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

[Arseniy Krasnov]

On 06.03.2023 14:57, Stefano Garzarella wrote:
> 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?
Yes, i think passing explicit integer value to 'virtio_transport_inc/dec_rx_pkt'
is more clear solution. I had this variant, but thought that current will be
smaller. Current version with skb argument forces to call 'skb_pull()' before
each 'virtio_transport_dec_rx_pkt()' as 'rx_bytes'/'fwd_cnt' new value relies
on skb parameters - otherwise 'rx_bytes' become invalid. 'skb_pull()' will be
used only to update 'skb->len' which shows rest of the data. I'll do it in v3.

Thanks, Arseniy
> 
> Thanks,
> Stefano
>