Subject: Re: [PATCH net v4 2/4] virtio-net: Ensure hdr_len is not set unless the header is forwarded to the device.

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 9 Nov 2025 16:37:50 -0500

On Wed, Oct 29, 2025 at 11:09:11AM +0800, Xuan Zhuo wrote:
> Although `virtio_net_hdr_from_skb` is used in several places outside of
> `virtio-net.c`, the `hdr_len` field is only utilized by the device
> according to the specification. Therefore, we do not need to set
> `hdr_len` unless the header is actually passed to the device.
> 
> Signed-off-by: Xuan Zhuo <[email-masked]>

maybe ...  but what is this patch trying to achieve?
it does not seem harmful to set it ...

> ---
>  include/linux/virtio_net.h | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 4d1780848d0e..710ae0d2d336 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -218,9 +218,14 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
>  	if (skb_is_gso(skb)) {
>  		struct skb_shared_info *sinfo = skb_shinfo(skb);
>  
> -		/* This is a hint as to how much should be linear. */
> -		hdr->hdr_len = __cpu_to_virtio16(little_endian,
> -						 skb_headlen(skb));
> +		/* In certain code paths (such as the af_packet.c receive path),
> +		 * this function may be called without a transport header.
> +		 * In this case, we do not need to set the hdr_len.
> +		 */
> +		if (skb_transport_header_was_set(skb))
> +			hdr->hdr_len = __cpu_to_virtio16(little_endian,
> +							 skb_headlen(skb));
> +
>  		hdr->gso_size = __cpu_to_virtio16(little_endian,
>  						  sinfo->gso_size);
>  		if (sinfo->gso_type & SKB_GSO_TCPV4)
> -- 
> 2.32.0.3.g01195cf9f