@@ -959,6 +959,18 @@ 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
+--------------------------------
+
+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
------------------------------------------
@@ -16,10 +16,18 @@ 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 $(call cc-option,-mstack-protector-guard=global) \
+ -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 := $(call cc-option,-lgcc)
+
user_ccflags = -Wp,-MMD,$(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))
ifdef CONFIG_WERROR
KBUILD_USERCFLAGS += -Werror
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 C 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. --- Documentation/kbuild/makefiles.rst | 12 ++++++++++++ scripts/Makefile.userprogs | 14 +++++++++++--- 2 files changed, 23 insertions(+), 3 deletions(-)