feat: add xt.Comment (#260)

Signed-off-by: thediveo <thediveo@gmx.eu>

xt/comment.go

@@ -0,0 +1,34 @@
+package xt
+
+import (
+	"bytes"
+	"fmt"
+)
+
+// CommentSize is the fixed size of a comment info xt blob, see:
+// https://elixir.bootlin.com/linux/v6.8.7/source/include/uapi/linux/netfilter/xt_comment.h#L5
+const CommentSize = 256
+
+// Comment gets marshalled and unmarshalled as a fixed-sized char array, filled
+// with zeros as necessary, see:
+// https://elixir.bootlin.com/linux/v6.8.7/source/include/uapi/linux/netfilter/xt_comment.h#L7
+type Comment string
+
+func (c *Comment) marshal(fam TableFamily, rev uint32) ([]byte, error) {
+	if len(*c) >= CommentSize {
+		return nil, fmt.Errorf("comment must be less than %d bytes, got %d bytes",
+			CommentSize, len(*c))
+	}
+	data := make([]byte, CommentSize)
+	copy(data, []byte(*c))
+	return data, nil
+}
+
+func (c *Comment) unmarshal(fam TableFamily, rev uint32, data []byte) error {
+	if len(data) != CommentSize {
+		return fmt.Errorf("malformed comment: got %d bytes, expected exactly %d bytes",
+			len(data), CommentSize)
+	}
+	*c = Comment(bytes.TrimRight(data, "\x00"))
+	return nil
+}

xt/comment_test.go

@@ -0,0 +1,62 @@
+package xt
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestComment(t *testing.T) {
+	t.Parallel()
+	payload := Comment("The quick brown fox jumps over the lazy dog.")
+	oversized := Comment(strings.Repeat("foobar", 100))
+	tests := []struct {
+		name   string
+		info   InfoAny
+		errmsg string
+	}{
+		{
+			name: "un/marshal Comment round-trip",
+			info: &payload,
+		},
+		{
+			name:   "marshal oversized Comment",
+			info:   &oversized,
+			errmsg: "comment must be less than 256 bytes, got 600 bytes",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			data, err := tt.info.marshal(0, 0)
+			if err != nil {
+				if tt.errmsg != "" && err.Error() == tt.errmsg {
+					return
+				}
+				t.Fatalf("marshal error: %+v", err)
+
+			}
+			if len(data) != CommentSize {
+				t.Fatalf("marshal error: invalid size %d", len(data))
+			}
+			if data[len(data)-1] != 0 {
+				t.Fatalf("marshal error: invalid termination")
+			}
+			var comment Comment
+			var recoveredInfo InfoAny = &comment
+			err = recoveredInfo.unmarshal(0, 0, data)
+			if err != nil {
+				t.Fatalf("unmarshal error: %+v", err)
+			}
+			if !reflect.DeepEqual(tt.info, recoveredInfo) {
+				t.Fatalf("original %+v and recovered %+v are different", tt.info, recoveredInfo)
+			}
+		})
+	}
+
+	oversizeddata := []byte(oversized)
+	var comment Comment
+	if err := (&comment).unmarshal(0, 0, oversizeddata); err == nil {
+		t.Fatalf("unmarshal: expected error, but got nil")
+	}
+}

xt/info.go

@@ -34,6 +34,9 @@ func Unmarshal(name string, fam TableFamily, rev uint32, data []byte) (InfoAny,
 		case 1:
 			i = &AddrTypeV1{}
 		}
+	case "comment":
+		var c Comment
+		i = &c
 	case "conntrack":
 		switch rev {
 		case 1: