Re: [RFC PATCH v2 1/4] virtio/vsock: fix 'rx_bytes'/'fwd_cnt' calculation

[Stefano Garzarella]

On Sun, Mar 05, 2023 at 11:06:26PM +0300, Arseniy Krasnov wrote:
> Substraction of 'skb->len' is redundant here: 'skb_headroom()' is delta
> between 'data' and 'head' pointers, e.g. it is number of bytes returned
> to user (of course accounting size of header). 'skb->len' is number of
> bytes rest in buffer.
>
> Fixes: 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")
> Signed-off-by: Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx>
> ---
> net/vmw_vsock/virtio_transport_common.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index a1581c77cf84..2e2a773df5c1 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -255,7 +255,7 @@ static void virtio_transport_dec_rx_pkt(struct virtio_vsock_sock *vvs,
> {
> 	int len;
>
> -	len = skb_headroom(skb) - sizeof(struct virtio_vsock_hdr) - skb->len;
> +	len = skb_headroom(skb) - sizeof(struct virtio_vsock_hdr);

IIUC virtio_transport_dec_rx_pkt() is always called after skb_pull(),
so skb_headroom() is returning the amount of space we removed.

Looking at the other patches in this series, I think maybe we should
change virtio_transport_dec_rx_pkt() and virtio_transport_inc_rx_pkt()
by passing the value to subtract or add directly.
Since some times we don't remove the whole payload, so it would be
better to call it with the value in hdr->len.

I mean something like this (untested):

index a1581c77cf84..9e69ae7a9a96 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -241,21 +241,18 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
 }

 static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
-                                       struct sk_buff *skb)
+                                       u32 len)
 {
-       if (vvs->rx_bytes + skb->len > vvs->buf_alloc)
+       if (vvs->rx_bytes + len > vvs->buf_alloc)
                return false;

-       vvs->rx_bytes += skb->len;
+       vvs->rx_bytes += len;
        return true;
 }

 static void virtio_transport_dec_rx_pkt(struct virtio_vsock_sock *vvs,
-                                       struct sk_buff *skb)
+                                       u32 len)
 {
-       int len;
-
-       len = skb_headroom(skb) - sizeof(struct virtio_vsock_hdr) - skb->len;
        vvs->rx_bytes -= len;
        vvs->fwd_cnt += len;
 }
@@ -388,7 +385,7 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
                skb_pull(skb, bytes);

                if (skb->len == 0) {
-                       virtio_transport_dec_rx_pkt(vvs, skb);
+                       virtio_transport_dec_rx_pkt(vvs, bytes);
                        consume_skb(skb);
                } else {
                        __skb_queue_head(&vvs->rx_queue, skb);
@@ -437,17 +434,17 @@ static int virtio_transport_seqpacket_do_dequeue(struct vsock_sock *vsk,

        while (!msg_ready) {
                struct virtio_vsock_hdr *hdr;
+               size_t pkt_len;

                skb = __skb_dequeue(&vvs->rx_queue);
                if (!skb)
                        break;
                hdr = virtio_vsock_hdr(skb);
+               pkt_len = (size_t)le32_to_cpu(hdr->len);

                if (dequeued_len >= 0) {
-                       size_t pkt_len;
                        size_t bytes_to_copy;

-                       pkt_len = (size_t)le32_to_cpu(hdr->len);
                        bytes_to_copy = min(user_buf_len, pkt_len);

                        if (bytes_to_copy) {
@@ -484,7 +481,7 @@ static int virtio_transport_seqpacket_do_dequeue(struct vsock_sock *vsk,
                                msg->msg_flags |= MSG_EOR;
                }

-               virtio_transport_dec_rx_pkt(vvs, skb);
+               virtio_transport_dec_rx_pkt(vvs, pkt_len);
                kfree_skb(skb);
        }

@@ -1040,7 +1037,7 @@ virtio_transport_recv_enqueue(struct vsock_sock *vsk,

        spin_lock_bh(&vvs->rx_lock);

-       can_enqueue = virtio_transport_inc_rx_pkt(vvs, skb);
+       can_enqueue = virtio_transport_inc_rx_pkt(vvs, len);
        if (!can_enqueue) {
                free_pkt = true;
                goto out;

When we used vsock_pkt, we were passing the structure because the `len`
field was immutable (and copied from the header), whereas with skb it
can change and so we introduced these errors.

What do you think?

Thanks,
Stefano