linux-kernel - Re: [PATCH 2/3] linux/compiler-clang.h: define function

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Date:   Tue, 5 Apr 2022 17:16:02 +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 2/3] linux/compiler-clang.h: define function_nocfi

On Fri, Apr 01, 2022 at 01:19:15PM -0700, Sami Tolvanen wrote:
> Use __builtin_function_start() to implement the function_nocfi() macro
> when CONFIG_CFI_CLANG is selected.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  include/linux/compiler-clang.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index babb1347148c..c84fec767445 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -69,6 +69,16 @@
>  #define __nocfi		__attribute__((__no_sanitize__("cfi")))
>  #define __cficanonical	__attribute__((__cfi_canonical_jump_table__))
>  
> +#if defined(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)	__builtin_function_start(x)
> +#endif

As a heads-up, with patches 1 and 2 applied (but not patch 3), we have two
definitions of function_nocfi(), which casues the compiler to scream for every
object:

| In file included from <built-in>:3:
| In file included from ././include/linux/compiler_types.h:107:
| ./arch/arm64/include/asm/compiler.h:33:9: warning: 'function_nocfi' macro redefined [-Wmacro-redefined]
| #define function_nocfi(x) ({                                            \
|         ^
| ./include/linux/compiler-clang.h:79:9: note: previous definition is here
| #define function_nocfi(x)       __builtin_function_start(x)
|         ^
| 1 warning generated.

... which would be painful for bisection.

I suspect It's probably worth folding patches 2 and 3 together to avoid that?

Arguably we could fold the entire series together into one commit at that
point.

Thanks,
Mark.