On 2011-07-12 13:08, Gilles Chanteperdrix wrote:
> On 07/12/2011 01:06 PM, Jan Kiszka wrote:
>> On 2011-07-12 13:04, Gilles Chanteperdrix wrote:
>>> On 07/12/2011 01:00 PM, Jan Kiszka wrote:
>>>> On 2011-07-12 12:59, Gilles Chanteperdrix wrote:
>>>>> On 07/12/2011 09:22 AM, Jan Kiszka wrote:
>>>>>> On 2011-07-12 08:41, Gilles Chanteperdrix wrote:
>>>>>>> On 07/11/2011 10:12 PM, Jan Kiszka wrote:
>>>>>>>> On 2011-07-11 22:09, Gilles Chanteperdrix wrote:
>>>>>>>>> On 07/11/2011 10:06 PM, Jan Kiszka wrote:
>>>>>>>>>> On 2011-07-11 22:02, Gilles Chanteperdrix wrote:
>>>>>>>>>>> On 07/11/2011 09:59 PM, Jan Kiszka wrote:
>>>>>>>>>>>> On 2011-07-11 21:51, Gilles Chanteperdrix wrote:
>>>>>>>>>>>>> On 07/11/2011 09:16 PM, Jan Kiszka wrote:
>>>>>>>>>>>>>> On 2011-07-11 21:10, Jan Kiszka wrote:
>>>>>>>>>>>>>>> On 2011-07-11 20:53, Gilles Chanteperdrix wrote:
>>>>>>>>>>>>>>>> On 07/08/2011 06:29 PM, GIT version control wrote:
>>>>>>>>>>>>>>>>> @@ -2528,6 +2534,22 @@ static inline void do_taskexit_event(struct task_struct *p)
>>>>>>>>>>>>>>>>>  	magic = xnthread_get_magic(thread);
>>>>>>>>>>>>>>>>>  
>>>>>>>>>>>>>>>>>  	xnlock_get_irqsave(&nklock, s);
>>>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>>>> +	gksched = thread->gksched;
>>>>>>>>>>>>>>>>> +	if (gksched) {
>>>>>>>>>>>>>>>>> +		xnlock_put_irqrestore(&nklock, s);
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Are we sure irqs are on here? Are you sure that what is needed is not an
>>>>>>>>>>>>>>>> xnlock_clear_irqon?
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> We are in the context of do_exit. Not only IRQs are on, also preemption.
>>>>>>>>>>>>>>> And surely no nklock is held.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Furthermore, I do not understand how we
>>>>>>>>>>>>>>>> "synchronize" with the gatekeeper, how is the gatekeeper garanteed to
>>>>>>>>>>>>>>>> wait for this assignment?
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> The gatekeeper holds the gksync token while it's active. We request it,
>>>>>>>>>>>>>>> thus we wait for the gatekeeper to become idle again. While it is idle,
>>>>>>>>>>>>>>> we reset the queued reference - but I just realized that this may tramp
>>>>>>>>>>>>>>> on other tasks' values. I need to add a check that the value to be
>>>>>>>>>>>>>>> null'ified is actually still ours.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Thinking again, that's actually not a problem: gktarget is only needed
>>>>>>>>>>>>>> while gksync is zero - but then we won't get hold of it anyway and,
>>>>>>>>>>>>>> thus, can't cause any damage.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Well, you make it look like it does not work. From what I understand,
>>>>>>>>>>>>> what you want is to set gktarget to null if a task being hardened is
>>>>>>>>>>>>> destroyed. But by waiting for the semaphore, you actually wait for the
>>>>>>>>>>>>> harden to be complete, so setting to NULL is useless. Or am I missing
>>>>>>>>>>>>> something else?
>>>>>>>>>>>>
>>>>>>>>>>>> Setting to NULL is probably unneeded but still better than rely on the
>>>>>>>>>>>> gatekeeper never waking up spuriously and then dereferencing a stale
>>>>>>>>>>>> pointer.
