Re: [!!Mass Mail KSE][MASSMAIL KLMS] Re: [RFC PATCH v1 0/7] virtio/vsock: introduce MSG_EOR flag for SEQPACKET

[Stefano Garzarella]

On Thu, Aug 05, 2021 at 12:21:57PM +0300, Arseny Krasnov wrote:
> On 05.08.2021 12:06, Stefano Garzarella wrote:
> > Caution: This is an external email. Be cautious while opening links or attachments.
> >
> >
> >
> > 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]:
>
> Yes, but also from POSIX: such calls like send() and sendmsg()
>
> operates with "message" and if we check recvmsg() we will
>
> find the following thing:
>
>
> For message-based sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the entire
>
> message shall be read in a single operation. If a message is too long to fit in the supplied
>
> buffers, and MSG_PEEK is not set in the flags argument, the excess bytes shall be discarded.
>
>
> I understand this, that send() boundaries also must be maintained.
>
> I've checked SEQPACKET in AF_UNIX and AX_25 - both doesn't support
>
> MSG_EOR, so send() boundaries must be supported.
>
> >    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.
> You are right, if we talking about "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
>
> P.S.: seems SEQPACKET is too exotic thing that everyone implements it 
> in
>
> own manner, because i've tested SCTP seqpacket implementation, and 
> found
>
> that:
>
> 1) It doesn't support MSG_EOR bit at send side, but uses MSG_EOR at 
> receiver
>
> side to mark MESSAGE boundary.
>
> 2) According POSIX any extra bytes that didn't fit in user's buffer 
> must be dropped,
>
> but SCTP doesn't drop it - you can read rest of datagram in next calls.

Thanks for this useful information, now I see the differences and why we 
should support both.

I think is better to include them in the cover letter.

I'm going to review the paches right now :-)

Stefano