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

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Date:	Mon, 11 May 2009 11:22:19 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <mhiramat@...hat.com>
cc:	Ingo Molnar <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	kvm <kvm@...r.kernel.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>
Subject: Re: [PATCH -tip v5 2/7] kprobes: checks probe address is instruction
 boudary on x86


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
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