reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git/commit/?id=82b0945ce2c2d636d5e893ad50210875c929f257
        git remote add char-misc https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git
        git fetch --no-tags char-misc char-misc-next
        git checkout 82b0945ce2c2d636d5e893ad50210875c929f257
        make W=1 O=/tmp/kselftest headers
        make W=1 O=/tmp/kselftest -C tools/testing/selftests
        make W=1 O=/tmp/kselftest -C tools/testing/selftests/bpf