Re: [PATCH 1/2] io_uring: ensure recv and recvmsg handle MSG_WAITALL correctly

Jens Axboe
2022-03-23T21:24:50Z

On 3/23/22 2:52 PM, Pavel Begunkov wrote:
> On 3/23/22 20:45, Constantine Gavrilov wrote:
>> On Wed, Mar 23, 2022 at 10:14 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>>>
>>> On 3/23/22 15:39, Jens Axboe wrote:
>>>> We currently don't attempt to get the full asked for length even if
>>>> MSG_WAITALL is set, if we get a partial receive. If we do see a partial
>>>> receive, then just note how many bytes we did and return -EAGAIN to
>>>> get it retried.
>>>>
>>>> The iov is advanced appropriately for the vector based case, and we
>>>> manually bump the buffer and remainder for the non-vector case.
>>>
>>> How datagrams work with MSG_WAITALL? I highly doubt it coalesces 2+
>>> packets to satisfy the length requirement (e.g. because it may move
>>> the address back into the userspace). I'm mainly afraid about
>>> breaking io_uring users who are using the flag just to fail links
>>> when there is not enough data in a packet.
>>>
>>> --
>>> Pavel Begunkov
>>
>> Pavel:
>>
>> Datagrams have message boundaries and the MSG_WAITALL flag does not
>> make sense there. I believe it is ignored by receive code on daragram
>> sockets. MSG_WAITALL makes sends only on stream sockets, like TCP. The
>> manual page says "This flag has no effect for datagram sockets.".
>
> Missed the line this in mans, thanks, and it's exactly as expected.
> The problem is on the io_uring side where with the patch it might
> blindly do a second call into the network stack consuming 2+ packets.

Right, it should not be applied for datagrams.

--
Jens Axboe