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

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Date:   Mon, 29 Nov 2021 09:40:30 +0800
From:   "liuqi (BA)" <liuqi115@...wei.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
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>,
        <linuxarm@...wei.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/2] arm64: kprobe: Enable OPTPROBE for arm64



On 2021/11/27 20:23, Masami Hiramatsu wrote:
> On Fri, 26 Nov 2021 18:31:06 +0800
> "liuqi (BA)" <liuqi115@...wei.com> wrote:
> 
>>
>>
>> On 2021/8/24 18:50, Mark Rutland wrote:
>>>> diff --git a/arch/arm64/kernel/probes/optprobe_trampoline.S b/arch/arm64/kernel/probes/optprobe_trampoline.S
>>>> new file mode 100644
>>>> index 000000000000..24d713d400cd
>>>> --- /dev/null
>>>> +++ b/arch/arm64/kernel/probes/optprobe_trampoline.S
>>>> @@ -0,0 +1,37 @@
>>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>>> +/*
>>>> + * trampoline entry and return code for optprobes.
>>>> + */
>>>> +
>>>> +#include <linux/linkage.h>
>>>> +#include <asm/asm-offsets.h>
>>>> +#include <asm/assembler.h>
>>>> +
>>>> +	.global optprobe_template_entry
>>>> +optprobe_template_entry:
>>> Please use SYM_*(); see arch/arm64/kernel/entry-ftrace.S for examples of
>>> how to use that for trampolines.
>>>
>>> This should be:
>>>
>>> SYM_CODE_START(optprobe_template)
>>>
>> 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?

Thanks,
Qi

>>
>> So how to reuse SYM_CODE_START  in this situation, does anyone has a
>> good idea?
>>
>> Thanks,
>> Qi
>>> ... and note the matching end below.
>>>
>>>> +	sub sp, sp, #PT_REGS_SIZE
>>>> +	save_all_base_regs
>>>> +	/* Get parameters to optimized_callback() */
>>>> +	ldr	x0, 1f
>>>> +	mov	x1, sp
>>>> +	/* Branch to optimized_callback() */
>>>> +	.global optprobe_template_call
>>>> +optprobe_template_call:
>>> SYM_INNER_LABEL(optprobe_template_call, SYM_L_GLOBAL)
>>>
>>> ...and likewise for all the other labels.
>>>
>>>> +	nop
>>>> +	restore_all_base_regs
>>>> +	ldr lr, [sp, #S_LR]
>>>> +        add sp, sp, #PT_REGS_SIZE
>>>> +	.global optprobe_template_restore_orig_insn
>>>> +optprobe_template_restore_orig_insn:
>>>> +	nop
>>>> +	.global optprobe_template_restore_end
>>>> +optprobe_template_restore_end:
>>>> +	nop
>>>> +	.global optprobe_template_end
>>>> +optprobe_template_end:
>>>> +	.global optprobe_template_val
>>>> +optprobe_template_val:
>>>> +	1:	.long 0
>>>> +		.long 0
>>>> +	.global optprobe_template_max_length
>>>> +optprobe_template_max_length:
>>> SYM_INNER_LABEL(optprobe_template_end, SYM_L_GLOBAL)
>>> SYM_CODE_END(optprobe_template)
>>>
>>> Thanks,
>>> Mark.
>>>
>>>> -- 
> 
>