On Thu, Aug 05, 2021 at 12:21:57PM +0300, Arseny Krasnov wrote:

On 05.08.2021 12:06, Stefano Garzarella wrote:

quoted

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

quoted

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]:

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.

quoted

   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".

quoted

 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