reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git/commit/?id=2bc5a98f47c0329ca2e8eef172500d49eb7c98c2
        git remote add viro-vfs https://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git
        git fetch --no-tags viro-vfs work.filename
        git checkout 2bc5a98f47c0329ca2e8eef172500d49eb7c98c2
        # apt-get install sparse
        # sparse version: v0.6.4-73-gfbdde312

        # 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=um olddefconfig
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=um SHELL=/bin/bash fs/