linux-kernel - Re: [PATCH v3 08/12] um: add infrastructure to build files using nolibc

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Message-ID: <43a71f2c-2d3d-4b66-b755-d182eefc90d2@t-8ch.de>
Date: Thu, 25 Sep 2025 18:36:36 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Benjamin Berg <benjamin@...solutions.net>
Cc: linux-um@...ts.infradead.org, Willy Tarreau <w@....eu>, 
	linux-kselftest@...r.kernel.org, Arnaldo Carvalho de Melo <acme@...hat.com>, 
	linux-kernel@...r.kernel.org, Benjamin Berg <benjamin.berg@...el.com>
Subject: Re: [PATCH v3 08/12] um: add infrastructure to build files using
 nolibc

On 2025-09-24 16:20:55+0200, Benjamin Berg wrote:
> From: Benjamin Berg <benjamin.berg@...el.com>
> 
> Add NOLIBC_CFLAGS and NOLIBC_OBJS to build files against nolibc rather
> than libc. With this it is possible to move to nolibc in smaller steps.
> 
> Set NOLIBC_IGNORE_ERRNO, as the nolibc errno implementation is overly
> simple and cannot handle threading. nolibc provides sys_* functions that
> do not emulate the libc errno behaviour and can be used instead.
> 
> Leave the syscall definition for kernel code for now, but guard it as it
> is a macro in nolibc.
> 
> Signed-off-by: Benjamin Berg <benjamin.berg@...el.com>
> 
> ---
> v3:
> - Change the syscall guard to use __KERNEL__
> 
> v2:
> - Do not include nolibc.h via CFLAGS
> - Make syscall guard more explicit
> - Remove __UM_NOLIBC__ define, it is not needed
> - Fix out-of-tree building
> ---
>  arch/um/Makefile               | 20 +++++++++++++++++++-
>  arch/um/include/shared/os.h    |  2 ++
>  arch/um/include/shared/user.h  |  1 -
>  arch/um/scripts/Makefile.rules |  8 +++++++-
>  4 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/um/Makefile b/arch/um/Makefile
> index f7c509262568..c3a81df50911 100644
> --- a/arch/um/Makefile
> +++ b/arch/um/Makefile
> @@ -78,6 +78,24 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
>  		-include $(srctree)/include/linux/kern_levels.h \
>  		-include $(srctree)/$(ARCH_DIR)/include/shared/user.h
>  
> +NOLIBC_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
> +		$(ARCH_INCLUDE) $(MODE_INCLUDE) $(filter -I%,$(CFLAGS)) \
> +		-I $(srctree)/tools/include \
> +		-D__EXPORTED_HEADERS__ \
> +		-D__UM_HOST__ \
> +		-DNOLIBC_NO_RUNTIME \
> +		-DNOLIBC_IGNORE_ERRNO \
> +		-nostdlib -nostdinc -static \
> +		-I$(srctree)/include/uapi \
> +		-I$(srctree)/$(HOST_DIR)/include/uapi \
> +		-I$(objtree)/$(HOST_DIR)/include/generated/uapi \
> +		-I $(srctree)/tools/include/nolibc \
> +		-I $(srctree)/usr/include \

This looks incorrect. The UAPI headers would be installed to $(objtree).
But UML does not even use CONFIG_HEADERS_INSTALL.
Also the whitespace is inconsistent between different lines.

> +		-include $(objtree)/include/generated/autoconf.h \
> +		-include $(srctree)/tools/include/linux/kconfig.h \
> +		-include $(srctree)/include/linux/kern_levels.h \
> +		-include $(srctree)/$(ARCH_DIR)/include/shared/user.h
> +
>  #This will adjust *FLAGS accordingly to the platform.
>  include $(srctree)/$(ARCH_DIR)/Makefile-os-Linux
>  
> @@ -160,4 +178,4 @@ archclean:
>  		-o -name '*.gcov' \) -type f -print | xargs rm -f
>  	$(Q)$(MAKE) -f $(srctree)/Makefile ARCH=$(HEADER_ARCH) clean
>  
> -export HEADER_ARCH SUBARCH USER_CFLAGS CFLAGS_NO_HARDENING DEV_NULL_PATH
> +export HEADER_ARCH SUBARCH USER_CFLAGS NOLIBC_CFLAGS CFLAGS_NO_HARDENING DEV_NULL_PATH

(...)