Re: [Patch net] vsock: improve tap delivery accuracy

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, May 02, 2023 at 10:44:04AM -0700, Cong Wang wrote:
> From: Cong Wang <cong.wang@bytedance.com>
> 
> When virtqueue_add_sgs() fails, the skb is put back to send queue,
> we should not deliver the copy to tap device in this case. So we
> need to move virtio_transport_deliver_tap_pkt() down after all
> possible failures.
> 
> Fixes: 82dfb540aeb2 ("VSOCK: Add virtio vsock vsockmon hooks")
> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> Cc: Stefano Garzarella <sgarzare@redhat.com>
> Cc: Bobby Eshleman <bobby.eshleman@bytedance.com>
> Signed-off-by: Cong Wang <cong.wang@bytedance.com>
> ---
>  net/vmw_vsock/virtio_transport.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index e95df847176b..055678628c07 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -109,9 +109,6 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>  		if (!skb)
>  			break;
>  
> -		virtio_transport_deliver_tap_pkt(skb);
> -		reply = virtio_vsock_skb_reply(skb);
> -
>  		sg_init_one(&hdr, virtio_vsock_hdr(skb), sizeof(*virtio_vsock_hdr(skb)));
>  		sgs[out_sg++] = &hdr;
>  		if (skb->len > 0) {
> @@ -128,6 +125,8 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>  			break;
>  		}
>  
> +		virtio_transport_deliver_tap_pkt(skb);
> +		reply = virtio_vsock_skb_reply(skb);

I don't remember the reason for the ordering, but I'm pretty sure it was
deliberate. Probably because the payload buffers could be freed as soon
as virtqueue_add_sgs() is called.

If that's no longer true with Bobby's skbuff code, then maybe it's safe
to monitor packets after they have been sent.

Stefan

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