example xdp: The IP address output by xdp demo is empty

[BillyChen1]

Describe the bug
The IP address output by xdp demo is empty (NIL)

To Reproduce

Env:

~ go version
go version go1.21.1 linux/amd64
~ uname -a
Linux cqm-OptiPlex-7040 5.15.0-83-generic #92~20.04.1-Ubuntu SMP Mon Aug 21 14:00:49 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
~ cat /etc/issue
Ubuntu 20.04.5 LTS \n \l

Behavior:

xdp git:(main) ✗ sudo ./xdp enp0s31f6
2023/10/19 16:56:15 Attached XDP program to iface "enp0s31f6" (index 2)
2023/10/19 16:56:15 Press Ctrl-C to exit and remove the program
2023/10/19 16:56:16 Map contents:
        <nil> => 1
        <nil> => 1
        <nil> => 61
2023/10/19 16:56:17 Map contents:
        <nil> => 1
        <nil> => 5
        <nil> => 2
        <nil> => 1
        <nil> => 164

Expected behavior
My expectation was to see the IP address string.
I found that if I use the uint32 type in examples/xdp/main.go to accept the val attribute, and use internal.NativeEndian.PutUint32 to convert it to a byte array, I can solve the problem.
The following is my modification to the formatMapContents method in examples/xdp/main.go, which can solve this problem.

func formatMapContents(m *ebpf.Map) (string, error) {
	var (
		sb strings.Builder
		key uint32
		val uint32
	)
	iter := m.Iterate()
	for iter.Next(&key, &val) {
		b := make([]byte, 4)
		internal.NativeEndian.PutUint32(b, key)
		sourceIP := net.IP(b) // IPv4 source address in network byte order.
		packetCount := val
		sb.WriteString(fmt.Sprintf("\t%s => %d\n", sourceIP, packetCount))
	}
	return sb.String(), iter.Err()
}

After modification, the problem was solved.

➜  xdp git:(main) ✗ sudo ./xdp enp0s31f6
2023/10/19 17:08:20 Attached XDP program to iface "enp0s31f6" (index 2)
2023/10/19 17:08:20 Press Ctrl-C to exit and remove the program
2023/10/19 17:08:21 Map contents:
        114.212.81.84 => 16
        100.103.31.185 => 5
        172.27.144.197 => 1
2023/10/19 17:08:22 Map contents:
        114.212.81.84 => 43
        100.103.31.185 => 5
        120.55.98.112 => 2
        172.27.144.197 => 6
2023/10/19 17:08:23 Map contents:
        114.212.81.84 => 43
        100.103.31.185 => 7
        114.212.87.49 => 1
        120.55.98.112 => 2
        172.27.144.197 => 9

[rgo3]

I can confirm that this is also broken for me.
@markpash already suggested on the PR for this example, that we could use netip.Addr as the type for the key, which fixed it for me:

func formatMapContents(m *ebpf.Map) (string, error) {
	var (
		sb strings.Builder
		key netip.Addr
		val uint32
	)
	iter := m.Iterate()
	for iter.Next(&key, &val) {
                sourceIP := key
		packetCount := val
		sb.WriteString(fmt.Sprintf("\t%s => %d\n", sourceIP, packetCount))
	}
	return sb.String(), iter.Err()
}

[BillyChen1]

I can confirm that this is also broken for me. @markpash already suggested on the PR for this example, that we could use netip.Addr as the type for the key, which fixed it for me:

func formatMapContents(m *ebpf.Map) (string, error) {
	var (
		sb strings.Builder
		key netip.Addr
		val uint32
	)
	iter := m.Iterate()
	for iter.Next(&key, &val) {
                sourceIP := key
		packetCount := val
		sb.WriteString(fmt.Sprintf("\t%s => %d\n", sourceIP, packetCount))
	}
	return sb.String(), iter.Err()
}

cool! It worked for me, too!

[rgo3]

Feel free to open a PR with the fix @BillyChen1

[BillyChen1]

@rgo3 #1178

[rgo3]

@lmb I had some suspicions and this broke with 4609dc7. So my suggested fix fixes the issue described here but there might be some underlying bug with the zero allocation stuff?