reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/martin.lau/bpf-next.git/commit/?id=32e27daf9a1563276f88eb17d076f61f8cc8b502
        git remote add martin-lau-bpf-next https://git.kernel.org/pub/scm/linux/kernel/git/martin.lau/bpf-next.git
        git fetch --no-tags martin-lau-bpf-next cgrp.struct_ops.v8
        git checkout 32e27daf9a1563276f88eb17d076f61f8cc8b502

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash kernel/bpf/