Stefano Garzarella
2021-Aug-06 07:16 UTC
[!!Mass Mail KSE][MASSMAIL KLMS] Re: [RFC PATCH v1 0/7] virtio/vsock: introduce MSG_EOR flag for SEQPACKET
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