[PATCH net v3 2/3] if_ether: Add an accessor to read the raw ethertype

[Linus Walleij <linus.walleij@linaro.org>]

There are circumstances where the skb->protocol can not be
trusted, such as when using DSA switches that add a custom
ethertype to the ethernet packet, which is later on supposed
to be stripped by the switch hardware connected to the
conduit ethernet interface.

Since ethernet drivers transmitting such frames with alien
ethertypes can have hardware that will get confused by
custom ethertypes they need a way to retrieve and act
on any such type.

The new eth_skb_raw_ethertype() helper will extract the
ethertype directly from the skb->data using the ethernet
and (if necessary) VLAN helper functions, and return the
ethertype actually found inside the raw buffer.

Suggested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 include/linux/if_ether.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/include/linux/if_ether.h b/include/linux/if_ether.h
index 8a9792a6427a..6bf265979757 100644
--- a/include/linux/if_ether.h
+++ b/include/linux/if_ether.h
@@ -37,6 +37,22 @@ static inline struct ethhdr *inner_eth_hdr(const struct sk_buff *skb)
 	return (struct ethhdr *)skb_inner_mac_header(skb);
 }
 
+/* This determines the ethertype incoded into the skb data without
+ * relying on skb->protocol which is not always identical.
+ */
+static inline u16 skb_eth_raw_ethertype(struct sk_buff *skb)
+{
+	struct ethhdr *hdr;
+
+	/* If we can't extract a header, return invalid type */
+	if (!pskb_may_pull(skb, ETH_HLEN))
+		return 0x0000U;
+
+	hdr = skb_eth_hdr(skb);
+
+	return ntohs(hdr->h_proto);
+}
+
 int eth_header_parse(const struct sk_buff *skb, unsigned char *haddr);
 
 extern ssize_t sysfs_format_mac(char *buf, const unsigned char *addr, int len);

-- 
2.34.1