reproduce:
        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests
        # apt-get install sparse
        # sparse version: v0.6.4-66-g0196afe1
        # https://github.com/amir73il/linux/commit/e37f0a8ea49c2287a7c722abb1d4d97619bc7271
        git remote add amir73il https://github.com/amir73il/linux
        git fetch --no-tags amir73il ovl_real_file
        git checkout e37f0a8ea49c2287a7c722abb1d4d97619bc7271
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        ~/lkp-tests/kbuild/make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=hexagon olddefconfig
        ~/lkp-tests/kbuild/make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=hexagon SHELL=/bin/bash fs/overlayfs/