Re: [PATCH v4 2/2] arm64: kprobe: Enable OPTPROBE for arm64

[Masami Hiramatsu]

On Tue, 30 Nov 2021 14:48:06 +0800
"liuqi (BA)" <liuqi115@xxxxxxxxxx> wrote:

> 
> 
> On 2021/11/29 22:35, Masami Hiramatsu wrote:
> >>>>>> Hi all,
> >>>>>>
> >>>>>> I meet a problem when I use SYM_CODE_START(optprobe_template) to replace
> >>>>>> optprobe_template_entry.
> >>>>>>
> >>>>>> If SYM_CODE_START is used, all optprobe will share one trampoline space.
> >>>>>> Under this circumstances, if user register two optprobes, trampoline
> >>>>>> will be overwritten by the newer one, and this will cause kernel panic
> >>>>>> when the old optprobe is trigger.
> >>>>> Hm, this is curious, because the template should be copied to the
> >>>>> trampoline buffer for each optprobe and be modified.
> >>>>>
> >>>>>> Using optprobe_template_entry will not have this problem, as each
> >>>>>> optprobe has its own trampoline space (alloced in get_opinsn_slot()).
> >>>>> Yes, it is designed to do so.
> >>>>>
> >>>>> Thank you,
> >>>>>
> >>>> Hi Masami,
> >>>>
> >>>> Thanks for your reply. But I also met a problem when using
> >>>> get_opinsn_slot() to alloc trampoline buffer.
> >>>>
> >>>> As module_alloc(like x86) is used to alloc buffer, trampoline is in
> >>>> module space, so if origin insn is in kernel space, the range between
> >>>> origin insn and trampoline is out of 128M.
> >>>>
> >>>> As module PLT cannot used here, I have no idea to achieve long jump in
> >>>> this situation. Do you have any good idea?
> >> Hi Masami,
> >>
> >> Thanks so much for your reply.
> >>
> >>> One possible solution is to use pre-allocated trampoline space in
> >>> the text area, as same as ppc64 does.
> >>> (See arch/powerpc/kernel/optprobes_head.S, it embeds a space at "optinsn_slot")
> >>>
> >> I find something interesting in arch/powerpc/kernel/optprobes.c, it use
> >> "optinsn_slot" as a public buffer, and use a static "insn_page_in_use"
> >> to make sure there is only one optprobe in kernel.
> >>
> >> If we use this solution , users could only register one optprobe each
> >> time. This will also be a limitation for users, what's your opinion
> >> about this?
> > No, that is just a memory area for pooling trampoline buffer. So optprobe
> > can allocate the buffer from that area. Please see kernel/kprobes.c:344.
> > optprobe allocates "insn_slot" from kprobe_optinsn_slots, which uses
> > alloc_optinsn_page() to allocate the pool of slots.
> > 
> > Thank you,
> > 
> 
> Hi,
> 
> Thanks for your reply, I test alloc_optinsn_page() and it does work well 
> to alloc the pool of slots.
> 
> But when I tried to use module PLT, something seems wrong here.
> Arm64 Module PLT in mod->arch.ftrace_trampolines is set in 
> module_finalize, after that, mod->arch.ftrace_trampolines seems to be a 
> read-only memory. But in arch_optimize_kprobes() we need to modify the 
> destination of PLT (as each optprobe has its own trampoline buffer), if 
> so, we cannot get rid of the 128M branch limit :(

Hmm, OK, we need to introduce trampoline buffer allocation pool for modules
for such arch. But that should be another story. I think you should start
from the core-kernel. At this moment, if the probe address is in the module,
please return -ERANGE from arch_prepare_optimized_kprobe().
Module support must be done in the next step (series), since that will involve
the kprobes generic code change.

Thank you,

> 
> Thanks,
> Qi


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>