net: Allow ethtool to set interface in loopback mode.

This patch enables ethtool to set the loopback mode on a given interface.
By configuring the interface in loopback mode in conjunction with a policy
route / rule, a userland application can stress the egress / ingress path
exposing the flows of the change in progress and potentially help developer(s)
understand the impact of those changes without even sending a packet out
on the network.

Following set of commands illustrates one such example -
    a) ip -4 addr add 192.168.1.1/24 dev eth1
    b) ip -4 rule add from all iif eth1 lookup 250
    c) ip -4 route add local 0/0 dev lo proto kernel scope host table 250
    d) arp -Ds 192.168.1.100 eth1
    e) arp -Ds 192.168.1.200 eth1
    f) sysctl -w net.ipv4.ip_nonlocal_bind=1
    g) sysctl -w net.ipv4.conf.all.accept_local=1
    # Assuming that the machine has 8 cores
    h) taskset 000f netserver -L 192.168.1.200
    i) taskset 00f0 netperf -t TCP_CRR -L 192.168.1.100 -H 192.168.1.200 -l 30

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Acked-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 5 insertions, 3 deletions

diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index d70fb76edb77..4ce9e5f2c069 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -174,7 +174,8 @@ static void loopback_setup(struct net_device *dev)
                 | NETIF_F_HIGHDMA
                 | NETIF_F_LLTX
                 | NETIF_F_NETNS_LOCAL
-                | NETIF_F_VLAN_CHALLENGED;
+                | NETIF_F_VLAN_CHALLENGED
+                | NETIF_F_LOOPBACK;
         dev->ethtool_ops        = &loopback_ethtool_ops;
         dev->header_ops         = &eth_header_ops;
         dev->netdev_ops         = &loopback_ops;
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index d5de66af46f9..e7244ed1f9a8 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1067,6 +1067,7 @@ struct net_device {
 #define NETIF_F_RXHASH          (1 << 28) /* Receive hashing offload */
 #define NETIF_F_RXCSUM          (1 << 29) /* Receive checksumming offload */
 #define NETIF_F_NOCACHE_COPY    (1 << 30) /* Use no-cache copyfromuser */
+#define NETIF_F_LOOPBACK        (1 << 31) /* Enable loopback */
 
         /* Segmentation offload features */
 #define NETIF_F_GSO_SHIFT       16
@@ -1082,7 +1083,7 @@ struct net_device {
         /* = all defined minus driver/device-class-related */
 #define NETIF_F_NEVER_CHANGE    (NETIF_F_VLAN_CHALLENGED | \
                                   NETIF_F_LLTX | NETIF_F_NETNS_LOCAL)
-#define NETIF_F_ETHTOOL_BITS    (0x7f3fffff & ~NETIF_F_NEVER_CHANGE)
+#define NETIF_F_ETHTOOL_BITS    (0xff3fffff & ~NETIF_F_NEVER_CHANGE)
 
         /* List of features with software fallbacks. */
 #define NETIF_F_GSO_SOFTWARE    (NETIF_F_TSO | NETIF_F_TSO_ECN | \
diff --git a/net/core/ethtool.c b/net/core/ethtool.c
index 927819d92248..b6f405888538 100644
--- a/net/core/ethtool.c
+++ b/net/core/ethtool.c
@@ -362,7 +362,7 @@ static const char netdev_features_strings[ETHTOOL_DEV_FEATURE_WORDS * 32][ETH_GS
         /* NETIF_F_RXHASH */          "rx-hashing",
         /* NETIF_F_RXCSUM */          "rx-checksum",
         /* NETIF_F_NOCACHE_COPY */    "tx-nocache-copy"
-        "",
+        /* NETIF_F_LOOPBACK */        "loopback",
 };
 
 static int __ethtool_get_sset_count(struct net_device *dev, int sset)