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Re: [PATCH tracing/kprobes] kprobes: Disable booster when CONFIG_PREEMPT=y
- From: Masami Hiramatsu <mhiramat at redhat dot com>
- To: Mathieu Desnoyers <compudj at krystal dot dyndns dot org>
- Cc: Peter Zijlstra <peterz at infradead dot org>, Ingo Molnar <mingo at elte dot hu>, Thomas Gleixner <tglx at linutronix dot de>, Fr??d??ric Weisbecker <fweisbec at gmail dot com>, Steven Rostedt <rostedt at goodmis dot org>, lkml <linux-kernel at vger dot kernel dot org>, systemtap <systemtap at sources dot redhat dot com>, DLE <dle-develop at lists dot sourceforge dot net>, Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, Jim Keniston <jkenisto at us dot ibm dot com>
- Date: Fri, 29 Jan 2010 12:32:58 -0500
- Subject: Re: [PATCH tracing/kprobes] kprobes: Disable booster when CONFIG_PREEMPT=y
- References: <4B5E476C.9030303@redhat.com> <20100127215531.24775.26807.stgit@dhcp-100-2-132.bos.redhat.com> <20100129092135.GE10878@elte.hu> <4B62F61D.5060203@redhat.com> <20100129170817.GA2283@Krystal> <1264785338.24455.14.camel@laptop> <20100129172757.GA4281@Krystal>
Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@infradead.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@redhat.com