On Mon, May 26, 2025 at 04:19:53PM +0200, Nicolas Schier wrote:
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 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.
(out-of-tree == external kmods; out-of-source == build-dir != source-dir)
you probably meant out-of-source.
I *did* mean out-of-tree.
Out-of-source already works with the current patchset. It is the default setup of kunit.py.
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
Ack.
+--------------------------------
+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.
Currently this dependency is encoded in Kconfig. If CONFIG_HEADERS_INSTALL=y then the headers are installed in the 'prepare' phase and already available when building any userprog. To me this seems like the easier and nicer implementation.