On Mon, Apr 07, 2025 at 09:42:38AM +0200, Thomas Weißschuh 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 <[email protected]>
>
> ---
> 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.
(out-of-tree == external kmods; out-of-source == build-dir != source-dir)
you probably meant out-of-source.
> For that tools/include/nolibc/*.h and usr/include/*.h would need to be
> installed into the build directory.
Out-of-source builds could be achieved by adding 'headers' as
dependency, see below.
> ---
> Documentation/kbuild/makefiles.rst | 12 ++++++++++++
> scripts/Makefile.userprogs | 16 +++++++++++++---
> 2 files changed, 25 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/kbuild/makefiles.rst
> b/Documentation/kbuild/makefiles.rst
> index
> 3b9a8bc671e2e92126857059e985d6e5b2c43fd4..f905a6f77c965311c491cfd7ab3103185af7e82e
> 100644
> --- a/Documentation/kbuild/makefiles.rst
> +++ b/Documentation/kbuild/makefiles.rst
> @@ -970,6 +970,18 @@ When linking bpfilter_umh, it will be passed the extra
> option -static.
>
> From command line, :ref:`USERCFLAGS and USERLDFLAGS <userkbuildflags>` will
> also be used.
>
> +Bulding userprogs against nolibc
Bulding -> Building
> +--------------------------------
> +
> +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.
> +
> +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..a1447c02b948901631098b585f5cf4d3ea383a57
> 100644
> --- a/scripts/Makefile.userprogs
> +++ b/scripts/Makefile.userprogs
> @@ -16,10 +16,20 @@ user-csingle := $(addprefix $(obj)/, $(user-csingle))
> user-cmulti := $(addprefix $(obj)/, $(user-cmulti))
> user-cobjs := $(addprefix $(obj)/, $(user-cobjs))
>
> +user-libgcc := $(call try-run,$(CC) -Werror $(KBUILD_USERCFLAGS) -lgcc
> -x c -shared /dev/null -o "$$TMP",-lgcc)
> +
> +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/
> +user_nolibc_ldflags := -nostdlib -nostdinc -static
> +user_nolibc_ldlibs := $(user-libgcc)
> +
> user_ccflags = -Wp,-MMD,$(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] $@
Adding another hunk for scripts/Makefile.userprogs would allow to build
out-of-source:
@@ -39,5 +49,5 @@ $(call multi_depend, $(user-cmulti), , -objs)
# Create .o file from a .c file
quiet_cmd_user_cc_o_c = CC [U] $@
cmd_user_cc_o_c = $(CC) $(user_ccflags) -c -o $@ $<
-$(user-cobjs): $(obj)/%.o: $(src)/%.c FORCE
+$(user-cobjs): $(obj)/%.o: $(src)/%.c headers FORCE
$(call if_changed_dep,user_cc_o_c)
But I am unsure if it is ok to add 'headers' as a build dependency for
userprogs. For me, it feels a bit odd, but I think it really makes
sense here.
--
Nicolas Schier
