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

On Tue, Oct 20, 2026 at 08:05:29PM -0400, Willem de Bruijn wrote:

Michael S. Tsirkin wrote:

quoted

On Tue, Oct 20, 2026 at 03:44:35PM -0400, Willem de Bruijn wrote:

quoted

quoted

+/* 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