linux-kernel - Re: [PATCH] parisc/kernel/ftrace: Remove function callback casts

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Message-ID: <97375269-f6f1-6157-c8a8-3b925232f00c@gmx.de>
Date:   Sat, 27 Jun 2020 22:09:40 +0200
From:   Helge Deller <deller@....de>
To:     Oscar Carter <oscar.carter@....com>,
        Kees Cook <keescook@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        "James E . J . Bottomley" <James.Bottomley@...senPartnership.com>
Cc:     kernel-hardening@...ts.openwall.com, linux-parisc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] parisc/kernel/ftrace: Remove function callback casts

On 27.06.20 15:43, Oscar Carter wrote:
> In an effort to enable -Wcast-function-type in the top-level Makefile to
> support Control Flow Integrity builds, remove all the function callback
> casts.
>
> To do this remove the cast to a function pointer type in the comparison
> statement and add to the right and left operand a cast to unsigned long
> type. This can be done since the comparison is against function address
> (these operands are not function calls).

On some architectures (namely ia64, ppc64 and parisc64) function pointers
actually refer to function descriptors, which in turn point to the real
function address. The compiler usually takes care of such comparism.
That said, casting to "unsigned long" might break this...

> Signed-off-by: Oscar Carter <oscar.carter@....com>
> ---
>  arch/parisc/kernel/ftrace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/parisc/kernel/ftrace.c b/arch/parisc/kernel/ftrace.c
> index 1df0f67ed667..86b49a5fc049 100644
> --- a/arch/parisc/kernel/ftrace.c
> +++ b/arch/parisc/kernel/ftrace.c
> @@ -64,7 +64,7 @@ void notrace __hot ftrace_function_trampoline(unsigned long parent,
>  				function_trace_op, regs);
>
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> -	if (ftrace_graph_return != (trace_func_graph_ret_t) ftrace_stub ||
> +	if ((unsigned long)ftrace_graph_return != (unsigned long)ftrace_stub ||

Untested, but maybe better use:
	dereference_function_descriptor(ftrace_graph_return) !=
		dereference_function_descriptor(ftrace_stub)

?
Helge