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

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Date:   Tue, 30 Nov 2021 14:48:06 +0800
From:   "liuqi (BA)" <liuqi115@...wei.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Linuxarm <linuxarm@...wei.com>
CC:     Mark Rutland <mark.rutland@....com>, <catalin.marinas@....com>,
        <will@...nel.org>, <naveen.n.rao@...ux.ibm.com>,
        <anil.s.keshavamurthy@...el.com>, <davem@...emloft.net>,
        <linux-arm-kernel@...ts.infradead.org>,
        <song.bao.hua@...ilicon.com>, <prime.zeng@...ilicon.com>,
        <robin.murphy@....com>, <f.fangjian@...wei.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/2] arm64: kprobe: Enable OPTPROBE for arm64



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 :(

Thanks,
Qi