Re: [dpdk-dev] [PATCH v1] devtools: update abi ignore for cryptodev

["Kinsella, Ray" <mdr@ashroe.eu>]

On 21/01/2021 15:58, Thomas Monjalon wrote:
> 21/01/2021 16:15, Dodji Seketeli:
>> Hello Thomas and others,
>>
>> Thomas Monjalon <thomas@monjalon.net> writes:
>>
>>> Question to an expert, Dodji,
>>
>> Thanks for the kind words, but I am not an expert in anything, sadly.  I
>> am just trying to keep learning about these things ;-)
>>
>>> We have this structure:
>>>
>>> struct rte_cryptodev {
>>> 	lot of fields...
>>> 	uint8_t attached : 1;
>>> } __rte_cache_aligned;
>>>
>>> Because of the cache alignment, there is enough padding in the struct
>>> (no matter the size of the cache line) for adding two more pointers:
>>>
>>> struct rte_cryptodev {
>>> 	lot of fields...
>>> 	uint8_t attached : 1;
>>> 	struct rte_cryptodev_cb_rcu *enq_cbs;
>>> 	struct rte_cryptodev_cb_rcu *deq_cbs;
>>> } __rte_cache_aligned;
>>>
>>> We checked manually that the ABI is still compatible.
>>
>> Right.
>>
>> I am curious, but normally, libabigail should raise the addition of
>> structures, but then it'll tell you that there was no size or offset
>> change between the two structures.  If it doesn't, then that's a bug.  I
>> hope it does :-)
> 
> Yes it was raising a problem, that's why we are adding a rule.
> 
> 
>>> Then I've added (quickly) a libabigail exception rule:
>>>
>>> [suppress_type]
>>> 	name = rte_cryptodev
>>> 	has_data_member_inserted_between = {0, 1023}
>>>
>>> Now we want to improve this rule to restrict the offsets
>>> to the padding at the end of the struct only,
>>> so we keep forbidding changes in existing fields,
>>> and forbidding additions further the current struct size.
>>> Is this new rule good?
>>>
>>> 	has_data_member_inserted_between = {offset_after(attached), end}
>>
>>
>> Yes, this rule should do what you think it says.
>>
>>> Do you confirm that the keyword "end" means the old reference size?
>>
>> Yes I do.
>>
>>
>>> What else do we need to check for adding a new field in a padding?
>>
>> Actually, that rule will work independantly of it there is enough
>> padding or not.  It'll shut down the change report, even if the added
>> data exceeds the padding.
> 
> I don't understand why.
> If "end" means the old reference size, then addition after the old size
> should be reported, isn't it?

yes - this comment confuses me also.

If "end" refers to the size original data-structure (position of the end), 
which in this case had some padding. If the additions fall fully within the 
padding I would expect this rule to work - as long as the data-structure size
is still the same. 

However if the additions fall beyond the size of the original data-structure,
the data-structure's size will have changed, I would not expect this rule to 
condone a change in the size of the data-structure.  

> 
> 
>> You just made me think of an idea of a new feature there.
>>
>> Maybe we'd need a new property for the [suppress_type] directive that
>> would suppress changes only if said changes don't modify the size of the
>> type or any offset of any member of the type?
>>
>> Maybe something like:
>>
>>     [suppress_type]
>>        ; lots of properties can go here.
>>
>>        ; ...
>>
>>        ; If the type has any size or offset change
>>        ; then this suppression directive will fail
>>        ; and the change report will be emitted
>>        has_no_size_or_offset_change
>>
>> Would that be useful to you in this case,
>>
>> Cheers,
> 
> 
>