Re: [PATCH] kprobes: x86: cleanup __recover_probed_insn().

[Wang Nan <wangnan0@huawei.com>]

On 2015/3/3 16:21, Masami Hiramatsu wrote:
> (2015/03/03 15:39), Wang Nan wrote:
>> Since kernel kconfig forbids turning off KPROBES_ON_FTRACE for x86, we
>> don't need to consider the situation that a kprobe probing on a ftrace
>> location. The only exception should be early kprobe with
>> KPROBES_ON_FTRACE enabled. However, it is still impossible for it to get
>> a tainted by ftrace if it is registered before ftrace is ready.
>>
>> Thus this patch removes unneed logic to make code simpler.
> 
> Nak.
> Please make sure why this is introduced (and try to check by reproducing it).
> https://lkml.org/lkml/2015/2/20/208
> 
> Thank you,
> 

Thank you for your response. I didn't realize it is newly introduced code.

It breaks my early kprobes on ftrace code. I provided a fix on it:

https://lkml.org/lkml/2015/3/3/4

Do you have any suggestion on it?

Thank you.

>>
>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
>> ---
>>  arch/x86/kernel/kprobes/core.c | 62 ++++++++----------------------------------
>>  1 file changed, 12 insertions(+), 50 deletions(-)
>>
>> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
>> index 4e3d5a9..88a99c0 100644
>> --- a/arch/x86/kernel/kprobes/core.c
>> +++ b/arch/x86/kernel/kprobes/core.c
>> @@ -219,55 +219,6 @@ retry:
>>  	}
>>  }
>>  
>> -static unsigned long
>> -__recover_probed_insn(kprobe_opcode_t *buf, unsigned long addr)
>> -{
>> -	struct kprobe *kp;
>> -	unsigned long faddr;
>> -
>> -	kp = get_kprobe((void *)addr);
>> -	faddr = ftrace_location(addr);
>> -	/*
>> -	 * Addresses inside the ftrace location are refused by
>> -	 * arch_check_ftrace_location(). Something went terribly wrong
>> -	 * if such an address is checked here.
>> -	 */
>> -	if (WARN_ON(faddr && faddr != addr))
>> -		return 0UL;
>> -	/*
>> -	 * Use the current code if it is not modified by Kprobe
>> -	 * and it cannot be modified by ftrace.
>> -	 */
>> -	if (!kp && !faddr)
>> -		return addr;
>> -
>> -	/*
>> -	 * Basically, kp->ainsn.insn has an original instruction.
>> -	 * However, RIP-relative instruction can not do single-stepping
>> -	 * at different place, __copy_instruction() tweaks the displacement of
>> -	 * that instruction. In that case, we can't recover the instruction
>> -	 * from the kp->ainsn.insn.
>> -	 *
>> -	 * On the other hand, in case on normal Kprobe, kp->opcode has a copy
>> -	 * of the first byte of the probed instruction, which is overwritten
>> -	 * by int3. And the instruction at kp->addr is not modified by kprobes
>> -	 * except for the first byte, we can recover the original instruction
>> -	 * from it and kp->opcode.
>> -	 *
>> -	 * In case of Kprobes using ftrace, we do not have a copy of
>> -	 * the original instruction. In fact, the ftrace location might
>> -	 * be modified at anytime and even could be in an inconsistent state.
>> -	 * Fortunately, we know that the original code is the ideal 5-byte
>> -	 * long NOP.
>> -	 */
>> -	memcpy(buf, (void *)addr, MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
>> -	if (faddr)
>> -		memcpy(buf, ideal_nops[NOP_ATOMIC5], 5);
>> -	else
>> -		buf[0] = kp->opcode;
>> -	return (unsigned long)buf;
>> -}
>> -
>>  /*
>>   * Recover the probed instruction at addr for further analysis.
>>   * Caller must lock kprobes by kprobe_mutex, or disable preemption
>> @@ -282,7 +233,18 @@ unsigned long recover_probed_instruction(kprobe_opcode_t *buf, unsigned long add
>>  	if (__addr != addr)
>>  		return __addr;
>>  
>> -	return __recover_probed_insn(buf, addr);
>> +	/*
>> +	 * If KPROBES_ON_FTRACE is off, we are not allowed probing at
>> +	 * ftrace location. If it is on, we should use
>> +	 * arm_kprobe_ftrace() and never get here.  As a result, there
>> +	 * is no need to care about confliction between kprobe and
>> +	 * ftrace. The only exception should be early kprobes. However,
>> +	 * for such kprobes registered before ftrace is ready, it is
>> +	 * impossible to get a tainted instruction; for such kprobes
>> +	 * registered after ftrace ready, it will use
>> +	 * arm_kprobe_ftrace() and won't get here.
>> +	 */
>> +	return addr;
>>  }
>>  
>>  /* Check if paddr is at an instruction boundary */
>>
> 
>