unionfs below: only read-only?

[Andriy Gapon]

$ uname -srm
FreeBSD 7.1-PRERELEASE amd64

$ mount -t unionfs -o below -o rw /usr/ports/distfiles
/export/j386/usr/ports/distfiles
mount_unionfs: /export/j386/usr/ports/distfiles: : Operation not supported
Exit 71

$ mount -t unionfs -o below -o ro /usr/ports/distfiles
/export/j386/usr/ports/distfiles
Exit 0

My intention was to mount /usr/ports/distfiles into a jail but in such
way that jail can not modify any "global" files but can add some files
of its own.
This is similar to the last example in mount_unionfs(8):
mount -t unionfs -o noatime -o below /sys $HOME/sys

But it seems that with -o below I can mount a filesystem only in RO mode
which is totally useless because I can not write anything to "uniondir"
i.e. /export/j386/usr/ports/distfiles:
$ cp ~/tar/diablo-caffe-freebsd6-i386-1.5.0_07-b01.tar.bz2
/export/j386/usr/ports/distfiles/
cp:
/export/j386/usr/ports/distfiles/diablo-caffe-freebsd6-i386-1.5.0_07-b01.tar.bz2:
Read-only file system
Exit 1

So effectively this is a nullfs ro mount.

Is there a buglet or am I missing something?

-- 
Andriy Gapon