selftests: net: Create veth pair for testing in networkless kernel

Check if the netdev list is empty and create veth pair to be used for
feature on/off testing.
Remove the veth pair after testing is complete.
Signed-off-by: Abhinav Jain <jain.abhinav177@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240821171903.118324-2-jain.abhinav177@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/netdevice.sh

@@ -129,6 +129,7 @@ kci_netdev_ethtool()
 
 	kci_netdev_ethtool_test 74 'dump' "ethtool -d $netdev"
 	kci_netdev_ethtool_test 94 'stats' "ethtool -S $netdev"
+
 	return 0
 }
 
@@ -196,10 +197,24 @@ if [ ! -e "$TMP_LIST_NETDEV" ];then
 fi
 
 ip link show |grep '^[0-9]' | grep -oE '[[:space:]].*eth[0-9]*:|[[:space:]].*enp[0-9]s[0-9]:' | cut -d\  -f2 | cut -d: -f1> "$TMP_LIST_NETDEV"
+
+if [ ! -s "$TMP_LIST_NETDEV" ]; then
+	echo "No valid network device found, creating veth pair"
+	ip link add veth0 type veth peer name veth1
+	echo "veth0" > "$TMP_LIST_NETDEV"
+	veth_created=1
+fi
+
 while read netdev
 do
 	kci_test_netdev "$netdev"
 done < "$TMP_LIST_NETDEV"
 
+#clean up veth interface pair if it was created
+if [ "$veth_created" ]; then
+	ip link delete veth0
+	echo "Removed veth pair"
+fi
+
 rm "$TMP_LIST_NETDEV"
 exit 0