linux-hardening - Re: [PATCH v4 06/15] ftrace: Use an opaque type for functions not callable from C

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Date:   Tue, 5 Oct 2021 20:29:45 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, Kees Cook <keescook@...omium.org>,
        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,
        llvm@...ts.linux.dev, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v4 06/15] ftrace: Use an opaque type for functions not
 callable from C

On Thu, Sep 30, 2021 at 11:05:22AM -0700, Sami Tolvanen wrote:
> With CONFIG_CFI_CLANG, the compiler changes function references to point
> to the CFI jump table. As ftrace_call, ftrace_regs_call, and mcount_call
> are not called from C, use DECLARE_ASM_FUNC_SYMBOL to declare them.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  include/linux/ftrace.h | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 832e65f06754..67de28464aeb 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -578,9 +578,10 @@ extern void ftrace_replace_code(int enable);
>  extern int ftrace_update_ftrace_func(ftrace_func_t func);
>  extern void ftrace_caller(void);
>  extern void ftrace_regs_caller(void);
> -extern void ftrace_call(void);
> -extern void ftrace_regs_call(void);
> -extern void mcount_call(void);
> +
> +DECLARE_ASM_FUNC_SYMBOL(ftrace_call);
> +DECLARE_ASM_FUNC_SYMBOL(ftrace_regs_call);
> +DECLARE_ASM_FUNC_SYMBOL(mcount_call);

I'm thinking DECLARE_ASM_FUNC_SYMBOL needs a better name. It's not clear
from reading it why some asm symbols need the macro and others don't.

I guess it means "an asm text symbol which isn't callable from C code
(not including alternatives)"?

DECLARE_UNCALLED_SYMBOL() maybe?

-- 
Josh