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

[Thomas Monjalon <thomas@monjalon.net>]

03/02/2020 18:40, Ferruh Yigit:
> 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?

_LIST_MAX is range of value that DPDK can return in the ABI contract.
So the appplication can rely on the range 0.._LIST_MAX.

> 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.

No, ABI compatibility contract means you can compile your application
with DPDK 19.11.0 and run it with DPDK 20.02.
So _LIST_END comes from 19.11 and does not include ChachaPoly.

> 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.

If we define _LIST_MAX as a bigger value than current _LIST_END,
we have some room to add values in between.

If (as of now) we don't have _LIST_MAX room, then yes we must version
the functions returning the enum.
In this case, the proper solution is to implement
rte_cryptodev_info_get_v1911() so it filters out
RTE_CRYPTO_AEAD_CHACHA20_POLY1305 capability.
With this solution, an application compiled with DPDK 19.11 will keep
seeing the same range as before, while a 20.02 application could
see and use ChachaPoly.
This is another proposal that I was expecting from the crypto team,
instead of claiming there is no issue (and wasting precious time).


> 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.

I don't understand what you mean. I think you misunderstood what is ABI compat.


> > 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