Re: [RFC PATCH v2 5/9] vsock: SO_RCVLOWAT transport set callback

[Stefano Garzarella]

On Mon, Jul 25, 2022 at 08:05:28AM +0000, Arseniy Krasnov wrote:
> 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@xxxxxxxxxxxxxx>
> ---
> include/net/af_vsock.h   |  1 +
> net/vmw_vsock/af_vsock.c | 19 +++++++++++++++++++
> 2 files changed, 20 insertions(+)
>
> diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
> index f742e50207fb..eae5874bae35 100644
> --- a/include/net/af_vsock.h
> +++ b/include/net/af_vsock.h
> @@ -134,6 +134,7 @@ 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 *, int);
>
> 	/* SEQ_PACKET. */
> 	ssize_t (*seqpacket_dequeue)(struct vsock_sock *vsk, struct msghdr *msg,
> diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
> index 63a13fa2686a..b7a286db4af1 100644
> --- a/net/vmw_vsock/af_vsock.c
> +++ b/net/vmw_vsock/af_vsock.c
> @@ -2130,6 +2130,24 @@ 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;
> +	int err = 0;
> +
> +	vsk = vsock_sk(sk);
> +	transport = vsk->transport;

`transport` can be NULL if the user call SO_RCVLOWAT before we assign 
it, so we should check it.

I think if the transport implements `set_rcvlowat`, maybe we should set 
there sk->sk_rcvlowat, so I would do something like that:

    if (transport && transport->set_rcvlowat)
        err = transport->set_rcvlowat(vsk, val);
    else
        WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);

    return err;

In addition I think we should check that val does not exceed 
vsk->buffer_size, something similar of what tcp_set_rcvlowat() does.

Thanks,
Stefano