linux-hardening - Re: [PATCH 04/15] static_call: Use global functions for the self-test

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Date:   Fri, 16 Apr 2021 23:37:16 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sami Tolvanen <samitolvanen@...gle.com>, x86@...nel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH 04/15] static_call: Use global functions for the self-test

On Fri, Apr 16 2021 at 13:38, Sami Tolvanen wrote:

> With CONFIG_CFI_CLANG, the compiler renames static functions. This
> breaks static_call users using static functions, because the current
> implementation assumes functions have stable names by hardcoding them
> in inline assembly. Make the self-test functions global to prevent the
> compiler from renaming them.

Sorry, no.

> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  kernel/static_call.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/static_call.c b/kernel/static_call.c
> index 723fcc9d20db..d09f500c2d2a 100644
> --- a/kernel/static_call.c
> +++ b/kernel/static_call.c
> @@ -503,12 +503,12 @@ long __static_call_return0(void)
>  
>  #ifdef CONFIG_STATIC_CALL_SELFTEST
>  
> -static int func_a(int x)
> +int func_a(int x)
>  {
>  	return x+1;
>  }
>  
> -static int func_b(int x)
> +int func_b(int x)
>  {
>  	return x+2;
>  }

Did you even compile that?

Global functions without a prototype are generating warnings, but we can
ignore them just because of sekurity, right?

Aside of that polluting the global namespace with func_a/b just to work
around a tool shortcoming is beyond hillarious.

Fix the tool not the perfectly correct code.

Thanks,

        tglx