linux-kernel - Re: [PATCH v2 03/10] kbuild: allow architectures to override CC_CAN

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Message-ID: <20251114043122.GC3582402@ax162>
Date: Thu, 13 Nov 2025 21:31:22 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Nicolas Schier <nicolas.schier@...ux.dev>,
	Nicolas Schier <nsc@...nel.org>, Paul Walmsley <pjw@...nel.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	Andreas Larsson <andreas@...sler.com>, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Masahiro Yamada <masahiroy@...nel.org>,
	linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-mips@...r.kernel.org,
	sparclinux@...r.kernel.org
Subject: Re: [PATCH v2 03/10] kbuild: allow architectures to override
 CC_CAN_LINK

On Tue, Oct 14, 2025 at 03:05:18PM +0200, Thomas Weißschuh wrote:
> The generic test for CC_CAN_LINK assumes that all architectures use -m32
> and -m64 to switch between 32-bit and 64-bit compilation. This is overly
> simplistic. Architectures may use other flags (-mabi, -m31, etc.) or may
> also require byte order handling (-mlittle-endian, -EL). Expressing all
> of the different possibilities will be very complicated and brittle.
> Instead allow architectures to supply their own logic which will be
> easy to understand and evolve.
> 
> Both the boolean ARCH_HAS_CC_CAN_LINK and the string ARCH_USERFLAGS need
> to be implemented as kconfig does not allow the reuse of string options.
> 
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>

Modulo Nicolas's existing comment about CONFIG_ARCH_USERPROGS_CFLAGS:

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  Makefile     | 13 +++++++++++--
>  init/Kconfig |  4 ++++
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 17cfa11ca7163aa3297101ceb3c9e85f4878f12d..26e5bca534e27034c355939bca4c90db435ecdbd 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1134,8 +1134,17 @@ ifneq ($(CONFIG_ARCH_VMLINUX_NEEDS_RELOCS),)
>  LDFLAGS_vmlinux	+= --emit-relocs --discard-none
>  endif
>  
> -# Align the bit size of userspace programs with the kernel
> -USERFLAGS_FROM_KERNEL := -m32 -m64 --target=%
> +# Align the architecture of userspace programs with the kernel
> +USERFLAGS_FROM_KERNEL := --target=%
> +
> +ifdef CONFIG_ARCH_USERPROGS_CFLAGS
> +KBUILD_USERCFLAGS += $(CONFIG_ARCH_USERFLAGS)
> +KBUILD_USERLDFLAGS += $(CONFIG_ARCH_USERFLAGS)
> +else
> +# If not overridden also inherit the bit size
> +USERFLAGS_FROM_KERNEL += -m32 -m64
> +endif
> +
>  KBUILD_USERCFLAGS  += $(filter $(USERFLAGS_FROM_KERNEL), $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS))
>  KBUILD_USERLDFLAGS += $(filter $(USERFLAGS_FROM_KERNEL), $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS))
>  
> diff --git a/init/Kconfig b/init/Kconfig
> index 7b722e714d5c5a0580467914b226dc0700ec0797..4a2ae3cfbf26525a60936d9b29a74ef4319ba3a5 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -82,8 +82,12 @@ config RUSTC_LLVM_VERSION
>  	int
>  	default $(rustc-llvm-version)
>  
> +config ARCH_HAS_CC_CAN_LINK
> +	bool
> +
>  config CC_CAN_LINK
>  	bool
> +	default ARCH_CC_CAN_LINK if ARCH_HAS_CC_CAN_LINK
>  	default $(cc_can_link_user,$(m64-flag)) if 64BIT
>  	default $(cc_can_link_user,$(m32-flag))
>  
> 
> -- 
> 2.51.0
>