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

[Thomas Monjalon <thomas@monjalon.net>]

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.

> 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 */
};

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