Re: [PATCH v3 0/3] kernel/notifier.c: avoid duplicate registration

[Xiaoming Ni]

On 2019/7/12 22:07, gregkh@xxxxxxxxxxxxxxxxxxx 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

> 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.

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

Xiaoming Ni