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