linux-kernel - Re: [PATCH tracing/kprobes] kprobes: Disable booster when CONFIG

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Date:	Fri, 29 Jan 2010 12:32:58 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
CC:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>
Subject: Re: [PATCH tracing/kprobes] kprobes: Disable booster when	CONFIG_PREEMPT=y

Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@...radead.org) wrote:
>> On Fri, 2010-01-29 at 12:08 -0500, Mathieu Desnoyers wrote:
>>>
>>> If a task is set as stopped, and the preempted before calling schedule,
>>> can this result in a preempted task staying in that state for an
>>> arbitrary long period of time ? Or is there some mechanism prohibiting
>>> that in the scheduler ? 
>>
>> PREEMPT_ACTIVE does that:
>>
>> preempt_schedule()
>>                 add_preempt_count(PREEMPT_ACTIVE);
>>                 schedule();
>>
>>
>> schedule()
>>         if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
>>                 if (unlikely(signal_pending_state(prev->state, prev)))
>>                         prev->state = TASK_RUNNING;
>>                 else
>>                         deactivate_task(rq, prev, 1);
>>                 switch_count = &prev->nvcsw;
>>         }
> 
> OK, it looks safe for preemption. Is there any unforeseen weird way a
> task can be scheduled out and stopped that would permit it to either:
> 
> - stall the algorithm forever (DoS)
> - appear as quiescent to the algorithm while its stack would hold return
>   pointers to incorrect locations
> 
> ?
> 
> I'm concerned about page faults here.

booster also checks whether the instruction can cause page-fault,
if it will cause it, kprobes doesn't boost it.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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