Re: [PATCH v5] virtio/vsock: replace virtio_vsock_pkt with sk_buff

[Paolo Abeni <pabeni@redhat.com>]

Hello,

On Fri, 2022-12-02 at 09:35 -0800, Bobby Eshleman wrote:
[...]
> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> index 35d7eedb5e8e..6c0b2d4da3fe 100644
> --- a/include/linux/virtio_vsock.h
> +++ b/include/linux/virtio_vsock.h
> @@ -3,10 +3,129 @@
>  #define _LINUX_VIRTIO_VSOCK_H
>  
>  #include <uapi/linux/virtio_vsock.h>
> +#include <linux/bits.h>
>  #include <linux/socket.h>
>  #include <net/sock.h>
>  #include <net/af_vsock.h>
>  
> +#define VIRTIO_VSOCK_SKB_HEADROOM (sizeof(struct virtio_vsock_hdr))
> +
> +enum virtio_vsock_skb_flags {
> +	VIRTIO_VSOCK_SKB_FLAGS_REPLY		= BIT(0),
> +	VIRTIO_VSOCK_SKB_FLAGS_TAP_DELIVERED	= BIT(1),
> +};
> +
> +static inline struct virtio_vsock_hdr *virtio_vsock_hdr(struct sk_buff *skb)
> +{
> +	return (struct virtio_vsock_hdr *)skb->head;
> +}
> +
> +static inline bool virtio_vsock_skb_reply(struct sk_buff *skb)
> +{
> +	return skb->_skb_refdst & VIRTIO_VSOCK_SKB_FLAGS_REPLY;
> +}

I'm sorry for the late feedback. The above is extremelly risky: if the
skb will land later into the networking stack, we could experience the
most difficult to track bugs.

You should use the skb control buffer instead (skb->cb), with the
additional benefit you could use e.g. bool - the compiler could emit
better code to manipulate such fields - and you will not need to clear
the field before release nor enqueue.

[...]

> @@ -352,37 +360,38 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>  				   size_t len)
>  {
>  	struct virtio_vsock_sock *vvs = vsk->trans;
> -	struct virtio_vsock_pkt *pkt;
>  	size_t bytes, total = 0;
> -	u32 free_space;
> +	struct sk_buff *skb;
>  	int err = -EFAULT;
> +	u32 free_space;
>  
>  	spin_lock_bh(&vvs->rx_lock);
> -	while (total < len && !list_empty(&vvs->rx_queue)) {
> -		pkt = list_first_entry(&vvs->rx_queue,
> -				       struct virtio_vsock_pkt, list);
> +	while (total < len && !skb_queue_empty_lockless(&vvs->rx_queue)) {
> +		skb = __skb_dequeue(&vvs->rx_queue);

Here the locking schema is confusing. It looks like vvs->rx_queue is
under vvs->rx_lock protection, so the above should be skb_queue_empty()
instead of the lockless variant.

[...]

> @@ -858,16 +873,11 @@ static int virtio_transport_reset_no_sock(const struct virtio_transport *t,
>  static void virtio_transport_remove_sock(struct vsock_sock *vsk)
>  {
>  	struct virtio_vsock_sock *vvs = vsk->trans;
> -	struct virtio_vsock_pkt *pkt, *tmp;
>  
>  	/* We don't need to take rx_lock, as the socket is closing and we are
>  	 * removing it.
>  	 */
> -	list_for_each_entry_safe(pkt, tmp, &vvs->rx_queue, list) {
> -		list_del(&pkt->list);
> -		virtio_transport_free_pkt(pkt);
> -	}
> -
> +	virtio_vsock_skb_queue_purge(&vvs->rx_queue);

Still assuming rx_queue is under the rx_lock, given you don't need the
locking here as per the above comment, you should use the lockless
purge variant.

Thanks!

Paolo