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

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Message-ID: <20090819004627.GH5231@nowhere>
Date:	Wed, 19 Aug 2009 02:46:29 +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 08:19:33PM -0400, Masami Hiramatsu wrote:
> 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. :)



Ah ok. I'll just add a small comment about that then, and apply
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?
> 
> Yes, it doesn't modify kernel text, just recover an original
> instruction from kernel text and backup byte on a buffer.


Ok.


> > 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,



Right, thanks!



> -- 
> Masami Hiramatsu
> 
> Software Engineer
> Hitachi Computer Products (America), Inc.
> Software Solutions Division
> 
> e-mail: mhiramat@...hat.com
> 

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