Date:   Mon, 15 Jul 2019 08:38:07 +0300
From:   Vasily Averin <vvs@...tuozzo.com>
To:     Xiaoming Ni <nixiaoming@...wei.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
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Subject: Re: [PATCH v3 0/3] kernel/notifier.c: avoid duplicate registration

On 7/14/19 5:45 AM, Xiaoming Ni wrote:
> On 2019/7/12 22:07, gregkh@...uxfoundation.org wrote:
>> On Fri, Jul 12, 2019 at 09:11:57PM +0800, Xiaoming Ni wrote:
>>> On 2019/7/11 21:57, Vasily Averin wrote:
>>>> On 7/11/19 4:55 AM, Nixiaoming wrote:
>>>>> On Wed, July 10, 2019 1:49 PM Vasily Averin wrote:
>>>>>> On 7/10/19 6:09 AM, Xiaoming Ni wrote:
>>>>>>> Registering the same notifier to a hook repeatedly can cause the hook
>>>>>>> list to form a ring or lose other members of the list.
>>>>>>
>>>>>> I think is not enough to _prevent_ 2nd register attempt,
>>>>>> it's enough to detect just attempt and generate warning to mark host in bad state.
>>>>>>
>>>>>
>>>>> Duplicate registration is prevented in my patch, not just "mark host in bad state"
>>>>>
>>>>> Duplicate registration is checked and exited in notifier_chain_cond_register()
>>>>>
>>>>> Duplicate registration was checked in notifier_chain_register() but only 
>>>>> the alarm was triggered without exiting. added by commit 831246570d34692e 
>>>>> ("kernel/notifier.c: double register detection")
>>>>>
>>>>> My patch is like a combination of 831246570d34692e and notifier_chain_cond_register(),
>>>>>  which triggers an alarm and exits when a duplicate registration is detected.
>>>>>
>>>>>> Unexpected 2nd register of the same hook most likely will lead to 2nd unregister,
>>>>>> and it can lead to host crash in any time: 
>>>>>> you can unregister notifier on first attempt it can be too early, it can be still in use.
>>>>>> on the other hand you can never call 2nd unregister at all.
>>>>>
>>>>> Since the member was not added to the linked list at the time of the second registration, 
>>>>> no linked list ring was formed. 
>>>>> The member is released on the first unregistration and -ENOENT on the second unregistration.
>>>>> After patching, the fault has been alleviated
>>>>
>>>> You are wrong here.
>>>> 2nd notifier's registration is a pure bug, this should never happen.
>>>> If you know the way to reproduce this situation -- you need to fix it. 
>>>>
>>>> 2nd registration can happen in 2 cases:
>>>> 1) missed rollback, when someone forget to call unregister after successfull registration, 
>>>> and then tried to call register again. It can lead to crash for example when according module will be unloaded.
>>>> 2) some subsystem is registered twice, for example from  different namespaces.
>>>> in this case unregister called during sybsystem cleanup in first namespace will incorrectly remove notifier used 
>>>> in second namespace, it also can lead to unexpacted behaviour.
>>>>
>>> So in these two cases, is it more reasonable to trigger BUG() directly when checking for duplicate registration ?
>>> But why does current notifier_chain_register() just trigger WARN() without exiting ?
>>> notifier_chain_cond_register() direct exit without triggering WARN() ?
>>
>> It should recover from this, if it can be detected.  The main point is
>> that not all apis have to be this "robust" when used within the kernel
>> as we do allow for the callers to know what they are doing :)
>>
> In the notifier_chain_register(), the condition ( (*nl) == n) is the same registration of the same hook.
>  We can intercept this situation and avoid forming a linked list ring to make the API more rob

Once again -- yes, you CAN prevent list corruption, but you CANNOT recover the host and return it back to safe state.
If double register event was detected -- it means you have bug in kernel.

Yes, you can add BUG here and crash the host immediately, but I prefer to use warning in such situations.

>> If this does not cause any additional problems or slow downs, it's
>> probably fine to add.
>>
> Notifier_chain_register() is not a system hotspot function.
> At the same time, there is already a WARN_ONCE judgment. There is no new judgment in the new patch.
> It only changes the processing under the condition of (*nl) == n, which will not cause performance problems.
> At the same time, avoiding the formation of a link ring can make the system more robust.

I disagree, 
yes, node will have correct list, but anyway node will work wrong and can crash the host in any time.

>> thanks,
>>
>> greg k-h
>>
>> .
>>
> Thanks
> 
> Xiaoming Ni
> 
> 
> 

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