Re: Chaining accept+read

[Pavel Begunkov <asml.silence@gmail.com>]

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. Jens, are there any plans to
backport it?

-- 
Pavel Begunkov