reproduce:
        # apt-get install sparse
        # sparse version: v0.6.4-66-g0196afe1
        # https://github.com/congwang/linux/commit/8c55ea4b2a55101e73f58a10025db899f02c9efa
        git remote add congwang https://github.com/congwang/linux.git
        git fetch --no-tags congwang bpf-next
        git checkout 8c55ea4b2a55101e73f58a10025db899f02c9efa
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=i386 olddefconfig
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=i386 SHELL=/bin/bash