Re: [dpdk-dev] [PATCH v2 4/4] add ABI checks

[Ray Kinsella <mdr@ashroe.eu>]

On 03/02/2020 17:34, Thomas Monjalon wrote:
> 03/02/2020 18:09, Thomas Monjalon:
>> 03/02/2020 10:30, Ferruh Yigit:
>>> On 2/2/2020 2:41 PM, Ananyev, Konstantin wrote:
>>>> 02/02/2020 14:05, Thomas Monjalon:
>>>>> 31/01/2020 15:16, Trahe, Fiona:
>>>>>> On 1/30/2020 8:18 PM, Thomas Monjalon wrote:
>>>>>>> If library give higher value than expected by the application,
>>>>>>> if the application uses this value as array index,
>>>>>>> there can be an access out of bounds.
>>>>>>
>>>>>> [Fiona] All asymmetric APIs are experimental so above shouldn't be a problem.
>>>>>> But for the same issue with sym crypto below, I believe Ferruh's explanation makes
>>>>>> sense and I don't see how there can be an API breakage.
>>>>>> So if an application hasn't compiled against the new lib it will be still using the old value
>>>>>> which will be within bounds. If it's picking up the higher new value from the lib it must
>>>>>> have been compiled against the lib so shouldn't have problems.
>>>>>
>>>>> You say there is no ABI issue because the application will be re-compiled
>>>>> for the updated library. Indeed, compilation fixes compatibility issues.
>>>>> But this is not relevant for ABI compatibility.
>>>>> ABI compatibility means we can upgrade the library without recompiling
>>>>> the application and it must work.
>>>>> You think it is a false positive because you assume the application
>>>>> "picks" the new value. I think you miss the case where the new value
>>>>> is returned by a function in the upgraded library.
>>>>>
>>>>>> There are also no structs on the API which contain arrays using this
>>>>>> for sizing, so I don't see an opportunity for an appl to have a
>>>>>> mismatch in memory addresses.
>>>>>
>>>>> Let me demonstrate where the API may "use" the new value
>>>>> RTE_CRYPTO_AEAD_CHACHA20_POLY1305 and how it impacts the application.
>>>>>
>>>>> Once upon a time a DPDK application counting the number of devices
>>>>> supporting each AEAD algo (in order to find the best supported algo).
>>>>> It is done in an array indexed by algo id:
>>>>> int aead_dev_count[RTE_CRYPTO_AEAD_LIST_END];
>>>>> The application is compiled with DPDK 19.11,
>>>>> where RTE_CRYPTO_AEAD_LIST_END = 3.
>>>>> So the size of the application array aead_dev_count is 3.
>>>>> This binary is run with DPDK 20.02,
>>>>> where RTE_CRYPTO_AEAD_CHACHA20_POLY1305 = 3.
>>>>> When calling rte_cryptodev_info_get() on a device QAT_GEN3,
>>>>> rte_cryptodev_info.capabilities.sym.aead.algo is set to
>>>>> RTE_CRYPTO_AEAD_CHACHA20_POLY1305 (= 3).
>>>>> The application uses this value:
>>>>> ++ aead_dev_count[info.capabilities.sym.aead.algo];
>>>>> The application is crashing because of out of bound access.
>>>>
>>>> I'd say this is an example of bad written app.
>>>> It probably should check that returned by library value doesn't
>>>> exceed its internal array size.
>>>
>>> +1
>>>
>>> Application should ignore values >= MAX.
>>
>> Of course, blaming the API user is a lot easier than looking at the API.
>> Here the API has RTE_CRYPTO_AEAD_LIST_END which can be understood
>> as the max value for the application.
>> Value ranges are part of the ABI compatibility contract.
>> It seems you expect the application developer to be aware that
>> DPDK could return a higher value, so the application should
>> check every enum values after calling an API. CRAZY.
>>
>> When we decide to announce an ABI compatibility and do some marketing,
>> everyone is OK. But when we need to really make our ABI compatible,
>> I see little or no effort. DISAPPOINTING.
>>
>>> Do you suggest we don't extend any enum or define between ABI breakage releases
>>> to be sure bad written applications not affected?
>>
>> I suggest we must consider not breaking any assumption made on the API.
>> Here we are breaking the enum range because nothing mentions _LIST_END
>> is not really the absolute end of the enum.
>> The solution is to make the change below in 20.02 + backport in 19.11.1:
> 
> Thinking twice, merging such change before 20.11 is breaking the
> ABI assumption based on the API 19.11.0.
> I ask the release maintainers (Luca, Kevin, David and me) and
> the ABI maintainers (Neil and Ray) to vote for a or b solution:
> 	a) add comment and LIST_MAX as below in 20.02 + 19.11.1

That would still be an ABI breakage though right.

> 	b) wait 20.11 and revert Chacha-Poly from 20.02

Thanks for analysis above Fiona, Ferruh and all. 

That is a nasty one alright - there is no "good" answer here.
I agree with Ferruh's sentiments overall, we should rethink this API for 20.11. 
Could do without an enumeration?

There a c) though right.
We could work around the issue by api versioning rte_cryptodev_info_get() and friends.
So they only support/acknowledge the existence of Chacha-Poly for applications build against > 20.02.

It would be painful I know.
It would also mean that Chacha-Poly would only be available to those building against >= 20.02.


> 
> 
>> - _LIST_END
>> + _LIST_END, /* an ABI-compatible version may increase this value */
>> + _LIST_MAX = _LIST_END + 42 /* room for ABI-compatible additions */
>> };
>>
>> Then *_LIST_END values could be ignored by libabigail with such a change.
>>
>> If such a patch is not done by tomorrow, I will have to revert
>> Chacha-Poly commits before 20.02-rc2, because
>>
>> 1/ LIST_END, without any comment, means "size of range"
>> 2/ we do not blame users for undocumented ABI changes
>> 3/ we respect the ABI compatibility contract