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.

Jan