reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/cip/linux-cip.git/commit/?id=2c16891623cc7e8c0c4144f8665d117c7fc94552
        git remote add cip https://git.kernel.org/pub/scm/linux/kernel/git/cip/linux-cip.git
        git fetch --no-tags cip linux-4.19.y-cip-rt-rebase
        git checkout 2c16891623cc7e8c0c4144f8665d117c7fc94552
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um olddefconfig
        make W=1 O=build_dir ARCH=um SHELL=/bin/bash kernel/bpf/