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

[Thomas Monjalon <thomas@monjalon.net>]

04/02/2020 11:16, Akhil Goyal:
> Hi,
> > 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.
> > 
> 
> Can we have something like 
> enum rte_crypto_aead_algorithm {
>         RTE_CRYPTO_AEAD_AES_CCM = 1,
>         /**< AES algorithm in CCM mode. */
>         RTE_CRYPTO_AEAD_AES_GCM,
>         /**< AES algorithm in GCM mode. */
>         RTE_CRYPTO_AEAD_LIST_END,
>         /**< List end for 19.11 ABI compatibility */
>         RTE_CRYPTO_AEAD_CHACHA20_POLY1305,
>         /**< Chacha20 cipher with poly1305 authenticator */
>         RTE_CRYPTO_AEAD_LIST_END_2011
>         /**< List end for 20.11 ABI compatibility */
> };
> 
> And in 20.11 release we alter the RTE_CRYPTO_AEAD_LIST_END to the end and remove RTE_CRYPTO_AEAD_LIST_END_2011
> 
> I believe it will be ok for any application which need to use the chacha poly assume that this algo is
> Experimental and will move to formal list in 20.11. This can be documented in the documentation.
> I believe there is no way to add a new enum as experimental so far. This way we can formalize this
> requirement as well.
> 
> I believe this way effect of ABI breakage will be nullified.

This is a possibility in the (a) proposal.
But it breaks API (and ABI) because a high value is returned
while not expected by the application.

I guess ABI and release maintainers will vote no to such breakage.
Note: I vote no.


> > > 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