rpc: add method name length limit (ethereum#31711)

This change adds a limit for RPC method names to prevent potential abuse
where large method names could lead to large response sizes.

The limit is enforced in:
- handleCall for regular RPC method calls
- handleSubscribe for subscription method calls

Added tests in websocket_test.go to verify the length limit
functionality for both regular method calls and subscriptions.

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

Author: MatusKysel

rpc/handler.go

@@ -501,6 +501,10 @@ func (h *handler) handleCall(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage
 	if msg.isUnsubscribe() {
 		callb = h.unsubscribeCb
 	} else {
+		// Check method name length
+		if len(msg.Method) > maxMethodNameLength {
+			return msg.errorResponse(&invalidRequestError{fmt.Sprintf("method name too long: %d > %d", len(msg.Method), maxMethodNameLength)})
+		}
 		callb = h.reg.callback(msg.Method)
 	}
 	if callb == nil {
@@ -536,6 +540,11 @@ func (h *handler) handleSubscribe(cp *callProc, msg *jsonrpcMessage) *jsonrpcMes
 		return msg.errorResponse(ErrNotificationsUnsupported)
 	}
 
+	// Check method name length
+	if len(msg.Method) > maxMethodNameLength {
+		return msg.errorResponse(&invalidRequestError{fmt.Sprintf("subscription name too long: %d > %d", len(msg.Method), maxMethodNameLength)})
+	}
+
 	// Subscription method name is first argument.
 	name, err := parseSubscriptionName(msg.Params)
 	if err != nil {

rpc/json.go

@@ -35,6 +35,7 @@ const (
 	subscribeMethodSuffix    = "_subscribe"
 	unsubscribeMethodSuffix  = "_unsubscribe"
 	notificationMethodSuffix = "_subscription"
+	maxMethodNameLength      = 2048
 
 	defaultWriteTimeout = 10 * time.Second // used if context has no deadline
 )

rpc/websocket_test.go

@@ -391,3 +391,77 @@ func wsPingTestHandler(t *testing.T, conn *websocket.Conn, shutdown, sendPing <-
 		}
 	}
 }
+
+func TestWebsocketMethodNameLengthLimit(t *testing.T) {
+	t.Parallel()
+
+	var (
+		srv     = newTestServer()
+		httpsrv = httptest.NewServer(srv.WebsocketHandler([]string{"*"}))
+		wsURL   = "ws:" + strings.TrimPrefix(httpsrv.URL, "http:")
+	)
+	defer srv.Stop()
+	defer httpsrv.Close()
+
+	client, err := DialWebsocket(context.Background(), wsURL, "")
+	if err != nil {
+		t.Fatalf("can't dial: %v", err)
+	}
+	defer client.Close()
+
+	// Test cases
+	tests := []struct {
+		name           string
+		method         string
+		params         []interface{}
+		expectedError  string
+		isSubscription bool
+	}{
+		{
+			name:           "valid method name",
+			method:         "test_echo",
+			params:         []interface{}{"test", 1},
+			expectedError:  "",
+			isSubscription: false,
+		},
+		{
+			name:           "method name too long",
+			method:         "test_" + string(make([]byte, maxMethodNameLength+1)),
+			params:         []interface{}{"test", 1},
+			expectedError:  "method name too long",
+			isSubscription: false,
+		},
+		{
+			name:           "valid subscription",
+			method:         "nftest_subscribe",
+			params:         []interface{}{"someSubscription", 1, 2},
+			expectedError:  "",
+			isSubscription: true,
+		},
+		{
+			name:           "subscription name too long",
+			method:         string(make([]byte, maxMethodNameLength+1)) + "_subscribe",
+			params:         []interface{}{"newHeads"},
+			expectedError:  "subscription name too long",
+			isSubscription: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var result interface{}
+			err := client.Call(&result, tt.method, tt.params...)
+			if tt.expectedError == "" {
+				if err != nil {
+					t.Errorf("unexpected error: %v", err)
+				}
+			} else {
+				if err == nil {
+					t.Error("expected error, got nil")
+				} else if !strings.Contains(err.Error(), tt.expectedError) {
+					t.Errorf("expected error containing %q, got %q", tt.expectedError, err.Error())
+				}
+			}
+		})
+	}
+}