Re: [PATCH v3 04/16] kbuild: userprogs: add nolibc support

[Masahiro Yamada <masahiroy@kernel.org>]

On Wed, Jun 11, 2025 at 4:38 PM Thomas Weißschuh
<thomas.weissschuh@linutronix.de> wrote:
>
> Userprogs are built with the regular kernel compiler $CC.
> A kernel compiler does not necessarily contain a libc which is required
> for a normal userspace application.
> However the kernel tree does contain a minimal libc implementation
> "nolibc" which can be used to build userspace applications.
>
> Introduce support to build userprogs against nolibc instead of the
> default libc of the compiler, which may not exist.
>
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
>
> ---
> This could probably be moved out of the generic kbuild makefiles.
> I think the ergonimics would suffer and this functionality could be
> used by other users of userprogs.
>
> Also this does currently not support out-of-tree builds.
> For that tools/include/nolibc/*.h and usr/include/*.h would need to be
> installed into the build directory.
> ---
>  Documentation/kbuild/makefiles.rst | 13 +++++++++++++
>  scripts/Makefile.userprogs         | 13 ++++++++++---
>  2 files changed, 23 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
> index 8aef3650c1f32b6b197e0dc777e26775d371a081..4cc7a1b89f1803857a4723284613111e9ad71d92 100644
> --- a/Documentation/kbuild/makefiles.rst
> +++ b/Documentation/kbuild/makefiles.rst
> @@ -974,6 +974,19 @@ When linking bpfilter_umh, it will be passed the extra option -static.
>
>  From command line, :ref:`USERCFLAGS and USERLDFLAGS <userkbuildflags>` will also be used.
>
> +Building userprogs against nolibc
> +---------------------------------
> +
> +Not all kernel toolchains provide a libc.
> +Simple userprogs can be built against a very simple libc call "nolibc" provided
> +by the kernel source tree.
> +This requires ``CONFIG_HEADERS_INSTALL=y``.
> +
> +Example::
> +
> +  # lib/kunit/Makefile
> +  uapi-preinit-nolibc := $(CONFIG_ARCH_HAS_NOLIBC)
> +
>  When userspace programs are actually built
>  ------------------------------------------
>
> diff --git a/scripts/Makefile.userprogs b/scripts/Makefile.userprogs
> index f3a7e1ef3753b54303718fae97f4b3c9d4eac07c..b1633a9de6c86a023c70a717bac0b80b89d01431 100644
> --- a/scripts/Makefile.userprogs
> +++ b/scripts/Makefile.userprogs
> @@ -16,10 +16,17 @@ user-csingle        := $(addprefix $(obj)/, $(user-csingle))
>  user-cmulti    := $(addprefix $(obj)/, $(user-cmulti))
>  user-cobjs     := $(addprefix $(obj)/, $(user-cobjs))
>
> +user_nolibc_ccflags := -nostdlib -nostdinc -static -fno-ident -fno-asynchronous-unwind-tables \
> +                     -ffreestanding -fno-stack-protector \
> +                     -isystem $(objtree)/usr/include -include $(srctree)/tools/include/nolibc/nolibc.h -isystem $(srctree)/tools/include/nolibc/

The tools/ directory is a different world, and Kbuild scripts do not know
anything about it.

And, you do not need to implement this in scripts/Makefile.userprogs
because you can move this to lib/kunit/Makefile.kunit-uapi or somewhere.









> +user_nolibc_ldflags := -nostdlib -nostdinc -static
> +
>  user_ccflags   = -Wp,-MMD,$(depfile) $(KBUILD_USERCFLAGS) $(userccflags) \
> -                       $($(target-stem)-userccflags)
> -user_ldflags   = $(KBUILD_USERLDFLAGS) $(userldflags) $($(target-stem)-userldflags)
> -user_ldlibs    = $(userldlibs) $($(target-stem)-userldlibs)
> +                       $($(target-stem)-userccflags) $(if $($(target-stem)-nolibc),$(user_nolibc_ccflags))
> +user_ldflags   = $(KBUILD_USERLDFLAGS) $(userldflags) $($(target-stem)-userldflags) \
> +                       $(if $($(target-stem)-nolibc),$(user_nolibc_ldflags))
> +user_ldlibs    = $(userldlibs) $($(target-stem)-userldlibs) \
> +                       $(if $($(target-stem)-nolibc),$(user_nolibc_ldlibs))
>
>  # Create an executable from a single .c file
>  quiet_cmd_user_cc_c = CC [U]  $@
>
> --
> 2.49.0
>
>


--
Best Regards
Masahiro Yamada