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

[Xiaoming Ni <nixiaoming@huawei.com>]

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() ?

Thanks

Xiaoming Ni

>> It may be more helpful to return an error code when someone tries to register the same
>> notification program a second time.
> 
> You are wrong again here, it is senseless.
> If you have detected 2nd register -- your node is already in bad state.
> 
>> But I noticed that notifier_chain_cond_register() returns 0 when duplicate registration 
>> is detected. At the same time, in all the existing export function comments of notify,
>> "Currently always returns zero"
>>
>> I am a bit confused: which is better?
>>
>>>
>>> Unfortunately I do not see any ways to handle such cases properly,
>>> and it seems for me your patches does not resolve this problem.
>>>
>>> Am I missed something probably?
>>>
>>>> case1: An infinite loop in notifier_chain_register() can cause soft lockup
>>>>         atomic_notifier_chain_register(&test_notifier_list, &test1);
>>>>         atomic_notifier_chain_register(&test_notifier_list, &test1);
>>>>         atomic_notifier_chain_register(&test_notifier_list, &test2);
>>
>> Thanks
>>
>> Xiaoming Ni
>>
> 
> .
>