Re: [PATCH 5.11] io_uring: don't take fs for recvmsg/sendmsg

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

On 18/11/2020 16:57, Stefan Metzmacher wrote:
> Am 18.11.20 um 17:27 schrieb Stefan Metzmacher:
>> Am 07.11.20 um 17:07 schrieb Pavel Begunkov:
>>> On 07/11/2020 16:02, Pavel Begunkov wrote:
>>>> On 07/11/2020 13:46, Stefan Metzmacher wrote:
>>>>> Hi Pavel,
>>>>>
>>>>>> We don't even allow not plain data msg_control, which is disallowed in __sys_{send,revb}msg_sock().
>>>>>
>>>>> Can't we better remove these checks and allow msg_control?
>>>>> For me it's a limitation that I would like to be removed.
>>>>
>>>> We can grab fs only in specific situations as you mentioned, by e.g.
>>>> adding a switch(opcode) in io_prep_async_work(), but that's the easy
>>>> part. All msg_control should be dealt one by one as they do different
>>>> things. And it's not the fact that they ever require fs.
>>>
>>> BTW, Jens mentioned that there is a queued patch that allows plain
>>> data msg_control. Are those not enough?
>>
>> You mean the PROTO_CMSG_DATA_ONLY check?
>>
>> It's not perfect, but better than nothing for a start.
> 
> What actually have in mind for my smbdirect socket driver [1]:
> 
> - I have a pipe that got filled by IORING_OP_SPLICE
> - The data in the pipe need to be "spliced" into a remote RDMA buffers,
>   but I can't use IORING_OP_SPLICE again, because the RDMA buffer descriptor [2]
>   array needs to be passed too.
> - I'd like to use IORING_OP_SENDMSG with MSG_OOB and msg_control.
>   msg_control would get the RDMA buffer descriptor array and the pipe fd.

If I get you right, you can't splice again because there is an RDMA header
that should go before payload data. Is that correct?

So you would need to do like in the pseudo-code below

payload = pipe.get_buffers();
iov[] = {&header, payload};
sendmsg(iov);

> The reverse operation (splicing data from remote RDMA buffers into a pipe)
> would be implemented with IORING_OP_RECVMSG with MSG_OOB and msg_control.
> 
> I guess my smbdirect socket driver would not qualify to be marked as PROTO_CMSG_DATA_ONLY, correct?
> 
> [1] https://git.samba.org/?p=metze/linux/smbdirect.git;a=blob;f=smbdirect_socket.c;h=a738854462b198e#l2076
> [2] https://docs.microsoft.com/ru-ru/openspecs/windows_protocols/ms-smbd/bee890cb-48f0-42a3-ba62-f1a3a19b0edc

-- 
Pavel Begunkov