Re: Sending CQE to a different ring

[Artyom Pavlov <newpavlov@gmail.com>]

>> I like IORING_OP_WAKEUP_RING with sqe->len being copied to
>> cqe->result. The only question I have is how should "negative" (i.e.
>> bigger or equal to 2^31) lengths be handled. They either should be
>> copied similarly to positive values or we should get an error.
> 
> Any IO transferring syscall in Linux supports INT_MAX as the maximum in
> one request, which is also why the res field is sized the way it is. So
> you cannot transfer more than that anyway, hence it should not be an
> issue (at least specific to io_uring).
> 
>> Also what do you think about registering ring fds? Could it be
>> beneficial?
> 
> Definitely, it'll make the overhead a bit lower for issuing the
> IORING_OP_MSG_RING (it's been renamed ;-) request.
> 

I mean the case when sqe->len is used to transfer arbitrary data set by 
user. I believe IORING_OP_MSG_RING behavior for this edge case should be 
explicitly documented.

It looks like we have 3 options:
1) Copy sqe->len to cqe->result without any checks. Disadvantage: 
user-provided value may collide with EBADFD and EOVERFLOW.
2) Submit error CQE to the submitter ring.
3) Submit error CQE to the receiver ring (cqe->result will contain error 
code).