Re: [PATCH -tip v5 2/7] kprobes: checks probe address is instruction boudary on x86

Masami Hiramatsu
2009-05-11T18:20:32Z

Steven Rostedt wrote:
> On Mon, 11 May 2009, Masami Hiramatsu wrote:
>>>> + * by fix_riprel().
>>>> + */
>>>> + memcpy(buf, kp->addr, MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
>>>> + buf[0] = kp->opcode;
>>> Why is it OK to copy addr to "buf" and then rewrite the first character of
>>> buf? Does it have something to do with the above "p"?
>> Yes, each kprobe copied probed instruction to kp->ainsn.insn,
>> which is an executable buffer for single stepping.
>> So, basically, kp->ainsn.insn has an original instruction.
>> However, RIP-relative instruction can not do single-stepping
>> at different place, fix_riprel() tweaks the displacement of
>> that instruction. In that case, we can't recover the instruction
>> from the kp->ainsn.insn.
>>
>> On the other hand, 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.
>
> For code that is awkward, complex or non-trivial, don't be afraid to put
> in a paragraph explaining the code. The above explanation should be a
> comment in the code. Otherwise people like me would just look at it and
> say "huh?".
>
> Note, I'm a bit cranky this morning, so I hope I don't offend anyone.

No, that's very helpful review for me. Thanks :-)