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

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Date:   Wed, 21 Apr 2021 10:37:32 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseny Krasnov <arseny.krasnov@...persky.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jorgen Hansen <jhansen@...are.com>,
        Andra Paraschiv <andraprs@...zon.com>,
        Colin Ian King <colin.king@...onical.com>,
        Norbert Slusarek <nslusarek@....net>,
        Alexander Popov <alex.popov@...ux.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, stsp2@...dex.ru, oxffffaa@...il.com
Subject: Re: [RFC PATCH v8 04/19] af_vsock: implement SEQPACKET receive loop

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@...persky.com>
>---
>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
>