linux-kernel - Re: [PATCH 3/3] arm64: Drop the inline assembly implementation of function

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Date:   Tue, 5 Apr 2022 17:23:13 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 3/3] arm64: Drop the inline assembly implementation of
 function_nocfi

On Fri, Apr 01, 2022 at 01:19:16PM -0700, Sami Tolvanen wrote:
> Drop the inline assembly version of function_nocfi() now that
> CONFIG_CFI_CLANG depends on __builtin_function_start(), which allows
> this macro to be implemented more cleanly and in a way that works also
> with static initializers.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  arch/arm64/include/asm/compiler.h | 16 ----------------
>  1 file changed, 16 deletions(-)

It's very nice to see the asm go here!

I grabbed the LLVM 14.0.0 binary release from llvm.org and gave this a spin. I
tested with ThinLTO and CFI_CLANG, and verified that CPU hotplug and ftrace
worked as expected (given those rely on function_nocfi()), and that the LKDTM
CFI_FORWARD_PROTO caught the mismatched prototype as expected.

FWIW:

Reviewed-by: Mark Rutland <mark.rutland@....com>
Tested-by: Mark Rutland <mark.rutland@....com>

As noted on patch 2 I think we should squash the series together, but those
tags apply either way.

Thanks,
Mark.

> 
> diff --git a/arch/arm64/include/asm/compiler.h b/arch/arm64/include/asm/compiler.h
> index dc3ea4080e2e..6fb2e6bcc392 100644
> --- a/arch/arm64/include/asm/compiler.h
> +++ b/arch/arm64/include/asm/compiler.h
> @@ -23,20 +23,4 @@
>  #define __builtin_return_address(val)					\
>  	(void *)(ptrauth_clear_pac((unsigned long)__builtin_return_address(val)))
>  
> -#ifdef CONFIG_CFI_CLANG
> -/*
> - * With CONFIG_CFI_CLANG, the compiler replaces function address
> - * references with the address of the function's CFI jump table
> - * entry. The function_nocfi macro always returns the address of the
> - * actual function instead.
> - */
> -#define function_nocfi(x) ({						\
> -	void *addr;							\
> -	asm("adrp %0, " __stringify(x) "\n\t"				\
> -	    "add  %0, %0, :lo12:" __stringify(x)			\
> -	    : "=r" (addr));						\
> -	addr;								\
> -})
> -#endif
> -
>  #endif /* __ASM_COMPILER_H */
> -- 
> 2.35.0
>