Re: RFC (V2) the proposal to resolve the missing dependency issue for counted_by attribute

[Qing Zhao <qing.zhao@oracle.com>]

> On Nov 9, 2023, at 11:50 AM, Jose Marchesi <jose.marchesi@oracle.com> wrote:
> 
>> 
>> On Thu, Nov 09, 2023 at 03:49:49PM +0000, Qing Zhao wrote:
>>> Is it reasonable to add one option to disable the “counted_by” attribute?
>>> (then no insertion of the new .ACCESS_WITH_SIZE into IL).  
>>> 
>>> The major reason is: some users might want to ignore all the “counted_by” attribute added in the source code,
>>> We need to provide them a way to disable this feature.
>> 
>> -D'counted_by(x)='
>> and/or
>> -D'__counted_by__(x)='
>> ?
> 
> The insertion of .ACCESS_WITH_SIZE collides with the BPF CO-RE
> preserve_access_index implementation.
> 
> I don't think this will be a problem in practice (the BPF program can
> define counted_by to the empty string as Jakub suggests) but we ought to
> at least detect when a data structure featuring a counted_by FMA is
> accessed with access index preservation (either attribute or builtin)
> and either error out or warning out and try to accomodate by turning the
> .ACCESS_WTIH_INDEX back to plain accesses.  We can do either with BPF
> specific backend code.

Yes, I agree that handling this in BPF backend code might be a better approach
 since this is really a BPF CO-RE specific issue.

For the counted_by implementation, I will keep the current design.

But I will add this identified BPF CO-RE issue into the proposal as a known issue for record purpose.

Thanks a lot for raising this issue and the possible solutions.

Qing