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).
#1 should not be an issue, as cqe->result for those values is the
original ring result code, not the target ring.
I'd say the application should just case it to u32 and intepret it like
that, if it's worried about the signed nature of it?
Ah, indeed. I've missed that EBADFD and EOVERFLOW errors only can happen
in the submitter ring, so the receiver ring can always interpret CQE
with the IORING_CQE_F_MSG flag as a successfully received message from
another ring.
