Re: [PATCH v3 04/16] kbuild: userprogs: add nolibc support

[Masahiro Yamada]

On Tue, Jun 17, 2025 at 4:59 PM Thomas Weißschuh
<thomas.weissschuh@xxxxxxxxxxxxx> wrote:
>
> On Tue, Jun 17, 2025 at 12:35:07AM +0900, Masahiro Yamada wrote:
> > 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.
>
> <snip>
>
> > > --- 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.
>
> Ack.
>
> How does this statement affect the next patch which creates
> tools/include/nolibc/Kconfig.nolibc ?
> Is it fine to create the Kconfig file in tools/ or should I move it?
> I do want to maintain this file as part of nolibc and not KUnit.
> The possibilities I see are init/Kconfig.nolibc or lib/Kconfig.nolibc.

Personally I like init/Kconfig.nolibc.
(I am even fine with adding this new entry to init/Kconfig)
But, this file is not what I maintain. It is up to you after all.



> > 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.
>
> Understood. This is not unexpected, as hinted in the commit message.

One existing example is lib/vdso/Makefile.include, which is included
from multiple Makefiles.





> > > +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]  $@
>
>
> Thomas


--
Best Regards
Masahiro Yamada