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

[David Marchand <david.marchand@redhat.com>]

On Mon, Feb 3, 2020 at 7:56 PM Ray Kinsella <mdr@ashroe.eu> wrote:
> 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.

Yes.


>
> >       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.

It has a lot of friends, but it sounds like the right approach.
Is someone looking into this?


> 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.

Yes.


--
David Marchand