Building in a read only repository

["D'Arcy J.M. Cain" <darcy%NetBSD.org@localhost>]

First, here is my setup.

I have a directory /usr/NetBSD/cvs where I checked out and regularly
update current NetBSD as myself.  I then null mount from here
to /usr/src and /usr/pkgsrc.  In /etc/mk.conf I have the following
directory related entries:

WRKDIRPREFIX = /usr/obj
DISTDIR = /usr/distfiles
BUILDIR = /usr/obj/build
WRKOBJDIR = /usr/obj
PKGSRCDIR = /usr/pkgsrc
TOOLDIR = /usr/obj/tools
BSDOBJDIR = /usr/obj
RELEASEDIR = /usr/obj/release   
DESTDIR = /usr/obj/destdir      

I then go into /usr/src and build everything.  The following two
commands work fine:

# ./build.sh -M /usr/obj tools
# ./build.sh -M /usr/obj kernel=GENERIC

However this fails.  Error follows.

# ./build.sh -M /usr/obj distribution

...
do-compat-lib ===> .
build_install ===> compat       (with: BOOTSTRAP_SUBDIRS=../../../lib)
build_install ===> compat/dirshack
build_install ===> compat/amd64/i386
build_install ===> compat/amd64/i386/../../../lib
dependall-csu ===> compat/amd64/i386/../../../lib
dependall ===> compat/amd64/i386/../../../lib/csu
#   compile  csu/crt0.o
/usr/obj/tools/bin/x86_64--netbsd-gcc  -m32   --sysroot=/usr/obj/destdir 
-DELFSIZE=32 -I/usr/src/libexec/ld.elf_so -I/usr/src/lib/csu/common -I.  -x 
assembler-with-cpp -c /usr/src/lib/csu/arch/i386/crt0.S -o crt0.o.S.o
Assembler messages:
Fatal error: can't create crt0.o.S.o: Read-only file system

Have I found a problem with the build system or have I missed
an entry in /etc/mk.conf?

-- 
D'Arcy J.M. Cain <darcy%NetBSD.org@localhost>
http://www.NetBSD.org/ IM:darcy%Vex.Net@localhost