net: optimize eth_type_trans() vs CONFIG_STACKPROTECTOR_STRONG=y

Some platforms exhibit very high costs with CONFIG_STACKPROTECTOR_STRONG=y
when a function needs to pass the address of a local variable to external
functions.

eth_type_trans() (and its callers) is showing this anomaly on AMD EPYC 7B12
platforms (and maybe others).

We could :

1) inline eth_type_trans()

This would help if its callers also has the same issue, and the canary cost
would be paid by the callers already.

This is a bit cumbersome because netdev_uses_dsa() is pulling
whole <net/dsa.h> definitions.

2) Compile net/ethernet/eth.c with -fno-stack-protector

This would weaken security.

3) Hack eth_type_trans() to temporarily use skb->dev as a place holder
if skb_header_pointer() needs to pull 2 bytes not present in skb->head.

This patch implements 3), and brings a 5% improvement on TX/RX intensive
workload (tcp_rr 10,000 flows) on AMD EPYC 7B12.

Removing CONFIG_STACKPROTECTOR_STRONG on this platform can improve
performance by 25 %.
This means eth_type_trans() issue is not an isolated artifact.

Signed-off-by: Eric Dumazet <[e-mail]>
Reviewed-by: Willem de Bruijn <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ethernet/eth.c

···
  */
 __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
 {
-	unsigned short _service_access_point;
 	const unsigned short *sap;
 	const struct ethhdr *eth;
+	__be16 res;
 
 	skb->dev = dev;
 	skb_reset_mac_header(skb);
···
 	 *      the protocol design and runs IPX over 802.3 without an 802.2 LLC
 	 *      layer. We look for FFFF which isn't a used 802.2 SSAP/DSAP. This
 	 *      won't work for fault tolerant netware but does for the rest.
+	 *	We use skb->dev as temporary storage to not hit
+	 *	CONFIG_STACKPROTECTOR_STRONG=y costs on some platforms.
 	 */
-	sap = skb_header_pointer(skb, 0, sizeof(*sap), &_service_access_point);
-	if (sap && *sap == 0xFFFF)
-		return htons(ETH_P_802_3);
+	sap = skb_header_pointer(skb, 0, sizeof(*sap), &skb->dev);
+	res = (sap && *sap == 0xFFFF) ? htons(ETH_P_802_3) : htons(ETH_P_802_2);
 
-	/*
-	 *      Real 802.2 LLC
-	 */
-	return htons(ETH_P_802_2);
+	/* restore skb->dev in case it was mangled by skb_header_pointer(). */
+	skb->dev = dev;
+	return res;
 }
 EXPORT_SYMBOL(eth_type_trans);