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

[Masami Hiramatsu]

On Thu, 14 Oct 2021 10:13:32 +0100
Mark Rutland <mark.rutland@xxxxxxx> wrote:

> 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@xxxxxxx> 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.

Ah, because SP is not used while unwinding frame. For the kretprobe,
either FP or SP is OK. But for the stacktrace.c, I can not use SP
and is easy to change to use FP. :)

So, when we introduce ORC unwinder on arm64, I think I have to reconsider
using SP based on the configuration.

Thank you,

> 
> 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@xxxxxxxxxx>
> > > 
> > > Regardless of the above:
> > > 
> > > Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
> > 
> > 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@xxxxxxxxxx>


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>