Re: [PATCH net 1/2] vsock/virtio: fix length and offset in tap skb for split packets

[Bobby Eshleman <bobbyeshleman@gmail.com>]

On Fri, May 08, 2026 at 06:44:10PM +0200, Stefano Garzarella wrote:
> From: Stefano Garzarella <sgarzare@redhat.com>
> 
> virtio_transport_build_skb() builds a new skb to be delivered to the
> vsockmon tap device. To build the new skb, it uses the original skb
> data length as payload length, but as the comment notes, the original
> packet stored in the skb may have been split in multiple packets, so we
> need to use the length in the header, which is correctly updated before
> the packet is delivered to the tap, and the offset for the data.
> 
> This was also similar to what we did before commit 71dc9ec9ac7d
> ("virtio/vsock: replace virtio_vsock_pkt with sk_buff") where we probably
> missed something during the skb conversion.
> 
> Also update the comment above, which was left stale by the skb
> conversion and still mentioned a buffer pointer that no longer exists.
> 
> Fixes: 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>  net/vmw_vsock/virtio_transport_common.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 9b8014516f4f..a678d5d75704 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -166,12 +166,12 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
>  	struct sk_buff *skb;
>  	size_t payload_len;
>  
> -	/* A packet could be split to fit the RX buffer, so we can retrieve
> -	 * the payload length from the header and the buffer pointer taking
> -	 * care of the offset in the original packet.
> +	/* A packet could be split to fit the RX buffer, so we use
> +	 * the payload length from the header, which has been updated
> +	 * by the sender to reflect the fragment size.
>  	 */
>  	pkt_hdr = virtio_vsock_hdr(pkt);
> -	payload_len = pkt->len;
> +	payload_len = le32_to_cpu(pkt_hdr->len);
>  
>  	skb = alloc_skb(sizeof(*hdr) + sizeof(*pkt_hdr) + payload_len,
>  			GFP_ATOMIC);
> @@ -219,7 +219,8 @@ static struct sk_buff *virtio_transport_build_skb(void *opaque)
>  
>  			virtio_transport_copy_nonlinear_skb(pkt, data, payload_len);
>  		} else {
> -			skb_put_data(skb, pkt->data, payload_len);
> +			skb_put_data(skb, pkt->data + VIRTIO_VSOCK_SKB_CB(pkt)->offset,
> +				     payload_len);
>  		}
>  	}
>  
> -- 
> 2.54.0
> 

Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>