linux-kernel - Re: [PATCH 3/8] arm64: kprobes: Record frame pointer with kretprobe instance

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Message-ID: <20211014091332.GA13770@C02TD0UTHF1T.local>
Date:   Thu, 14 Oct 2021 10:13:32 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Ananth N Mavinakayanahalli <ananth@...ux.ibm.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Sven Schnelle <svens@...ux.ibm.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/8] arm64: kprobes: Record frame pointer with kretprobe
 instance

On Thu, Oct 14, 2021 at 05:04:05PM +0900, Masami Hiramatsu wrote:
> On Wed, 13 Oct 2021 11:01:39 +0100
> Mark Rutland <mark.rutland@....com> wrote:
> 
> > On Fri, Oct 08, 2021 at 09:28:39PM +0900, Masami Hiramatsu wrote:
> > > Record the frame pointer instead of stack address with kretprobe
> > > instance as the identifier on the instance list.
> > > Since arm64 always enable CONFIG_FRAME_POINTER, we can use the
> > > actual frame pointer (x29).
> > 
> > Just to check, why do we need to use the FP rather than SP? It wasn't
> > clear to me if that's necessary later in the series, or if I'm missing
> > something here.
> 
> Actually, this is for finding correct return address from the per-task
> kretprobe instruction list (suppose it as a shadow stack) when it will
> be searched in stack-backtracing. At that point, the framepointer will
> be a reliable key.

Sure, my question was more "why isn't the SP a reliable key?", because both
the SP and FP should be balanced at function-entry and function-return
time. I'm asking because I think I'm missing a subtlety.

I'm perfectly happy to use the FP even if they're equivalent; I just
want to make sure there's not some issue I'm unaware of that could
affect unwinding.

Thanks,
Mark.

> > FWIW, I plan to rework arm64's ftrace bits to use FP for
> > HAVE_FUNCTION_GRAPH_RET_ADDR_PTR, so I'm happy to do likewise here.
> 
> Yes, I think you can use FP for that too.
> 
> > 
> > > Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> > 
> > Regardless of the above:
> > 
> > Acked-by: Mark Rutland <mark.rutland@....com>
> 
> Thank you!
> 
> > 
> > Mark.
> > 
> > > ---
> > >  arch/arm64/kernel/probes/kprobes.c |    4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> > > index e7ad6da980e8..d9dfa82c1f18 100644
> > > --- a/arch/arm64/kernel/probes/kprobes.c
> > > +++ b/arch/arm64/kernel/probes/kprobes.c
> > > @@ -401,14 +401,14 @@ int __init arch_populate_kprobe_blacklist(void)
> > >  
> > >  void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs)
> > >  {
> > > -	return (void *)kretprobe_trampoline_handler(regs, (void *)kernel_stack_pointer(regs));
> > > +	return (void *)kretprobe_trampoline_handler(regs, (void *)regs->regs[29]);
> > >  }
> > >  
> > >  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);
> > > +	ri->fp = (void *)regs->regs[29];
> > >  
> > >  	/* replace return addr (x30) with trampoline */
> > >  	regs->regs[30] = (long)&__kretprobe_trampoline;
> > > 
> 
> 
> -- 
> Masami Hiramatsu <mhiramat@...nel.org>