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

[liuqi (BA)]

On 2021/12/1 9:50, Masami Hiramatsu wrote:
> On Tue, 30 Nov 2021 14:48:06 +0800
> "liuqi (BA)" <liuqi115 at huawei.com> 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 :(
> 
Hi Masami,

> Hmm, OK, we need to introduce trampoline buffer allocation pool for modules
> for such arch. 

Yes, but will this expand the size of Image?

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.
> 

got it, I'll send a new patchset which support core-kernel optprobe 
soon. thanks a lot for your reply.

Qi
> Thank you,
> 
>>
>> Thanks,
>> Qi
> 
>