Subject: Re: [RFC PATCH v2 0/6] Improve SOCK_SEQPACKET receive logic

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 4 Jul 2021 04:30:50 -0400

On Sun, Jul 04, 2021 at 11:08:13AM +0300, Arseny Krasnov wrote:
> 	This patchset modifies receive logic for SOCK_SEQPACKET.
> Difference between current implementation and this version is that
> now reader is woken up when there is at least one RW packet in rx
> queue of socket and data is copied to user's buffer, while merged
> approach wake up user only when whole message is received and kept
> in queue. New implementation has several advantages:
>  1) There is no limit for message length. Merged approach requires
>     that length must be smaller than 'peer_buf_alloc', otherwise
>     transmission will stuck.
>  2) There is no need to keep whole message in queue, thus no
>     'kmalloc()' memory will be wasted until EOR is received.
> 
>     Also new approach has some feature: as fragments of message
> are copied until EOR is received, it is possible that part of
> message will be already in user's buffer, while rest of message
> still not received. And if user will be interrupted by signal or
> timeout with part of message in buffer, it will exit receive loop,
> leaving rest of message in queue. To solve this problem special
> callback was added to transport: it is called when user was forced
> to leave exit loop and tells transport to drop any packet until
> EOR met.

Sorry about commenting late in the game.  I'm a bit lost

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.

it's supposed to be reliable - how is it legal to drop packets?

> When EOR is found, this mode is disabled and normal packet
> processing started. Note, that when 'drop until EOR' mode is on,
> incoming packets still inserted in queue, reader will be woken up,
> tries to copy data, but nothing will be copied until EOR found.
> It was possible to drain such unneeded packets it rx work without
> kicking user, but implemented way is simplest. Anyway, i think
> such cases are rare.

>     New test also added - it tries to copy to invalid user's
> buffer.
> 
> Arseny Krasnov (16):
>  af_vsock/virtio/vsock: change seqpacket receive logic
>  af_vsock/virtio/vsock: remove 'seqpacket_has_data' callback
>  virtio/vsock: remove 'msg_count' based logic
>  af_vsock/virtio/vsock: add 'seqpacket_drop()' callback
>  virtio/vsock: remove record size limit for SEQPACKET
>  vsock_test: SEQPACKET read to broken buffer
> 
>  drivers/vhost/vsock.c                   |   2 +-
>  include/linux/virtio_vsock.h            |   7 +-
>  include/net/af_vsock.h                  |   4 +-
>  net/vmw_vsock/af_vsock.c                |  44 ++++----
>  net/vmw_vsock/virtio_transport.c        |   2 +-
>  net/vmw_vsock/virtio_transport_common.c | 103 ++++++++-----------
>  net/vmw_vsock/vsock_loopback.c          |   2 +-
>  tools/testing/vsock/vsock_test.c        | 120 ++++++++++++++++++++++
>  8 files changed, 193 insertions(+), 91 deletions(-)
> 
>  v1 -> v2:
>  Patches reordered and reorganized.
> 
> Signed-off-by: Arseny Krasnov <[email-masked]>
> ---
>  cv.txt | 0
>  1 file changed, 0 insertions(+), 0 deletions(-)
>  create mode 100644 cv.txt
> 
> diff --git a/cv.txt b/cv.txt
> new file mode 100644
> index 000000000000..e69de29bb2d1
> -- 
> 2.25.1