examples: add example for XDP

Add an example for attaching an eBPF program to a network
interface with XDP. The example program parses the IPv4
source address from packets (if available) and records
packet counts by IP address in an LRU hash map.

Issue: #645

Signed-off-by: Will Daly <[email protected]>

examples/README.md

@@ -16,6 +16,8 @@
   Like kprobes, but with better performance and usability, for kernels 5.5 and later.
   * [tcp_connect](fentry/) - Trace outgoing IPv4 TCP connections.
   * [tcp_close](tcprtt/) - Log RTT of IPv4 TCP connections using eBPF CO-RE helpers.
+* XDP - Attach a program to a network interface to process incoming packets.
+  * [xdp](xdp/) - Print packet counts by IPv4 source address.
 * Add your use case(s) here!
 
 ## How to run

examples/headers/common.h

@@ -56,6 +56,23 @@ enum bpf_map_type {
 	BPF_MAP_TYPE_INODE_STORAGE         = 28,
 };
 
+enum xdp_action {
+	XDP_ABORTED = 0,
+	XDP_DROP = 1,
+	XDP_PASS = 2,
+	XDP_TX = 3,
+	XDP_REDIRECT = 4,
+};
+
+struct xdp_md {
+	__u32 data;
+	__u32 data_end;
+	__u32 data_meta;
+	__u32 ingress_ifindex;
+	__u32 rx_queue_index;
+	__u32 egress_ifindex;
+};
+
 enum {
 	BPF_ANY     = 0,
 	BPF_NOEXIST = 1,

examples/xdp/main.go

@@ -0,0 +1,83 @@
+//go:build linux
+// +build linux
+
+// This program demonstrates attaching an eBPF program to a network interface
+// with XDP (eXpress Data Path). The program parses the IPv4 source address
+// from packets and writes the packet count by IP to an LRU hash map.
+// The userspace program (Go code in this file) prints the contents
+// of the map to stdout every second.
+// It is possible to modify the XDP program to drop or redirect packets
+// as well -- give it a try!
+package main
+
+import (
+	"fmt"
+	"log"
+	"net"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/cilium/ebpf"
+	"github.com/cilium/ebpf/link"
+)
+
+// $BPF_CLANG and $BPF_CFLAGS are set by the Makefile.
+//go:generate go run github.com/cilium/ebpf/cmd/bpf2go -cc $BPF_CLANG -cflags $BPF_CFLAGS bpf xdp.c -- -I../headers
+
+func main() {
+	if len(os.Args) < 2 {
+		log.Fatalf("Please specify a network interface")
+	}
+
+	// Look up the network interface by name.
+	ifaceName := os.Args[1]
+	iface, err := net.InterfaceByName(ifaceName)
+	if err != nil {
+		log.Fatalf("lookup network iface %q: %s", ifaceName, err)
+	}
+
+	// Load pre-compiled programs into the kernel.
+	objs := bpfObjects{}
+	if err := loadBpfObjects(&objs, nil); err != nil {
+		log.Fatalf("loading objects: %s", err)
+	}
+	defer objs.Close()
+
+	// Attach the program.
+	l, err := link.AttachXDP(link.XDPOptions{
+		Program:   objs.XdpProgFunc,
+		Interface: iface.Index,
+	})
+	if err != nil {
+		log.Fatalf("could not attach XDP program: %s", err)
+	}
+	defer l.Close()
+
+	log.Printf("Attached XDP program to iface %q (index %d)", iface.Name, iface.Index)
+	log.Printf("Press Ctrl-C to exit and remove the program")
+
+	// Print the contents of the BPF hash map (source IP address -> packet count).
+	ticker := time.NewTicker(1 * time.Second)
+	defer ticker.Stop()
+	for range ticker.C {
+		s, err := formatMapContents(objs.XdpStatsMap)
+		if err != nil {
+			log.Printf("Error reading map: %s", err)
+			continue
+		}
+		log.Printf("Map contents:\n%s", s)
+	}
+}
+
+func formatMapContents(m *ebpf.Map) (string, error) {
+	var sb strings.Builder
+	var key, val uint32
+	iter := m.Iterate()
+	for iter.Next(&key, &val) {
+		sourceIP := net.IPv4(byte(key>>24), byte(key>>16), byte(key>>8), byte(key))
+		packetCount := val
+		sb.WriteString(fmt.Sprintf("\t%s => %d\n", sourceIP, packetCount))
+	}
+	return sb.String(), iter.Err()
+}