Re: [PATCH -tip v14 03/12] kprobes: checks probe address is instruction boudary on x86

[Masami Hiramatsu <mhiramat@redhat.com>]

Frederic Weisbecker wrote:
> On Tue, Aug 18, 2009 at 07:17:39PM -0400, Masami Hiramatsu wrote:
>> Frederic Weisbecker wrote:
>>>> +	while (addr < paddr) {
>>>> +		kernel_insn_init(&insn, (void *)addr);
>>>> +		insn_get_opcode(&insn);
>>>> +
>>>> +		/* Check if the instruction has been modified. */
>>>> +		if (insn.opcode.bytes[0] == BREAKPOINT_INSTRUCTION) {
>>>> +			ret = recover_probed_instruction(buf, addr);
>>>
>>>
>>>
>>> I'm confused about the reason of this recovering. Is it to remove
>>> kprobes behind the current setting one in the current function?
>>
>> No, it recovers just an instruction which is probed by a kprobe,
>> because we need to know the first byte of this instruction for
>> decoding it.

Ah, sorry, it was not accurate. the function recovers an instruction
on the buffer(buf), not on the real kernel text. :)

>>
>> Perhaps we'd better to have more generic interface (text_peek?)
>> for it because another subsystem (e.g. kgdb) may want to insert int3...
>>
>> Thank you,
> 
> 
> Aah, I see now, it's to keep a sane check of the instructions
> boundaries without int 3 artifacts in the middle.
> 
> But in that case, you should re-arm the breakpoint after your
> check, right?
> 
> Or may be you could do the check without repatching?

Yes, it doesn't modify kernel text, just recover an original
instruction from kernel text and backup byte on a buffer.

> May be by doing a copy of insn.opcode.bytes and replacing bytes[0]
> with what a random kprobe has stolen?

Hm, no, this function is protected from other kprobes by kprobe_mutex.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com