Re: [RFC PATCH v1 0/7] virtio/vsock: introduce MSG_EOR flag for SEQPACKET

[Stefano Garzarella <sgarzare@redhat.com>]

On Thu, Aug 05, 2021 at 11:33:12AM +0300, Arseny Krasnov wrote:
>
>On 04.08.2021 15:57, Stefano Garzarella wrote:
>> Caution: This is an external email. Be cautious while opening links or attachments.
>>
>>
>>
>> Hi Arseny,
>>
>> On Mon, Jul 26, 2021 at 07:31:33PM +0300, Arseny Krasnov wrote:
>>>       This patchset implements support of MSG_EOR bit for SEQPACKET
>>> AF_VSOCK sockets over virtio transport.
>>>       Idea is to distinguish concepts of 'messages' and 'records'.
>>> Message is result of sending calls: 'write()', 'send()', 'sendmsg()'
>>> etc. It has fixed maximum length, and it bounds are visible using
>>> return from receive calls: 'read()', 'recv()', 'recvmsg()' etc.
>>> Current implementation based on message definition above.
>> Okay, so the implementation we merged is wrong right?
>> Should we disable the feature bit in stable kernels that contain it? Or
>> maybe we can backport the fixes...
>
>Hi,
>
>No, this is correct and it is message boundary based. Idea of this
>patchset is to add extra boundaries marker which i think could be
>useful when we want to send data in seqpacket mode which length
>is bigger than maximum message length(this is limited by transport).
>Of course we can fragment big piece of data too small messages, but 
>this
>requires to carry fragmentation info in data protocol. So In this case
>when we want to maintain boundaries receiver calls recvmsg() until 
>MSG_EOR found.
>But when receiver knows, that data is fit in maximum datagram length,
>it doesn't care about checking MSG_EOR just calling recv() or 
>read()(e.g.
>message based mode).

I'm not sure we should maintain boundaries of multiple send(), from 
POSIX standard [1]:

   SOCK_SEQPACKET
     Provides sequenced, reliable, bidirectional, connection-mode 
     transmission paths for records. A record can be sent using one or 
     more output operations and received using one or more input 
     operations, but a single operation never transfers part of more than 
     one record. Record boundaries are visible to the receiver via the 
     MSG_EOR flag.

 From my understanding a record could be sent with multiple send() and 
received, for example, with a single recvmsg().
The only boundary should be the MSG_EOR flag set by the user on the last 
send() of a record.

 From send() description [2]:

   MSG_EOR
     Terminates a record (if supported by the protocol).

 From recvmsg() description [3]:

   MSG_EOR
     End-of-record was received (if supported by the protocol).

Thanks,
Stefano

[1] 
https://pubs.opengroup.org/onlinepubs/9699919799/functions/socket.html
[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/send.html
[3] 
https://pubs.opengroup.org/onlinepubs/9699919799/functions/recvmsg.html