Building OpenBSD "make world" with /usr/src read-only

The setup is the following:

A SparcStation 5 running OpenBSD 3.6, "diskly challenged",
An Ultra 5 running OpenBSD 3.6 with ample disk space.

The idea is to build a new userland for OpenBSD 3.6/sparc and clearly I haven't got enough space to do so directly on the SparcStation 5.

Note: using a union mount would do the trick but it seems to be broken on OpenBSD 3.6 (loads of file errors).

Preliminary steps

Export /usr/src from the Ultra 5 read-only via NFS, this is trivally done by adding a suitable line in /etc/exports and then setting up NFS services
(if you don't know how to do this then you really shouldn't be rebuilding userland).
Export some suitable free space from the Ultra 5 read-write again via NFS.
On the SparcStation 5 mount /usr/src and the free space onto /usr/obj.

The meaty bits

If you now go into /usr/src and type make obj it won't work. This is because make obj needs to write symlinks into pretty much each directory under /usr/src.

Don't panic!

The work-around is trivial: on the NFS server go into /usr/src and type make obj which will place the suitable symlinks into most, but not all, the required directories.
What is missing?
Simple, the Ultra 5 (in my case) is a sparc64, the SparcStation 5 is a sparc which means that the files which are architecture specific will not have their obj symlinks made.

To solve this simply visit /usr/src/sys/arch/sparc/stand and in each subdirectory make a symlink into the /usr/obj hierarchy.
Here is some example output:

merlino:/usr/src/sys/arch/sparc/stand# find . -name obj -ls 
66078    0 lrwxr-xr-x    1 root     wsrc           38 Apr 27 16:01 ./binstall/obj -> /usr/obj/sys/arch/sparc/stand/binstall 
88566    0 lrwxr-xr-x    1 root     wsrc           34 Apr 27 17:08 ./boot/obj -> /usr/obj/sys/arch/sparc/stand/boot 
90907    0 lrwxr-xr-x    1 root     wsrc           36 Apr 27 17:08 ./bootxx/obj -> /usr/obj/sys/arch/sparc/stand/bootxx 
90911    0 lrwxr-xr-x    1 root     wsrc           36 Apr 27 17:08 ./common/obj -> /usr/obj/sys/arch/sparc/stand/common 
69371    0 lrwxr-xr-x    1 root     wsrc           41 Mar 16 15:34 ./installboot/obj -> /usr/obj/sys/arch/sparc/stand/installboot 
90913    0 lrwxr-xr-x    1 root     wsrc           37 Apr 27 17:08 ./libkern/obj -> /usr/obj/sys/arch/sparc/stand/libkern 
90914    0 lrwxr-xr-x    1 root     wsrc           35 Apr 27 17:08 ./libsa/obj -> /usr/obj/sys/arch/sparc/stand/libsa 
91102    0 lrwxr-xr-x    1 root     wsrc           34 Apr 27 17:08 ./libz/obj -> /usr/obj/sys/arch/sparc/stand/libz

At which point you can go back to the SparcStation 5 and happily make obj followed by make build.

Note for the impatient: it will take a long time for the make build to terminate.