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

[Thomas Monjalon <thomas@monjalon.net>]

RED FLAG

I don't see a lot of reactions, so I summarize the issue.
We need action TODAY!

API makes think that rte_cryptodev_info_get() cannot return
a value >= 3 (RTE_CRYPTO_AEAD_LIST_END in 19.11).
Current 20.02 returns 3 (RTE_CRYPTO_AEAD_CHACHA20_POLY1305).
The ABI compatibility contract is broken currently.

There are 3 possible outcomes:

a) Change the API comments and backport to 19.11.1
The details are discussed between Ferruh and me.
Either put responsibility on API user (with explicit comment),
or expose ABI extension allowance with a new API max value.
In both cases, this is breaking the implicit contract of 19.11.0.
This option can be chosen only if release and ABI maintainers
vote for it.

b) Revert Chacha-Poly from 20.02-rc2.

c) Add versioned function rte_cryptodev_info_get_v1911()
which calls rte_cryptodev_info_get() and filters out
RTE_CRYPTO_AEAD_CHACHA20_POLY1305 capability.
So Chacha-Poly capability would be seen and usable only
if compiling with DPDK 20.02.

I hope it is clear what are the actions for everybody:
- ABI and release maintainers must say yes or no to the proposal (a)
- In the meantime, crypto team must send a patch for the proposal (c)
- If (a) and (c) are not possible at the end of today, I will take (b)

Note: do not say it is too short for (c), as it was possible to work
on such solution since the issue was exposed on last Wednesday.


03/02/2020 22:07, Thomas Monjalon:
> 03/02/2020 19:55, Ray Kinsella:
> > 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.
> > 
> > > 	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.
> > So they only support/acknowledge the existence of Chacha-Poly for applications build against > 20.02.
> 
> I agree there is a c) as I proposed in another email:
> http://mails.dpdk.org/archives/dev/2020-February/156919.html
> "
> 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.
> "
> 
> > It would be painful I know.
> 
> Not so painful in my opinion.
> Just need to call rte_cryptodev_info_get() from
> rte_cryptodev_info_get_v1911() and filter the value
> in the 19.11 range: [0..AES_GCM].
> 
> > It would also mean that Chacha-Poly would only be available to
> > those building against >= 20.02.
> 
> Yes exactly.
> 
> The addition of comments and LIST_MAX like below are still valid
> to avoid versioning after 20.11.
> 
> > >> - _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.
> 
> In order to avoid ABI check complaining, the best is to completely
> remove LIST_END in DPDK 20.11.
> 
> 
> > >> 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