[PATCH net-next 1/2] IPv6/GRO: generic helper to remove temporary HBH/jumbo header in driver

[Coco Li <lixiaoyan@google.com>]

IPv6/TCP and GRO stacks can build big TCP packets with an added
temporary Hop By Hop header.

Is GSO is not involved, then the temporary header needs to be removed in
the driver. This patch provides a generic helper for drivers that need
to modify their headers in place.

Signed-off-by: Coco Li <lixiaoyan@google.com>
---
 include/net/ipv6.h | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index d383c895592a..a11d58c85c05 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -500,6 +500,39 @@ static inline int ipv6_has_hopopt_jumbo(const struct sk_buff *skb)
 	return jhdr->nexthdr;
 }
 
+/* Return 0 if HBH header is successfully removed
+ * Or if HBH removal is unnecessary (packet is not big TCP)
+ * Return error to indicate dropping the packet
+ */
+static inline int ipv6_hopopt_jumbo_remove(struct sk_buff *skb)
+{
+	const int hophdr_len = sizeof(struct hop_jumbo_hdr);
+	int nexthdr = ipv6_has_hopopt_jumbo(skb);
+	struct ipv6hdr *h6;
+
+	if (!nexthdr)
+		return 0;
+
+	if (skb_cow_head(skb, 0))
+		return -1;
+
+	/* Remove the HBH header.
+	 * Layout: [Ethernet header][IPv6 header][HBH][L4 Header]
+	 */
+	memmove(skb->data + hophdr_len,
+		skb->data,
+		ETH_HLEN + sizeof(struct ipv6hdr));
+
+	skb->data += hophdr_len;
+	skb->len -= hophdr_len;
+	skb->network_header += hophdr_len;
+
+	h6 = ipv6_hdr(skb);
+	h6->nexthdr = nexthdr;
+
+	return 0;
+}
+
 static inline bool ipv6_accept_ra(struct inet6_dev *idev)
 {
 	/* If forwarding is enabled, RA are not accepted unless the special
-- 
2.38.1.584.g0f3c55d4c2-goog