On 05/01/2018 12:31 PM, Toke Høiland-Jørgensen wrote:

Could you comment on specifically what you believe is broken in this,
please, so I can fix it in the same iteration?

Apart from the various pskb_may_pull() this helper should not change skb layout.

Ideally, the skb should be const and you would use skb_header_pointer() to make clear
you do not ever write this skb.

This would make the reviewer job pretty easy, as no side effect can possibly happen.

+static inline struct tcphdr *cake_get_tcphdr(struct sk_buff *skb)
+{
+	struct ipv6hdr *ipv6h;
+	struct iphdr *iph;
+
+	/* check IPv6 header size immediately, since for IPv4 we need the space
+	 * for the TCP header anyway
+	 */
+	if (!pskb_may_pull(skb, skb_network_offset(skb) +
+				sizeof(struct ipv6hdr)))
+		return NULL;
+
+	iph = ip_hdr(skb);
+
+	if (iph->version == 4) {
+		/* special-case 6in4 tunnelling, as that is a common way to get
+		 * v6 connectivity in the home
+		 */
+		if (iph->protocol == IPPROTO_IPV6) {
+			if (!pskb_may_pull(skb, (skb_network_offset(skb) +
+						 ip_hdrlen(skb) +
+						 sizeof(struct ipv6hdr))))
+				return NULL;
+
+			ipv6h = (struct ipv6hdr *)(skb_network_header(skb) +
+						   ip_hdrlen(skb));
+
+			if (ipv6h->nexthdr != IPPROTO_TCP)
+				return NULL;
+
+			skb_set_inner_network_header(skb,
+						     skb_network_offset(skb) +
+						     ip_hdrlen(skb));
+			skb_set_inner_transport_header(skb,
+						skb_inner_network_offset(skb) +
+						sizeof(struct ipv6hdr));

This is not allowed for a dissector.