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

[Masami Hiramatsu <mhiramat@kernel.org>]

On Wed, 13 Oct 2021 11:01:39 +0100
Mark Rutland <mark.rutland@arm.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.

> 
> 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@kernel.org>
> 
> Regardless of the above:
> 
> Acked-by: Mark Rutland <mark.rutland@arm.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@kernel.org>