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SubjectRe: [RFC PATCH v1 0/7] virtio/vsock: introduce MSG_EOR flag for SEQPACKET
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...
>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
>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
>message based mode).

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

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]:

Terminates a record (if supported by the protocol).

From recvmsg() description [3]:

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



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