linux-kernel - Re: [PATCH v4 18/24] riscv: vdso: Disable LTO for the vDSO

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Message-ID: <mhng-146EE95D-951E-41BB-898C-033437B27EA3@Palmers-Mini.rwc.dabbelt.com>
Date: Fri, 15 Aug 2025 08:07:33 -0700 (PDT)
From: Palmer Dabbelt <palmer@...belt.com>
To: thomas.weissschuh@...utronix.de
CC: Paul Walmsley <paul.walmsley@...ive.com>, aou@...s.berkeley.edu,
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  thomas.weissschuh@...utronix.de
Subject:     Re: [PATCH v4 18/24] riscv: vdso: Disable LTO for the vDSO

On Mon, 11 Aug 2025 22:44:31 PDT (-0700), thomas.weissschuh@...utronix.de wrote:
> All the other architectures supporting LTO (x86, arm64, loongarch) do not
> use it for the vDSO.
>
> Its is problematic for some upcoming compile-time validation of the
> generated object code.
> The LTO object files do not contain the necessary relocation information
> and -flto-fat-objects is not compatible with clang < 16.
>
> For consistency and to enable the mentioned compile-time checks,
> disable LTO for the vDSO.
> The vDSO heavily uses __always_inline anyways.
>
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
> Reviewed-by: Nathan Chancellor <nathan@...nel.org>
> Tested-by: Jan Stancek <jstancek@...hat.com>
> ---
>  arch/riscv/kernel/vdso/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> index c19c3c76f7c9f6b2f7523a59269de3b683656323..9f1bf5bae9bd473e43d9fd3022e9e1a185128b73 100644
> --- a/arch/riscv/kernel/vdso/Makefile
> +++ b/arch/riscv/kernel/vdso/Makefile
> @@ -49,7 +49,7 @@ CPPFLAGS_vdso.lds += -DHAS_VGETTIMEOFDAY
>  endif
>
>  # Disable -pg to prevent insert call site
> -CFLAGS_REMOVE_VDSO = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS)
> +CFLAGS_REMOVE_VDSO = $(CC_FLAGS_FTRACE) $(CC_FLAGS_SCS) $(CC_FLAGS_LTO)
>  CFLAGS_REMOVE_vgettimeofday.o = $(CFLAGS_REMOVE_VDSO)
>  CFLAGS_REMOVE_getrandom.o = $(CFLAGS_REMOVE_VDSO)
>  CFLAGS_REMOVE_hwprobe.o = $(CFLAGS_REMOVE_VDSO)

Acked-by: Palmer Dabbelt <palmer@...belt.com>

Also assuming this goes with the others.  Thanks!