linux-kernel - Re: [PATCH v2 01/11] kbuild: userprogs: add nolibc support

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Message-ID: <20250526163610-88b7aae6-7be4-4a02-be20-ec7fe74cbf31@linutronix.de>
Date: Mon, 26 May 2025 16:40:17 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Nicolas Schier <nicolas.schier@...ux.dev>
Cc: Masahiro Yamada <masahiroy@...nel.org>, 
	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>, 
	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
Subject: Re: [PATCH v2 01/11] kbuild: userprogs: add nolibc support

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@...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.
> 
> (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.