linux-hardening - Re: [PATCH v5 06/15] ftrace: Use an opaque type for functions not callable from C

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Date:   Wed, 13 Oct 2021 15:20:56 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, 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,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v5 06/15] ftrace: Use an opaque type for functions not
 callable from C

On Wed, 13 Oct 2021 11:16:49 -0700
Sami Tolvanen <samitolvanen@...gle.com> 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 by C code, but are trampolines injected
> as calls replacing the nops at the start of functions added by the
> compiler, use DECLARE_NOT_CALLED_FROM_C to declare them.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Sedat Dilek <sedat.dilek@...il.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..c53a00b96ba9 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_NOT_CALLED_FROM_C(ftrace_call);
> +DECLARE_NOT_CALLED_FROM_C(ftrace_regs_call);
> +DECLARE_NOT_CALLED_FROM_C(mcount_call);
>  
>  void ftrace_modify_all_code(int command);
>  

Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

-- Steve