Re: [PATCH net v3 5/8] net: resolve uncommon partial checksums at ingress

[Michael S. Tsirkin]

On Tue, Oct 20, 2026 at 03:44:35PM -0400, Willem de Bruijn wrote:
> > +/* Walk the L3 header at @nhoff: where does its transport header
> > + * start, and which protocol is it?  Returns false if the header
> > + * is truncated, malformed or fragmented.
> > + */
> > +static inline bool
> > +virtio_net_l4_start(struct sk_buff *skb, unsigned int nhoff, __be16 proto,
> > +		    unsigned int *thoff, int *ip_proto)
> > +{
> > +	if (proto == htons(ETH_P_IP)) {
> > +		const struct iphdr *iph = virtio_net_ip4_parse(skb, nhoff, thoff);
> > +
> > +		if (!iph || ip_is_fragment(iph))
> > +			return false;
> > +		*ip_proto = iph->protocol;
> > +		return true;
> > +	}
> > +
> > +	if (proto == htons(ETH_P_IPV6)) {
> > +		const struct ipv6hdr *iph;
> > +		int flags = 0;
> > +
> > +		if (!pskb_may_pull(skb, nhoff + sizeof(*iph)))
> > +			return false;
> > +		iph = (const void *)(skb->data + nhoff);
> > +		if (iph->version != 6)
> > +			return false;
> > +		*thoff = nhoff + sizeof(*iph);
> > +		*ip_proto = iph->nexthdr;
> > +		if (*ip_proto != IPPROTO_TCP && *ip_proto != IPPROTO_UDP) {
> > +			*thoff = nhoff;
> > +			*ip_proto = ipv6_find_hdr(skb, thoff, -1, NULL,
> > +						  &flags);
> > +			if (*ip_proto < 0 || flags & IP6_FH_F_FRAG)
> > +				return false;
> > +		}
> > +		return true;
> > +	}
> > +
> > +	return false;
> > +}
> 
> for all this parsing: could we use flow dissection, like the
> !VIRTIO_NET_HDR_F_NEEDS_CSUM case?

Hmm I don't see how.  That one trusts the caller's protocol and network
header. We could parse L2 ourselves and use the dissector for L3+. But
it stops at whatever it does not know, and every miss would turn into a
drop or a software checksum for a packet the stack delivers fine.  Our
parser deliberately covers everything the stack can deliver.

-- 
MST