Subject: Re: [PATCH RFC 3/5] tun: vringfd receive support.

From: Anthony Liguori <[email-masked]>
Date: Sat, 05 Apr 2008 12:26:57 -0500

Rusty Russell wrote:
> This patch modifies tun to allow a vringfd to specify the receive
> buffer.  Because we can't copy to userspace in bh context, we queue
> like normal then use the "pull" hook to actually do the copy.
> 
> More thought needs to be put into the possible races with ring
> registration and a simultaneous close, for example (see FIXME).
> 
> We use struct virtio_net_hdr prepended to packets in the ring to allow
> userspace to receive GSO packets in future (at the moment, the tun
> driver doesn't tell the stack it can handle them, so these cases are
> never taken).
> 
> Signed-off-by: Rusty Russell <[email-masked]>

<snip>

> +
> +#ifdef CONFIG_VRINGFD

I think the rest of the code needs these for it to actually work without 
VRINGFD.

> +static void unset_recv(void *_tun)
> +{
> +	struct tun_struct *tun = _tun;
> +
> +	tun->inring = NULL;
> +}
> +
> +/* Returns number of used buffers, or negative errno. */
> +static int pull_recv_skbs(void *_tun)
> +{
> +	struct tun_struct *tun = _tun;
> +	int err = 0, num_copied = 0;
> +	struct sk_buff *skb;
> +
> +	while ((skb = skb_dequeue(&tun->readq)) != NULL) {
> +		struct iovec iov[1+MAX_SKB_FRAGS];
> +		struct virtio_net_hdr gso = { 0 }; /* no info leak */
> +		unsigned int iovnum = ARRAY_SIZE(iov);
> +		unsigned long len;
> +		int id;
> +
> +		id = vring_get_buffer(tun->inring, iov, &iovnum, &len,
> +				      NULL, NULL, NULL);
> +		if (id <= 0) {
> +			err = id;
> +			break;
> +		}

Ah, I see now why you're passing something from the stack.

Regards,

Anthony Liguori