reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git/commit/?id=f46bfe6061c3fb76aabd27fb9dec3deb98d0fc75
        git remote add ast-bpf https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git
        git fetch --no-tags ast-bpf uptr_v3
        git checkout f46bfe6061c3fb76aabd27fb9dec3deb98d0fc75
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash