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

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Message-ID: <20251113105844-mutt-send-email-mst@kernel.org>
Date: Thu, 13 Nov 2025 10:59:17 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, netdev@...r.kernel.org,
	Richard Weinberger <richard@....at>,
	Anton Ivanov <anton.ivanov@...bridgegreys.com>,
	Johannes Berg <johannes@...solutions.net>,
	Willem de Bruijn <willemdebruijn.kernel@...il.com>,
	Jason Wang <jasowang@...hat.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Eugenio Pérez <eperezma@...hat.com>,
	Simon Horman <horms@...nel.org>, Jiri Pirko <jiri@...nulli.us>,
	Alvaro Karsz <alvaro.karsz@...id-run.com>,
	linux-um@...ts.infradead.org, virtualization@...ts.linux.dev
Subject: Re: [PATCH net v5 1/2] virtio-net: correct hdr_len handling for
 VIRTIO_NET_F_GUEST_HDRLEN

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 <xuanzhuo@...ux.alibaba.com>
> 
> 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?