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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20250522-fluorescent-liberal-pigeon-0404ed@l-nschier-aarch64>
Date: Mon, 26 May 2025 16:19:53 +0200
From: Nicolas Schier <nicolas.schier@...ux.dev>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
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, 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.

> 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

> +--------------------------------
> +
> +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.

-- 
Nicolas Schier