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

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Message-ID: <20200828133718.GB1362448@hirez.programming.kicks-ass.net>
Date:   Fri, 28 Aug 2020 15:37:18 +0200
From:   peterz@...radead.org
To:     Mark Rutland <mark.rutland@....com>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.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

On Fri, Aug 28, 2020 at 02:31:31PM +0100, Mark Rutland wrote:
> 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.

IIUC ri->rp is used for matching up the frame between the moment we
install the return trampoline on the stack and actually hitting that
trampoline.