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Date:   Sun, 11 Dec 2022 12:27:31 +0900
From:   Masami Hiramatsu (Google) <mhiramat@kernel.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        mark.rutland@arm.com, will@kernel.org, rostedt@goodmis.org,
        samitolvanen@google.com, keescook@chromium.org, mhiramat@kernel.org
Subject: Re: [PATCH] ftrace: Allow WITH_ARGS flavour of graph tracer with
 shadow call stack
Message-Id: <20221211122731.f909ac15e227675229f03ecf@kernel.org>
In-Reply-To: <20221209143402.3332369-1-ardb@kernel.org>
References: <20221209143402.3332369-1-ardb@kernel.org>
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On Fri,  9 Dec 2022 15:34:02 +0100
Ard Biesheuvel <ardb@kernel.org> wrote:

> The recent switch on arm64 from DYNAMIC_FTRACE_WITH_REGS to
> DYNAMIC_FTRACE_WITH_ARGS failed to take into account that we currently
> require the former in order to allow the function graph tracer to be
> enabled in combination with shadow call stacks. This means that this is
> no longer permitted at all, in spite of the fact that either flavour of
> ftrace works perfectly fine in this combination.
> 
> Given that arm64 is the only arch that implements shadow call stacks in
> the first place, let's update the condition to just reflect the arm64
> change. When other architectures adopt shadow call stack support, this
> can be revisited if needed.

This brings a question. Is the SCS safe if kretprobe(rethook) is enabled?
it also changes the stack entry after a calling function.

Thank you,

> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 072a1b39e3afd0d1..683f365b5e31c856 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -635,7 +635,7 @@ config ARCH_SUPPORTS_SHADOW_CALL_STACK
>  config SHADOW_CALL_STACK
>  	bool "Shadow Call Stack"
>  	depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
> -	depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER
> +	depends on DYNAMIC_FTRACE_WITH_ARGS || !FUNCTION_GRAPH_TRACER
>  	help
>  	  This option enables the compiler's Shadow Call Stack, which
>  	  uses a shadow stack to protect function return addresses from
> -- 
> 2.35.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>