Re: Chaining accept+read

[Jens Axboe <axboe@kernel.dk>]

On 9/28/22 5:59 AM, Pavel Begunkov wrote:
> On 9/28/22 11:55, Ben Noordhuis wrote:
>> On Wed, Sep 28, 2022 at 12:02 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>>>
>>> On 9/28/22 10:50, Ben Noordhuis wrote:
>>>> I'm trying to chain accept+read but it's not working.
>>>>
>>>> My code looks like this:
>>>>
>>>>       *sqe1 = (struct io_uring_sqe){
>>>>         .opcode     = IORING_OP_ACCEPT,
>>>>         .flags      = IOSQE_IO_LINK,
>>>>         .fd         = listenfd,
>>>>         .file_index = 42, // or 42+1
>>>>       };
>>>>       *sqe2 = (struct io_uring_sqe){
>>>>         .opcode     = IORING_OP_READ,
>>>>         .flags      = IOSQE_FIXED_FILE,
>>>>         .addr       = (u64) buf,
>>>>         .len        = len,
>>>>         .fd         = 42,
>>>>       };
>>>>       submit();
>>>>
>>>> Both ops fail immediately; accept with -ECANCELED, read with -EBADF,
>>>> presumably because fixed fd 42 doesn't exist at the time of submission.
>>>>
>>>> Would it be possible to support this pattern in io_uring or are there
>>>> reasons for why things are the way they are?
>>>
>>> It should already be supported. And errors look a bit odd, I'd rather
>>> expect -EBADF or some other for accept and -ECANCELED for the read.
>>> Do you have a test program / reporoducer? Hopefully in C.
>>
>> Of course, please see below. Error handling elided for brevity. Hope
>> I'm not doing anything stupid.
> 
> Perfect thanks
> 
>> For me it immediately prints this:
>>
>> 0 res=-125
>> 1 res=-9
> 
> The reason is that in older kernels we're resolving the read's
> file not after accept but when assembling the link, which was
> specifically fixed a bit later.

Right, IORING_FEAT_LINKED_FILE can be checked to see if this is
properly supported or not on the host.

> Jens, are there any plans to backport it?

If I recall I briefly looked at it, but it was a bit more involved
that I would've liked. But then it got simplified a bit after the
fact, so should probably be doable to get into 5.15-stable at least.
Anything earlier than that stable wise is too old anyway.

-- 
Jens Axboe