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

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 2/3/2020 5:09 PM, Thomas Monjalon wrote:
> 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:
>>>>>> 30/01/2020 17:09, Ferruh Yigit:
>>>>>>> On 1/29/2020 8:13 PM, Akhil Goyal wrote:
>>>>>>>>
>>>>>>>> I believe these enums will be used only in case of ASYM case which is experimental.
>>>>>>>
>>>>>>> Independent from being experiment and not, this shouldn't be a problem, I think
>>>>>>> this is a false positive.
>>>>>>>
>>>>>>> The ABI break can happen when a struct has been shared between the application
>>>>>>> and the library (DPDK) and the layout of that memory know differently by
>>>>>>> application and the library.
>>>>>>>
>>>>>>> Here in all cases, there is no layout/size change.
>>>>>>>
>>>>>>> As to the value changes of the enums, since application compiled with old DPDK,
>>>>>>> it will know only up to '6', 7 and more means invalid to the application. So it
>>>>>>> won't send these values also it should ignore these values from library. Only
>>>>>>> consequence is old application won't able to use new features those new enums
>>>>>>> provide but that is expected/normal.
>>>>>>
>>>>>> 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.

This is not to blame the user or to do less work, this is more sane approach
that library provides the _END/_MAX value and application uses it as valid range
check.

> 
>> 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:
> 
> - _LIST_END
> + _LIST_END, /* an ABI-compatible version may increase this value */
> + _LIST_MAX = _LIST_END + 42 /* room for ABI-compatible additions */
> };
> 

What is the point of "_LIST_MAX" here?

Application should know the "_LIST_END" of when it has been compiled for the
valid range check. Next time it is compiled "_LIST_END" may be different value
but same logic applies.

When "_LIST_END" is missing, application can't protect itself, in that case
library should send only the values application knows when it is compiled, this
means either we can't extend our enum/defines until next ABI breakage, or we
need to do ABI versioning to the functions that returns an enum each time enum
value extended.

I believe it is saner to provide _END/_MAX values to the application to use. And
if required comment them to clarify the expected usage.

But in above suggestion application can't use or rely on "_LIST_MAX", it doesn't
mean anything to application.

> 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
> 
>