++ b/sbin/mdmfs/mdmfs.8
@@ -200,6 +200,17 @@ Set the file (directory) permissions of the mount point
 .Ar mount-point
 to
 .Ar permissions .
+The
+.Ar permissions
+argument can be in any of the mode formats recognized by
+.Xr chmod 1 .
+If symbolic permissions are specified,
+the operation characters
+.Dq +
+and
+.Dq -
+are interpreted relative to the initial permissions of
+.Dq a=rwx .
 .It Fl S
 Do not enable soft-updates on the file system.
 .It Fl s Ar size
diff --git a/sbin/mdmfs/mdmfs.c b/sbin/mdmfs/mdmfs.c
index 035f1fe77c82..db143ccd7986 100644
--- a/sbin/mdmfs/mdmfs.c
+++ b/sbin/mdmfs/mdmfs.c
@@ -92,6 +92,7 @@ main(int argc, char **argv)
 	bool detach, softdep, autounit;
 	char *mtpoint, *unitstr;
 	char ch, *p;
+	void *set;
 
 	/* Misc. initialization. */
 	(void)memset(&mi, '\0', sizeof(mi));
@@ -191,11 +192,11 @@ main(int argc, char **argv)
 		case 'p':
 			if (compat)
 				usage();
-			if (*optarg >= '0' && *optarg <= '7')
-				mi.mi_mode = strtol(optarg, NULL, 8);
-			if ((mi.mi_mode & ~07777) != 0)
+			if ((set = setmode(optarg)) == NULL)
 				usage();
+			mi.mi_mode = getmode(set, S_IRWXU | S_IRWXG | S_IRWXO);
 			mi.mi_have_mode = true;
+			free(set);
 			break;
 		case 'S':
 			if (compat)