On Thu, Aug 05, 2021 at 11:33:12AM +0300, Arseny Krasnov wrote:

On 04.08.2021 15:57, Stefano Garzarella wrote:

quoted

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Hi Arseny,

On Mon, Jul 26, 2021 at 07:31:33PM +0300, Arseny Krasnov wrote:

quoted

      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