Dereference pointers in Layer's Stringer impl

Dereference any fields which are pointers before string formatting so that the
value pointed to ends up in the string representation.

Tested:
  Added TestLayerStringFormat to
  //third_party/gvisor/test/packetimpact/testbench:testbench_test
PiperOrigin-RevId: 305627821

test/packetimpact/testbench/BUILD

@@ -36,4 +36,5 @@ go_test(
     size = "small",
     srcs = ["layers_test.go"],
     library = ":testbench",
+    deps = ["//pkg/tcpip"],
 )

test/packetimpact/testbench/layers.go

@@ -118,7 +118,7 @@ func stringLayer(l Layer) string {
 		if v.IsNil() {
 			continue
 		}
-		ret = append(ret, fmt.Sprintf("%s:%v", t.Name, v))
+		ret = append(ret, fmt.Sprintf("%s:%v", t.Name, reflect.Indirect(v)))
 	}
 	return fmt.Sprintf("&%s{%s}", t, strings.Join(ret, " "))
 }

test/packetimpact/testbench/layers_test.go

@@ -16,6 +16,8 @@ package testbench
 
 import "testing"
 
+import "gvisor.dev/gvisor/pkg/tcpip"
+
 func TestLayerMatch(t *testing.T) {
 	var nilPayload *Payload
 	noPayload := &Payload{}
@@ -47,3 +49,96 @@ func TestLayerMatch(t *testing.T) {
 		}
 	}
 }
+
+func TestLayerStringFormat(t *testing.T) {
+	for _, tt := range []struct {
+		name string
+		l    Layer
+		want string
+	}{
+		{
+			name: "TCP",
+			l: &TCP{
+				SrcPort:    Uint16(34785),
+				DstPort:    Uint16(47767),
+				SeqNum:     Uint32(3452155723),
+				AckNum:     Uint32(2596996163),
+				DataOffset: Uint8(5),
+				Flags:      Uint8(20),
+				WindowSize: Uint16(64240),
+				Checksum:   Uint16(0x2e2b),
+			},
+			want: "&testbench.TCP{" +
+				"SrcPort:34785 " +
+				"DstPort:47767 " +
+				"SeqNum:3452155723 " +
+				"AckNum:2596996163 " +
+				"DataOffset:5 " +
+				"Flags:20 " +
+				"WindowSize:64240 " +
+				"Checksum:11819" +
+				"}",
+		},
+		{
+			name: "UDP",
+			l: &UDP{
+				SrcPort: Uint16(34785),
+				DstPort: Uint16(47767),
+				Length:  Uint16(12),
+			},
+			want: "&testbench.UDP{" +
+				"SrcPort:34785 " +
+				"DstPort:47767 " +
+				"Length:12" +
+				"}",
+		},
+		{
+			name: "IPv4",
+			l: &IPv4{
+				IHL:            Uint8(5),
+				TOS:            Uint8(0),
+				TotalLength:    Uint16(44),
+				ID:             Uint16(0),
+				Flags:          Uint8(2),
+				FragmentOffset: Uint16(0),
+				TTL:            Uint8(64),
+				Protocol:       Uint8(6),
+				Checksum:       Uint16(0x2e2b),
+				SrcAddr:        Address(tcpip.Address([]byte{197, 34, 63, 10})),
+				DstAddr:        Address(tcpip.Address([]byte{197, 34, 63, 20})),
+			},
+			want: "&testbench.IPv4{" +
+				"IHL:5 " +
+				"TOS:0 " +
+				"TotalLength:44 " +
+				"ID:0 " +
+				"Flags:2 " +
+				"FragmentOffset:0 " +
+				"TTL:64 " +
+				"Protocol:6 " +
+				"Checksum:11819 " +
+				"SrcAddr:197.34.63.10 " +
+				"DstAddr:197.34.63.20" +
+				"}",
+		},
+		{
+			name: "Ether",
+			l: &Ether{
+				SrcAddr: LinkAddress(tcpip.LinkAddress([]byte{0x02, 0x42, 0xc5, 0x22, 0x3f, 0x0a})),
+				DstAddr: LinkAddress(tcpip.LinkAddress([]byte{0x02, 0x42, 0xc5, 0x22, 0x3f, 0x14})),
+				Type:    NetworkProtocolNumber(4),
+			},
+			want: "&testbench.Ether{" +
+				"SrcAddr:02:42:c5:22:3f:0a " +
+				"DstAddr:02:42:c5:22:3f:14 " +
+				"Type:4" +
+				"}",
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.l.String(); got != tt.want {
+				t.Errorf("%s.String() = %s, want: %s", tt.name, got, tt.want)
+			}
+		})
+	}
+}