[PATCH] riscv/kprobe: Optimize the performance of patching instruction slot

[liaochang (A)]

在 2022/9/8 20:49, Masami Hiramatsu (Google) 写道:
> On Thu, 8 Sep 2022 09:43:45 +0800
> "liaochang (A)" <liaochang1 at huawei.com> wrote:
> 
>> Thanks for comment.
>>
>> 在 2022/9/8 1:21, Jisheng Zhang 写道:
>>> On Wed, Sep 07, 2022 at 10:33:27AM +0800, Liao Chang wrote:
>>>> Since no race condition occurs on each instruction slot, hence it is
>>>> safe to patch instruction slot without stopping machine.
>>>
>>> hmm, IMHO there's race when arming kprobe under SMP, so stopping
>>> machine is necessary here. Maybe I misundertand something.
>>>
>>
>> It is indeed necessary to stop machine when arm kprobe under SMP,
>> but i don't think it need to stop machine when prepare instruction slot,
>> two reasons:
>>
>> 1. Instruction slot is dynamically allocated data.
>> 2. Kernel would not execute instruction slot until original instruction
>>    is replaced by breakpoint.
> 
> Ah, this is for ss (single step out of line) slot. So until
> kprobe is enabled, this should not be used from other cores.
> OK, then it should be safe.

Exactly, Masami, and i find out this optimization could be applied to some other
architectures, such as arm64 and csky, do you think it is good time to do them all.

Thanks.

> 
> 
>>>>
>>>> Signed-off-by: Liao Chang <liaochang1 at huawei.com>
>>>> ---
>>>>  arch/riscv/kernel/probes/kprobes.c | 8 +++++---
>>>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
>>>> index e6e950b7cf32..eff7d7fab535 100644
>>>> --- a/arch/riscv/kernel/probes/kprobes.c
>>>> +++ b/arch/riscv/kernel/probes/kprobes.c
>>>> @@ -24,12 +24,14 @@ post_kprobe_handler(struct kprobe *, struct kprobe_ctlblk *, struct pt_regs *);
>>>>  static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
>>>>  {
>>>>  	unsigned long offset = GET_INSN_LENGTH(p->opcode);
>>>> +	const kprobe_opcode_t brk_insn = __BUG_INSN_32;
>>>> +	kprobe_opcode_t slot[MAX_INSN_SIZE];
>>>>  
>>>>  	p->ainsn.api.restore = (unsigned long)p->addr + offset;
>>>>  
>>>> -	patch_text(p->ainsn.api.insn, p->opcode);
>>>> -	patch_text((void *)((unsigned long)(p->ainsn.api.insn) + offset),
>>>> -		   __BUG_INSN_32);
>>>> +	memcpy(slot, &p->opcode, offset);
>>>> +	memcpy((void *)((unsigned long)slot + offset), &brk_insn, 4);
>>>> +	patch_text_nosync(p->ainsn.api.insn, slot, offset + 4);
> 
> BTW, didn't you have a macro for the size of __BUG_INSN_32?
> 
> Thank you,

I think you are saying GET_INSN_LENGTH, i will use it to caculate
the size of __BUG_INSN_32 in v2, instead of magic number '4'.

Thanks.

> 
> 
>>>>  }
>>>>  
>>>>  static void __kprobes arch_prepare_simulate(struct kprobe *p)
>>>> -- 
>>>> 2.17.1
>>>>
>>>>
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>>> .
>>
>> -- 
>> BR,
>> Liao, Chang
> 
> 

-- 
BR,
Liao, Chang