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

Steven Rostedt
2009-05-11T15:22:33Z

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.

-- Steve