Messages in this thread | | | Date | Thu, 14 Oct 2021 10:13:32 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH 3/8] arm64: kprobes: Record frame pointer with kretprobe instance |
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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>
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