linux-kernel - Re: [PATCH 7/9] um: add infrastructure to build files using nolibc

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Message-ID: <31a81a3a-e6f2-49b1-9a07-4b11df9e4091@t-8ch.de>
Date: Mon, 15 Sep 2025 10:57:51 +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, Tiwei Bie <tiwei.btw@...group.com>, 
	Benjamin Berg <benjamin.berg@...el.com>
Subject: Re: [PATCH 7/9] um: add infrastructure to build files using nolibc

On 2025-09-15 09:11:13+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.
> 
> Guard the syscall definition as it is a macro in nolibc.
> 
> Signed-off-by: Benjamin Berg <benjamin.berg@...el.com>
> ---
>  arch/um/Makefile               | 21 ++++++++++++++++++++-
>  arch/um/include/shared/os.h    |  2 ++
>  arch/um/scripts/Makefile.rules |  8 +++++++-
>  3 files changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/um/Makefile b/arch/um/Makefile
> index a3f27d791cc0..54aa26fcc32b 100644
> --- a/arch/um/Makefile
> +++ b/arch/um/Makefile
> @@ -78,6 +78,25 @@ 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 $(srctree)/tools/include \
> +		-D__EXPORTED_HEADERS__ \
> +		-D__UM_HOST__ -D__UM_NOLIBC__ \
> +		-DNOLIBC_NO_STARTCODE \
> +		-DNOLIBC_IGNORE_ERRNO \
> +		-nostdlib -nostdinc -static \
> +		-I$(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 \
> +		-include $(srctree)/include/generated/autoconf.h \
> +		-include $(srctree)/tools/include/linux/kconfig.h \
> +		-include $(srctree)/include/linux/kern_levels.h \
> +		-include $(srctree)/tools/include/nolibc/nolibc.h \

This is not needed. It doesn't hurt though.

> +		-include $(srctree)/$(ARCH_DIR)/include/shared/user.h
> +
>  #This will adjust *FLAGS accordingly to the platform.
>  include $(srctree)/$(ARCH_DIR)/Makefile-os-Linux
>  
> @@ -157,4 +176,4 @@ 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
> diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h
> index b35cc8ce333b..3e9f3ae61658 100644
> --- a/arch/um/include/shared/os.h
> +++ b/arch/um/include/shared/os.h
> @@ -327,7 +327,9 @@ extern int __ignore_sigio_fd(int fd);
>  /* tty.c */
>  extern int get_pty(void);
>  
> +#ifndef syscall

You could use 'ifndef NOLIBC' to make the reason clear.

>  long syscall(long number, ...);
> +#endif
>  
>  /* irqflags tracing */
>  extern void block_signals_trace(void);

(...)