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

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Date:	Wed, 19 Aug 2009 01:43:42 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	kvm <kvm@...r.kernel.org>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Avi Kivity <avi@...hat.com>, Andi Kleen <ak@...ux.intel.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	PrzemysławPawełczyk <przemyslaw@...elczyk.it>,
	Roland McGrath <roland@...hat.com>,
	Sam Ravnborg <sam@...nborg.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Vegard Nossum <vegard.nossum@...il.com>
Subject: Re: [PATCH -tip v14 03/12] kprobes: checks probe address is
	instruction boudary on x86

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

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