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 <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20251121061725.206675-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ethernet/eth.c b/net/ethernet/eth.c
index 43e211e611b1698cbec5f6256ffd59975584bf04..13a63b48b7eeb896dfe98eb0070a261eed2c384b 100644 (file)
--- a/net/ethernet/eth.c
+++ b/net/ethernet/eth.c
@@ -154,9 +154,9 @@ EXPORT_SYMBOL(eth_get_headlen);
  */
 __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);
@@ -181,15 +181,15 @@ __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
         *      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);