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

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Date:   Fri, 28 Aug 2020 14:31:31 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>, Eddy_Wu@...ndmicro.com,
        x86@...nel.org, davem@...emloft.net, rostedt@...dmis.org,
        naveen.n.rao@...ux.ibm.com, anil.s.keshavamurthy@...el.com,
        linux-arch@...r.kernel.org, cameron@...dycamel.com,
        oleg@...hat.com, will@...nel.org, paulmck@...nel.org
Subject: Re: [PATCH v4 04/23] arm64: kprobes: Use generic kretprobe
 trampoline handler

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@...nel.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.