>>>>>>>>>>>>
>>>>>>>>>>>> The key element of this fix is waitng on gksync, thus on the completion
>>>>>>>>>>>> of the non-RT part of the hardening. Actually, this part usually fails
>>>>>>>>>>>> as the target task received a termination signal at this point.
>>>>>>>>>>>
>>>>>>>>>>> Yes, but since you wait on the completion of the hardening, the test
>>>>>>>>>>> if (target &&...) in the gatekeeper code will always be true, because at
>>>>>>>>>>> this point the cleanup code will still be waiting for the semaphore.
>>>>>>>>>>
>>>>>>>>>> Yes, except we will ever wake up the gatekeeper later on without an
>>>>>>>>>> updated gktarget, ie. spuriously. Better safe than sorry, this is hairy
>>>>>>>>>> code anyway (hopefully obsolete one day).
>>>>>>>>>
>>>>>>>>> The gatekeeper is not woken up by posting the semaphore, the gatekeeper
>>>>>>>>> is woken up by the thread which is going to be hardened (and this thread
>>>>>>>>> is the one which waits for the semaphore).
>>>>>>>>
>>>>>>>> All true. And what is the point?
>>>>>>>
>>>>>>> The point being, would not something like this patch be sufficient?
>>>>>>>
>>>>>>> diff --git a/ksrc/nucleus/shadow.c b/ksrc/nucleus/shadow.c
>>>>>>> index 01f4200..4742c02 100644
>>>>>>> --- a/ksrc/nucleus/shadow.c
>>>>>>> +++ b/ksrc/nucleus/shadow.c
>>>>>>> @@ -2527,6 +2527,18 @@ static inline void do_taskexit_event(struct
>>>>>>> task_struct *p)
>>>>>>>  	magic = xnthread_get_magic(thread);
>>>>>>>
>>>>>>>  	xnlock_get_irqsave(&nklock, s);
>>>>>>> +	if (xnthread_test_info(thread, XNATOMIC)) {
>>>>>>> +		struct xnsched *gksched = xnpod_sched_slot(task_cpu(p));
>>>>>>
>>>>>> That's not reliable, the task might have been migrated by Linux in the
>>>>>> meantime. We must use the stored gksched.
>>>>>>
>>>>>>> +		xnlock_put_irqrestore(&nklock, s);
>>>>>>> +
>>>>>>> +		/* Thread is in flight to primary mode, wait for the
>>>>>>> +		   gatekeeper to be done with it. */
>>>>>>> +		down(&gksched->gksync);
>>>>>>> +		up(&gksched->gksync);
>>>>>>> +
>>>>>>> +		xnlock_get_irqsave(&nklock, s);
>>>>>>> +	}
>>>>>>> +
>>>>>>>  	/* Prevent wakeup call from xnshadow_unmap(). */
>>>>>>>  	xnshadow_thrptd(p) = NULL;
>>>>>>>  	xnthread_archtcb(thread)->user_task = NULL;
>>>>>>>
>>>>>>
>>>>>> Again, setting gktarget to NULL and testing for NULL is simply safer,
>>>>>> and I see no gain in skipping that. But if you prefer the
>>>>>> micro-optimization, I'll drop it.
>>>>>
>>>>> Could not we use an info bit instead of adding a pointer?
>>>>>
>>>>
>>>> "That's not reliable, the task might have been migrated by Linux in the
>>>> meantime. We must use the stored gksched."
>>>
>>> I mean add another info bit to mean that the task is queued for wakeup
>>> by the gatekeeper.
>>>
>>> XNGKQ, or something.
>>
>> What additional value does it provide to gksched != NULL? We need that
>> pointer anyway to identify the gatekeeper that holds a reference.
> 
> No, the scheduler which holds the reference is xnpod_sched_slot(task_cpu(p))

And that is still not reliable, the task may have been pushed to a
different CPU in the meantime.

Jan