Re: [PATCH v4 04/23] arm64: kprobes: Use generic kretprobe trampoline handler

From: Mark Rutland <mark.rutland@arm.com>
To: Masami Hiramatsu <mhiramat@kernel.org>
Cc: linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Eddy_Wu@trendmicro.com, x86@kernel.org, davem@davemloft.net,
	rostedt@goodmis.org, naveen.n.rao@linux.ibm.com,
	anil.s.keshavamurthy@intel.com, linux-arch@vger.kernel.org,
	cameron@moodycamel.com, oleg@redhat.com, will@kernel.org,
	paulmck@kernel.org
Subject: Re: [PATCH v4 04/23] arm64: kprobes: Use generic kretprobe trampoline handler
Date: Fri, 28 Aug 2020 14:31:31 +0100	[thread overview]
Message-ID: <20200828133131.GA71981@C02TD0UTHF1T.local> (raw)
In-Reply-To: <159861764221.992023.10214437014901668680.stgit@devnote2>

Hi,

On Fri, Aug 28, 2020 at 09:27:22PM +0900, Masami Hiramatsu wrote:
> Use the generic kretprobe trampoline handler, and use the
> kernel_stack_pointer(regs) for framepointer verification.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
>  arch/arm64/kernel/probes/kprobes.c |   78 +-----------------------------------
>  1 file changed, 3 insertions(+), 75 deletions(-)

> +	return (void *)kretprobe_trampoline_handler(regs, &kretprobe_trampoline,
> +					(void *)kernel_stack_pointer(regs));
>  }
>  
>  void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
>  				      struct pt_regs *regs)
>  {
>  	ri->ret_addr = (kprobe_opcode_t *)regs->regs[30];
> +	ri->fp = (void *)kernel_stack_pointer(regs);

This is probably a nomenclature problem, but what exactly is
kretprobe_instance::fp used for?

I ask because arm64's frame pointer lives in x29 (and is not necessarily
the same as the stack pointer at function boundaries), so the naming
is potentially misleading and possibly worth a comment or rename.

Thanks,
Mark.