reproduce (this is a W=1 build):
        # https://github.com/congwang/linux/commit/8325e39591226e0ab23a868609b5959cafe31b4c
        git remote add congwang https://github.com/congwang/linux.git
        git fetch --no-tags congwang bpf
        git checkout 8325e39591226e0ab23a868609b5959cafe31b4c
        # 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 net/ipv4/