net: Support RXFCS feature flag.

When set on hardware that supports the feature,
this causes the Ethernet FCS to be appended
to the end of the skb.

Useful for sniffing packets.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

Documentation/networking/netdev-features.txt

@@ -152,3 +152,9 @@ NETIF_F_VLAN_CHALLENGED should be set for devices which can't cope with VLAN
 headers. Some drivers set this because the cards can't handle the bigger MTU.
 [FIXME: Those cases could be fixed in VLAN code by allowing only reduced-MTU
 VLANs. This may be not useful, though.]
+
+*  rx-fcs
+
+This requests that the NIC append the Ethernet Frame Checksum (FCS)
+to the end of the skb data.  This allows sniffers and other tools to
+read the CRC recorded by the NIC on receipt of the packet.

include/linux/netdev_features.h

@@ -54,6 +54,7 @@ enum {
 	NETIF_F_RXCSUM_BIT,		/* Receive checksumming offload */
 	NETIF_F_NOCACHE_COPY_BIT,	/* Use no-cache copyfromuser */
 	NETIF_F_LOOPBACK_BIT,		/* Enable loopback */
+	NETIF_F_RXFCS_BIT,		/* Append FCS to skb pkt data */
 
 	/*
 	 * Add your fresh new feature above and remember to update
@@ -98,6 +99,7 @@ enum {
 #define NETIF_F_TSO		__NETIF_F(TSO)
 #define NETIF_F_UFO		__NETIF_F(UFO)
 #define NETIF_F_VLAN_CHALLENGED	__NETIF_F(VLAN_CHALLENGED)
+#define NETIF_F_RXFCS		__NETIF_F(RXFCS)
 
 /* Features valid for ethtool to change */
 /* = all defined minus driver/device-class-related */

net/core/ethtool.c

@@ -73,6 +73,7 @@ static const char netdev_features_strings[NETDEV_FEATURE_COUNT][ETH_GSTRING_LEN]
 	[NETIF_F_RXCSUM_BIT] =           "rx-checksum",
 	[NETIF_F_NOCACHE_COPY_BIT] =     "tx-nocache-copy",
 	[NETIF_F_LOOPBACK_BIT] =         "loopback",
+	[NETIF_F_RXFCS_BIT] =            "rx-fcs",
 };
 
 static int ethtool_get_features(struct net_device *dev, void __user *useraddr)