> Then, do a "make obj" first. (the OBJMACHINE is not necessaryly needed,
   > it'll give you a "obj.${MACHINE_ARCH}" dir instead of a "obj"). This will
   > give you "obj"-links to $BSDOBJDIR. Maybe see also
   > /usr/share/mk/bsd.README.
   
   Ah, close but backwards.  I'd like the src tree left read-only.


you can do read-only for everything except the "make obj" part.  i know
some who mount /usr/src rw for doing this but otherwise use ro /usr/src.


the alternative is union mounts or something :-)