@@ -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
+--------------------------------
+
+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 +16,20 @@ 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] $@
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 | 12 ++++++++++++ scripts/Makefile.userprogs | 16 +++++++++++++--- 2 files changed, 25 insertions(+), 3 deletions(-)