vsock: SO_RCVLOWAT transport set callback

This adds transport specific callback for SO_RCVLOWAT, because in some
transports it may be difficult to know current available number of bytes
ready to read. Thus, when SO_RCVLOWAT is set, transport may reject it.
Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/net/af_vsock.h

@@ -135,6 +135,7 @@ struct vsock_transport {
 	u64 (*stream_rcvhiwat)(struct vsock_sock *);
 	bool (*stream_is_active)(struct vsock_sock *);
 	bool (*stream_allow)(u32 cid, u32 port);
+	int (*set_rcvlowat)(struct vsock_sock *vsk, int val);
 
 	/* SEQ_PACKET. */
 	ssize_t (*seqpacket_dequeue)(struct vsock_sock *vsk, struct msghdr *msg,

net/vmw_vsock/af_vsock.c

@@ -2137,6 +2137,25 @@ vsock_connectible_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 	return err;
 }
 
+static int vsock_set_rcvlowat(struct sock *sk, int val)
+{
+	const struct vsock_transport *transport;
+	struct vsock_sock *vsk;
+
+	vsk = vsock_sk(sk);
+
+	if (val > vsk->buffer_size)
+		return -EINVAL;
+
+	transport = vsk->transport;
+
+	if (transport && transport->set_rcvlowat)
+		return transport->set_rcvlowat(vsk, val);
+
+	WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
+	return 0;
+}
+
 static const struct proto_ops vsock_stream_ops = {
 	.family = PF_VSOCK,
 	.owner = THIS_MODULE,
@@ -2156,6 +2175,7 @@ static const struct proto_ops vsock_stream_ops = {
 	.recvmsg = vsock_connectible_recvmsg,
 	.mmap = sock_no_mmap,
 	.sendpage = sock_no_sendpage,
+	.set_rcvlowat = vsock_set_rcvlowat,
 };
 
 static const struct proto_ops vsock_seqpacket_ops = {