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

[Stefano Garzarella]

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