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.

-- 
                                                                Gilles.

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