On Wed, Jun 11, 2025 at 4:38 PM Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx> 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@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. > > 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
