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

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Message-ID: <20250617095102-d3df1c46-3d51-4f77-af0a-8299f5e71ad9@linutronix.de>
Date: Tue, 17 Jun 2025 09:59:08 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Willy Tarreau <w@....eu>, 
	Thomas Weißschuh <linux@...ssschuh.net>, Brendan Higgins <brendan.higgins@...ux.dev>, 
	David Gow <davidgow@...gle.com>, Rae Moar <rmoar@...gle.com>, Shuah Khan <shuah@...nel.org>, 
	Jonathan Corbet <corbet@....net>, Nicolas Schier <nicolas.schier@...ux.dev>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>, 
	Christophe Leroy <christophe.leroy@...roup.eu>, linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com, linux-doc@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, workflows@...r.kernel.org
Subject: Re: [PATCH v3 04/16] kbuild: userprogs: add nolibc support

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@...utronix.de> 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@...utronix.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.

<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.

> 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.

> > +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