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

Mark Rutland
2021-10-14T09:16:18Z

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@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.

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@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>