linux-kernel - Re: [PATCH] riscv/kprobe: Optimize the performance of patching instruction slot

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Date:   Thu, 8 Sep 2022 09:43:45 +0800
From:   "liaochang (A)" <liaochang1@...wei.com>
To:     Jisheng Zhang <jszhang@...nel.org>
CC:     <paul.walmsley@...ive.com>, <palmer@...belt.com>,
        <aou@...s.berkeley.edu>, <mhiramat@...nel.org>,
        <rostedt@...dmis.org>, <linux-riscv@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] riscv/kprobe: Optimize the performance of patching
 instruction slot

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.

>>
>> Signed-off-by: Liao Chang <liaochang1@...wei.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);
>>  }
>>  
>>  static void __kprobes arch_prepare_simulate(struct kprobe *p)
>> -- 
>> 2.17.1
>>
>>
>> _______________________________________________
>> linux-riscv mailing list
>> linux-riscv@...ts.infradead.org
>> http://lists.infradead.org/mailman/listinfo/linux-riscv
> .

-- 
BR,
Liao, Chang

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