simplify and update tcx example to use Variable API

This commit updates the `example/tcx` program to use the new Variable API
rather than traditional bpf maps. The example already depends on `bpf_link`,
which requires kernel >= v6.6, therefore the Variable API is already
supported and working (since v5.5).

Signed-off-by: Simone Magnani <[email protected]>

examples/tcx/main.go

@@ -1,8 +1,7 @@
 // This program demonstrates attaching an eBPF program to a network interface
-// with Linux TC (Traffic Control). The program counts ingress and egress
-// packets using two ARRAY maps.
-// The userspace program (Go code in this file) prints the contents
-// of the two maps to stdout every second.
+// with Linux TCX (Traffic Control with eBPF). The program counts ingress and egress
+// packets using two variables. The userspace program (Go code in this file)
+// prints the contents of the two variables to stdout every second.
 // This example depends on tcx bpf_link, available in Linux kernel version 6.6 or newer.
 package main
 
@@ -78,20 +77,19 @@ func main() {
 	}
 }
 
-func formatCounters(ingressMap, egressMap *ebpf.Map) (string, error) {
+func formatCounters(ingressVar, egressVar *ebpf.Variable) (string, error) {
 	var (
 		ingressPacketCount uint64
 		egressPacketCount  uint64
-		key                int32
 	)
 
 	// retrieve value from the ingress map
-	if err := ingressMap.Lookup(&key, &ingressPacketCount); err != nil {
+	if err := ingressVar.Get(&ingressPacketCount); err != nil {
 		return "", err
 	}
 
 	// retrieve value from the egress map
-	if err := egressMap.Lookup(&key, &egressPacketCount); err != nil {
+	if err := egressVar.Get(&egressPacketCount); err != nil {
 		return "", err
 	}
 

examples/tcx/tcx.c

@@ -4,43 +4,17 @@
 
 char __license[] SEC("license") = "Dual MIT/GPL";
 
-/* Define an ARRAY map for storing ingress packet count */
-struct {
-	__uint(type, BPF_MAP_TYPE_ARRAY);
-	__type(key, __u32);
-	__type(value, __u64);
-	__uint(max_entries, 1);
-} ingress_pkt_count SEC(".maps");
-
-/* Define an ARRAY map for storing egress packet count */
-struct {
-	__uint(type, BPF_MAP_TYPE_ARRAY);
-	__type(key, __u32);
-	__type(value, __u64);
-	__uint(max_entries, 1);
-} egress_pkt_count SEC(".maps");
-
-/*
-Upon arrival of each network packet, retrieve and increment
-the packet count from the provided map.
-Returns TC_ACT_OK, allowing the packet to proceed.
-*/
-static __always_inline int update_map_pkt_count(void *map) {
-	__u32 key    = 0;
-	__u64 *count = bpf_map_lookup_elem(map, &key);
-	if (count) {
-		__sync_fetch_and_add(count, 1);
-	}
-
-	return TC_ACT_OK;
-}
+__u64 ingress_pkt_count = 0;
+__u64 egress_pkt_count = 0;
 
 SEC("tc")
 int ingress_prog_func(struct __sk_buff *skb) {
-	return update_map_pkt_count(&ingress_pkt_count);
+	ingress_pkt_count++;
+	return TC_ACT_OK;
 }
 
 SEC("tc")
 int egress_prog_func(struct __sk_buff *skb) {
-	return update_map_pkt_count(&egress_pkt_count);
+	egress_pkt_count++;
+	return TC_ACT_OK;
 }