reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=8646db238997df36c6ad71a9d7e0b52ceee221b2
        git remote add bpf-next https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git
        git fetch --no-tags bpf-next master
        git checkout 8646db238997df36c6ad71a9d7e0b52ceee221b2
        # 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 kernel/bpf/