Linux Virtualization - Mar 2023 - [RFC PATCH v2 1/4] virtio/vsock: fix 'rx_bytes'/'fwd_cnt' calculation

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 at sberdevices.ru>
>---
> 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