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

[Michael S. Tsirkin]

On Tue, Oct 20, 2026 at 08:05:29PM -0400, Willem de Bruijn wrote:
> Michael S. Tsirkin wrote:
> > 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.
> 
> You're right, the protocol and network offset need to be parsed
> manually. After that it could be passed to the flow dissector.

I posted v4 with the custom parser, since i had it tested. Let's do
cleanups on top? I spent too much time on this, to be frank.

-- 
MST