The RPM %post for the official SUN JRE will mount /proc/sys/fs/binfmt_misc, which causes all of the "umount -n $BUILD_ROOT/proc" in init_buildsystem and build to fail. Then, when the next build comes up /proc is still mounted and the clean of the buildroot tries to recursively rm -rf all of proc.
Attached is the straightforward patch to fix this for both 'build' and 'init_buildsystem'. I've been carrying these privately for > 6 months now. Please apply, I'm tired of breaking my build service every time I upgrade. :)
A better long-term fix would be to use the 'unshare(CLONE_NEWNS)' system call to detach from the parents FS so that mounts of /proc, etc wont outlive the build process. This would also completely eliminate the need to umount when complete. But this would require a much more intrusive reorganization of the build code.