Subject: Re: [PATCH net v5 1/2] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 13 Nov 2025 10:59:17 -0500

On Thu, Nov 13, 2025 at 03:39:35PM +0100, Paolo Abeni wrote:
> On 11/11/25 12:12 PM, Xuan Zhuo wrote:
> > The commit be50da3e9d4a ("net: virtio_net: implement exact header length
> > guest feature") introduces support for the VIRTIO_NET_F_GUEST_HDRLEN
> > feature in virtio-net.
> > 
> > This feature requires virtio-net to set hdr_len to the actual header
> > length of the packet when transmitting, the number of
> > bytes from the start of the packet to the beginning of the
> > transport-layer payload.
> > 
> > However, in practice, hdr_len was being set using skb_headlen(skb),
> > which is clearly incorrect. This commit fixes that issue.
> > 
> > Fixes: be50da3e9d4a ("net: virtio_net: implement exact header length guest feature")
> > Signed-off-by: Xuan Zhuo <[email-masked]>
> 
> IMHO this looks like more a new feature - namely,
> VIRTIO_NET_F_GUEST_HDRLEN support - than a fix.

I mean if guest negotiates VIRTIO_NET_F_GUEST_HDRLEN but the header
length is wrong then yes it is broken and this is a fix.

> [...]
> > @@ -2361,7 +2362,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
> >  	if (vnet_hdr_sz &&
> >  	    virtio_net_hdr_from_skb(skb, h.raw + macoff -
> >  				    sizeof(struct virtio_net_hdr),
> > -				    vio_le(), true, 0)) {
> > +				    vio_le(), true, false, 0)) {
> >  		if (po->tp_version == TPACKET_V3)
> >  			prb_clear_blk_fill_status(&po->rx_ring);
> >  		goto drop_n_account;
> To reduce the diffstat, what about creating a __virtio_net_hdr_from_skb()
> variant (please find a better name) allowing the extra `hdrlen_negotiated`
> argument, define virtio_net_hdr_from_skb() as a wrapper of such helper
> withthe extra arg == false, and use the helper in the few places that
> really could use hdrlen?