Re: [RFC PATCH v8 04/19] af_vsock: implement SEQPACKET receive loop

[Stefano Garzarella]

On Tue, Apr 13, 2021 at 03:42:47PM +0300, Arseny Krasnov wrote:
> This adds receive loop for SEQPACKET. It looks like receive loop for
> STREAM, but there is a little bit difference:
> 1) It doesn't call notify callbacks.
> 2) It doesn't care about 'SO_SNDLOWAT' and 'SO_RCVLOWAT' values, because
>   there is no sense for these values in SEQPACKET case.
> 3) It waits until whole record is received or error is found during
>   receiving.
> 4) It processes and sets 'MSG_TRUNC' flag.
>
> So to avoid extra conditions for two types of socket inside one loop, two
> independent functions were created.
>
> Signed-off-by: Arseny Krasnov <arseny.krasnov@xxxxxxxxxxxxx>
> ---
> v7 -> v8:
> - Length of message is now not returned by callback, it returns only
>   length of data read by each call.
> - Previous case, when EAGAIN is return and dequeue loop restarted now
>   removed(in this simplified version we consider that message could not
>   be corrupted).
> - MSG_TRUNC in input flags is now handled by callback.
>
> include/net/af_vsock.h   |  4 +++
> net/vmw_vsock/af_vsock.c | 66 +++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 69 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
> index b1c717286993..5175f5a52ce1 100644
> --- a/include/net/af_vsock.h
> +++ b/include/net/af_vsock.h
> @@ -135,6 +135,10 @@ struct vsock_transport {
> 	bool (*stream_is_active)(struct vsock_sock *);
> 	bool (*stream_allow)(u32 cid, u32 port);
>
> +	/* SEQ_PACKET. */
> +	ssize_t (*seqpacket_dequeue)(struct vsock_sock *vsk, struct msghdr *msg,
> +				     int flags, bool *msg_ready);
> +
> 	/* Notification. */
> 	int (*notify_poll_in)(struct vsock_sock *, size_t, bool *);
> 	int (*notify_poll_out)(struct vsock_sock *, size_t, bool *);
> diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
> index c4f6bfa1e381..d9fb4f9a3063 100644
> --- a/net/vmw_vsock/af_vsock.c
> +++ b/net/vmw_vsock/af_vsock.c
> @@ -1974,6 +1974,67 @@ static int __vsock_stream_recvmsg(struct sock *sk, struct msghdr *msg,
> 	return err;
> }
>
> +static int __vsock_seqpacket_recvmsg(struct sock *sk, struct msghdr *msg,
> +				     size_t len, int flags)
> +{
> +	const struct vsock_transport *transport;
> +	bool msg_ready;
> +	struct vsock_sock *vsk;
> +	ssize_t record_len;
> +	long timeout;
> +	int err = 0;
> +	DEFINE_WAIT(wait);
> +
> +	vsk = vsock_sk(sk);
> +	transport = vsk->transport;
> +
> +	timeout = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);
> +	msg_ready = false;
> +	record_len = 0;
> +
> +	while (1) {
> +		ssize_t tmp_record_len;

Maybe better a name like `partial_len`, `fragment_len`, or just `read`.

> +
> +		if (vsock_wait_data(sk, &wait, timeout, NULL, 0) <= 0) {
> +			/* In case of any loop break(timeout, signal
> +			 * interrupt or shutdown), we report user that
> +			 * nothing was copied.
> +			 */
> +			err = 0;
> +			break;
> +		}
> +
> +		tmp_record_len = transport->seqpacket_dequeue(vsk, msg, flags, &msg_ready);

I think we can avoid to pass 'flags' down to the transports.

We can require that seqpacket_dequeue() should always return the real 
size of the packet received, and then check below if 'MSG_TRUNC' was 
set...

> +
> +		if (tmp_record_len < 0) {
> +			err = -ENOMEM;
> +			break;
> +		}
> +
> +		record_len += tmp_record_len;
> +
> +		if (msg_ready)
> +			break;
> +	}
> +
> +	if (sk->sk_err)
> +		err = -sk->sk_err;
> +	else if (sk->sk_shutdown & RCV_SHUTDOWN)
> +		err = 0;
> +
> +	if (msg_ready && err == 0) {
> +		err = record_len;
> +
> +		/* Always set MSG_TRUNC if real length of packet is
> +		 * bigger than user's buffer.
> +		 */

...here:
		if (flags & MSG_TRUNC && record_len > len)

> +		if (record_len > len)
> +			msg->msg_flags |= MSG_TRUNC;
> +	}
> +
> +	return err;
> +}
> +
> static int
> vsock_connectible_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
> 			  int flags)
> @@ -2029,7 +2090,10 @@ vsock_connectible_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
> 		goto out;
> 	}
>
> -	err = __vsock_stream_recvmsg(sk, msg, len, flags);
> +	if (sk->sk_type == SOCK_STREAM)
> +		err = __vsock_stream_recvmsg(sk, msg, len, flags);
> +	else
> +		err = __vsock_seqpacket_recvmsg(sk, msg, len, flags);
>
> out:
> 	release_sock(sk);
> -- )
> 2.25.1