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Date:	Mon, 20 Oct 2014 10:55:30 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...hat.com>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Jiri Kosina <jkosina@...e.cz>, Jiri Slaby <jslaby@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] kprobes: introduce ARCH_HANDLES_KPROBES_ON_FTRACE

(2014/10/16 0:46), Heiko Carstens wrote:
> Allow architectures to implement handling of kprobes on function
> tracer call sites on their own, without depending on common code.
> 
> This patch removes the kprobes check if a kprobe is being placed
> on a function tracer call site and therefore gives full responsibility
> of handling this correctly to the architecture.
> 
> Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
> ---
>  arch/Kconfig     | 8 ++++++++
>  kernel/kprobes.c | 3 ++-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 05d7a8a458d5..e1a8e0edf03f 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -85,6 +85,14 @@ config KPROBES_ON_FTRACE
>  	 passing of pt_regs to function tracing, then kprobes can
>  	 optimize on top of function tracing.
>  
> +config ARCH_HANDLES_KPROBES_ON_FTRACE
> +	def_bool n
> +	help
> +	 If an architecture can handle kprobes on function tracer call
> +	 sites on own, then this option should be selected. This option
> +	 removes the check which otherwise prevents to set kprobes on
> +	 function tracer call sites.
> +
>  config UPROBES
>  	def_bool n
>  	select PERCPU_RWSEM
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 3995f546d0f3..4cc48aa67635 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1428,7 +1428,8 @@ static int check_kprobe_address_safe(struct kprobe *p,
>  			return -EILSEQ;
>  		p->flags |= KPROBE_FLAG_FTRACE;
>  #else	/* !CONFIG_KPROBES_ON_FTRACE */
> -		return -EINVAL;
> +		if (!IS_ENABLED(CONFIG_ARCH_HANDLES_KPROBES_ON_FTRACE))
> +			return -EINVAL;
>  #endif
>  	}

Hmm, this looks a bit not straight. Maybe we'd better introduce a local
check_ftrace_location() function which always returns 0 if
CONFIG_ARCH_HANDLES_KPROBES_ON_FTRACE(with a comment! :)) as below.

int check_ftrace_location(kp)
{
	unsigned long ftrace_address;

	/* If an architecture handles kprobes on ftrace, we don't check it */
	if (IS_ENABLED(CONFIG_ARCH_HANDLES_KPROBES_ON_FTRACE))
		return 0;

	...
}

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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