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

Thomas Monjalon thomas at monjalon.net
Sun Feb 2 14:05:56 CET 2020

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,
So the size of the application array aead_dev_count is 3.
This binary is run with DPDK 20.02,
When calling rte_cryptodev_info_get() on a device QAT_GEN3,
rte_cryptodev_info.capabilities.sym.aead.algo is set to
The application uses this value:
	++ aead_dev_count[info.capabilities.sym.aead.algo];
The application is crashing because of out of bound access.

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