[PATCH 04/12] kbuild: userprogs: add nolibc support

[Thomas Weißschuh]

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@xxxxxxxxxxxxx>

---
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(-)

diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
index d36519f194dc0d18b72a6f3ce7f3649f7c1b5637..cb859dc58b579def4995a5aa2efb45ab4ba3c93f 100644
--- a/Documentation/kbuild/makefiles.rst
+++ b/Documentation/kbuild/makefiles.rst
@@ -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
 ------------------------------------------
 
diff --git a/scripts/Makefile.userprogs b/scripts/Makefile.userprogs
index debbf083bcfa01c89c204a322a1b17e2bdbe95a1..e94f3f2250f531dadbe00f5f5ee6f780bee22487 100644
--- a/scripts/Makefile.userprogs
+++ b/scripts/Makefile.userprogs
@@ -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

-- 
2.48.1