reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git/commit/?id=6cba15e9daa2a0714c9159a46fd021947745eea0
        git remote add masahiroy https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git
        git fetch --no-tags masahiroy kbuild
        git checkout 6cba15e9daa2a0714c9159a46fd021947745eea0
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um olddefconfig
        make W=1 O=build_dir ARCH=um SHELL=/bin/bash