Re: [PATCH] arm64: Do not include __READ_ONCE() block in assembly files

[Nick Desaulniers <ndesaulniers@google.com>]

On Wed, Mar 9, 2022 at 11:19 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> When building arm64 defconfig + CONFIG_LTO_CLANG_{FULL,THIN}=y after
> commit 558c303c9734 ("arm64: Mitigate spectre style branch history side
> channels"), the following error occurs:
>
>   <instantiation>:4:2: error: invalid fixup for movz/movk instruction
>    mov w0, #ARM_SMCCC_ARCH_WORKAROUND_3
>    ^
>
> Marc figured out that moving "#include <linux/init.h>" in
> include/linux/arm-smccc.h into a !__ASSEMBLY__ block resolves it. The
> full include chain with CONFIG_LTO=y from include/linux/arm-smccc.h:
>
> include/linux/init.h
> include/linux/compiler.h
> arch/arm64/include/asm/rwonce.h
> arch/arm64/include/asm/alternative-macros.h
> arch/arm64/include/asm/assembler.h
>
> The asm/alternative-macros.h include in asm/rwonce.h only happens when
> CONFIG_LTO is set, which ultimately casues asm/assembler.h to be
> included before the definition of ARM_SMCCC_ARCH_WORKAROUND_3. As a
> result, the preprocessor does not expand ARM_SMCCC_ARCH_WORKAROUND_3 in
> __mitigate_spectre_bhb_fw, which results in the error above.
>
> Avoid this problem by just avoiding the CONFIG_LTO=y __READ_ONCE() block
> in asm/rwonce.h with assembly files, as nothing in that block is useful
> to assembly files, which allows ARM_SMCCC_ARCH_WORKAROUND_3 to be
> properly expanded with CONFIG_LTO=y builds.
>
> Cc: stable@vger.kernel.org
> Fixes: e35123d83ee3 ("arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG_LTO=y")
> Link: https://lore.kernel.org/r/20220309155716.3988480-1-maz@kernel.org/
> Reported-by: Marc Zyngier <maz@kernel.org>
> Acked-by: James Morse <james.morse@arm.com>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Thanks for taking point on all of the BHB fallout.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>
> This is based on current mainline; if it should be based on a specific
> arm64 branch, please let me know.
>
> As 558c303c9734 is going to stable, I marked this for stable as well to
> avoid breaking Android. I used e35123d83ee3 for the fixes tag to make it
> clear to the stable team this should only go where that commit is
> present. If a different fixes tag should be used, please feel free to
> substitute.
>
>  arch/arm64/include/asm/rwonce.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
> index 1bce62fa908a..56f7b1d4d54b 100644
> --- a/arch/arm64/include/asm/rwonce.h
> +++ b/arch/arm64/include/asm/rwonce.h
> @@ -5,7 +5,7 @@
>  #ifndef __ASM_RWONCE_H
>  #define __ASM_RWONCE_H
>
> -#ifdef CONFIG_LTO
> +#if defined(CONFIG_LTO) && !defined(__ASSEMBLY__)
>
>  #include <linux/compiler_types.h>
>  #include <asm/alternative-macros.h>
> @@ -66,7 +66,7 @@
>  })
>
>  #endif /* !BUILD_VDSO */
> -#endif /* CONFIG_LTO */
> +#endif /* CONFIG_LTO && !__ASSEMBLY__ */
>
>  #include <asm-generic/rwonce.h>
>
>
> base-commit: 330f4c53d3c2d8b11d86ec03a964b86dc81452f5
> --
> 2.35.1
>


-- 
Thanks,
~Nick Desaulniers