Currently, an mfs filesystem always inherits the permissions of it's mount
point.

This behaviour necessarily differs from disk-based filesystems, since mfs is
volatile.

The problem is that a common use of mfs is for mounting on /tmp, and this
requires 01777 permissions for correct operation.

So 'solve' this problem, users commonly chmod the /tmp directory on the root
filesystem to 01777, which is a bad thing to do because if for some reason the
mfs filesystem fails to mount or is later unmounted, it will leave a
world-writable directory on /.

This could be used by a malicious user to create hard links to various files
in /etc or elsewhere where they are not desired.

To avoid this, let's teach mount_mfs to accept an optional -p parameter to
specify the required permissions _after_ the mfs filesystem is mounted.

Note, the -p option was previously used in historical versions of newfs to
indicate a number of reserved sectors for each track.  This was removed from
OpenBSD in 2007, and since the new -p option is only valid for mount_mfs and
not newfs, it seems reasonable to re-use it.  The more obvious -m option as
used by mount_tmpfs is already in use in mount_mfs and newfs.

--- sbin/newfs/newfs.c.dist	Mon Apr  7 04:25:02 2025
+++ sbin/newfs/newfs.c	Wed Jul 16 07:47:33 2025
@@ -203,7 +203,7 @@
 		fatal("insane maxpartitions value %d", maxpartitions);
 
 	opstring = mfs ?
-	    "O:P:T:b:c:e:f:i:m:o:s:" :
+	    "O:P:T:b:c:e:f:i:m:o:p:s:" :
 	    "NO:S:T:b:c:e:f:g:h:i:m:o:qs:t:";
 	while ((ch = getopt(argc, argv, opstring)) != -1) {
 		switch (ch) {
@@ -286,6 +286,18 @@
 					    optarg);
 			}
 			break;
+		case 'p':
+			errno = 0;
+			mfsmode = strtoul(optarg, NULL, 8);
+			if (*optarg < '0' || *optarg > '7')
+				errno = EINVAL;
+			if ((mfsmode & ALLPERMS) != mfsmode)
+				fatal("unrecognised permission bits");
+			if (mfsmode == 0)
+				warnx("ignoring nonsensical permissions of 0x00");
+			if (errno)
+				fatal("invalid mode");
+			break;
 		case 'q':
 			quiet = 1;
 			break;
@@ -506,7 +518,8 @@
 			err(ECANCELED, "stat %s", node);
 		mfsuid = mountpoint.st_uid;
 		mfsgid = mountpoint.st_gid;
-		mfsmode = mountpoint.st_mode & ALLPERMS;
+		if (mfsmode == 0)
+			mfsmode = mountpoint.st_mode & ALLPERMS;
 	}
 #endif
 
--- sbin/newfs/newfs.8.dist	Mon Apr  7 04:25:02 2025
+++ sbin/newfs/newfs.8	Wed Jul 16 07:59:04 2025
@@ -67,6 +67,7 @@
 .Op Fl m Ar free-space
 .Op Fl O Ar filesystem-format
 .Op Fl o Ar options
+.Op Fl p Ar mode
 .Op Fl P Ar file
 .Op Fl s Ar size
 .Ar special node
@@ -270,7 +271,8 @@
 are as described for
 .Nm ,
 except for the
-.Fl o
+.Fl o ,
+.Fl p
 and
 .Fl P
 options.
@@ -284,6 +286,13 @@
 See the
 .Xr mount 8
 man page for possible options and their meanings.
+.It Fl p Ar mode
+Specifies the file mode bits to set on the mount point after the mfs
+filesystem has been mounted, overriding the default behaviour of inheriting
+the mode bits from those of the directory in the parent filesystem.
+Use of this option is preferred when mounting an mfs filesystem on /tmp
+as it avoids the potential security risks of creating a world writable
+directory on the root filesystem.
 .It Fl P Ar file
 If
 .Ar file