Mounting a read-only md FS as read-only

[perryh at pluto.rain.com]

I'm trying to mount an ext2fs image (in a file), following sec.
17.13.2 of the Handbook and the mdconfig(8) manpage.  Since I don't
want to change the image, just examine it, I specified -o readonly
to mdconfig and the equivalent to mount.  Is there some reason why
this should not work?  The backing file, and the mountpoint, do
exist.

# ls -ld [filename]
-rw-r--r--  1 perryh  perryh  104857600 Mar 16  2007 [filename]
# mdconfig -a -t vnode -f [filename] -o readonly
md0
# ls -ld /dev/md0 /[mountpoint]
crw-r-----  1 root  operator    1,  15 Nov 24 21:17 /dev/md0
drwxr-xr-x  2 perryh  perryh        512 Feb 17 21:57 /[mountpoint]
# mount -r -o noexec -t ext2fs /dev/md0 /[mountpoint]
mount_ext2fs: /dev/md0: Read-only file system
# mount -o ro,noexec -t ext2fs /dev/md0 /[mountpoint]
mount_ext2fs: /dev/md0: Read-only file system
# /sbin/mount_ext2fs -o ro,noexec /dev/md0 /[mountpoint]
mount_ext2fs: /dev/md0: Read-only file system
# mdconfig -l -u 0
md0     vnode     100M  /[mountpoint]
# uname -a
FreeBSD fbsd61 6.1-RELEASE FreeBSD 6.1-RELEASE #30: Mon Jan  1 23:01:34 PST 2007     perryh at fbsd61:/usr/src/sys/i386/compile/GENERIC  i386