lua: add recursive read-only bindings for borrowed slices

app/dns/lua.go

@@ -52,7 +52,7 @@ func luaServers(s *DNS) []luaDNSServer {
 
 func registerLua(L *lua.LState, servers []luaDNSServer, client featureDNS.Client) {
 	L.PreloadModule("xray.dns", func(L *lua.LState) int {
-		pushIPs := xlua.NewSlicePusher[net.IP](L)
+		pushIPs := xlua.NewReadOnlySlicePusher[net.IP](L)
 
 		serverList := L.CreateTable(len(servers), 0)
 		for i, client := range servers {

app/router/lua.go

@@ -62,7 +62,7 @@ func (r *Router) RegisterLua(L *lua.LState) {
 }
 
 func registerLuaContext(L *lua.LState) {
-	pushIPs := xlua.NewSlicePusher[net.IP](L)
+	pushIPs := xlua.NewReadOnlySlicePusher[net.IP](L)
 	attributes := L.NewTypeMetatable(luaAttributesType)
 	L.SetField(attributes, "__index", L.NewFunction(func(L *lua.LState) int {
 		values := L.CheckUserData(1).Value.(map[string]string)

common/geodata/lua.go

@@ -44,7 +44,7 @@ func RegisterLua(L *lua.LState) {
 				"Match":     luaIPMatch,
 				"AnyMatch":  luaIPAnyMatch,
 				"Matches":   luaIPMatches,
-				"FilterIPs": newLuaIPFilterIPs(xlua.NewSlicePusher[net.IP](L)),
+				"FilterIPs": newLuaIPFilterIPs(xlua.NewReadOnlySlicePusher[net.IP](L)),
 			}, "SetReverse", "ToggleReverse")
 			return 1
 		}))

common/lua/luar.go

@@ -1,6 +1,9 @@
 package lua
 
 import (
+	"net"
+	"reflect"
+
 	glua "github.com/yuin/gopher-lua"
 	luar "layeh.com/gopher-luar"
 )
@@ -22,6 +25,52 @@ func NewSlicePusher[T any](L *glua.LState) func(*glua.LState, []T) {
 	}
 }
 
+// NewReadOnlySlicePusher borrows slices without copying and recursively makes
+// nested slice types read-only through their shared metatables. IP byte views
+// and masks are also read-only. Other referenced elements retain their mutability.
+func NewReadOnlySlicePusher[T any](L *glua.LState) func(*glua.LState, []T) {
+	sliceMetatable := luar.MT(L, []T{})
+	if sliceMetatable.RawGetString("__readonly") != glua.LTrue {
+		readOnly := L.NewFunction(func(L *glua.LState) int {
+			L.RaiseError("slice is read-only")
+			return 0
+		})
+		setSliceReadOnly[[]T](L, readOnly)
+	}
+	return NewSlicePusher[T](L)
+}
+
+func setSliceReadOnly[S ~[]E, E any](L *glua.LState, readOnly *glua.LFunction) {
+	var visit func(reflect.Type)
+	visit = func(typ reflect.Type) {
+		if typ.Kind() != reflect.Slice {
+			return
+		}
+		metatable := luar.MT(L, reflect.Zero(typ).Interface())
+		if metatable.RawGetString("__readonly") == glua.LTrue {
+			return
+		}
+		// Mark before descending because named slice types can be recursive.
+		metatable.RawSetString("__readonly", glua.LTrue)
+		metatable.RawSetString("__newindex", readOnly)
+		// Reject explicitly so Lua cannot fall back to the other operand.
+		metatable.RawSetString("__add", readOnly)
+		if typ == reflect.TypeFor[net.IP]() {
+			// A nil receiver lets AppendText return its argument as a writable []byte.
+			methods := metatable.RawGetString("methods").(*glua.LTable)
+			appendText := methods.RawGetString("AppendText")
+			methods.ForEach(func(key, method glua.LValue) {
+				if method == appendText {
+					methods.RawSet(key, glua.LNil)
+				}
+			})
+			visit(reflect.TypeFor[net.IPMask]())
+		}
+		visit(typ.Elem())
+	}
+	visit(reflect.TypeFor[S]())
+}
+
 // DirectMethod handles a Lua call without luar's reflected method invocation.
 // It returns the result count and whether it handled the arguments. On false,
 // it must leave the stack unchanged for the original luar wrapper.

common/lua/luar_test.go

@@ -47,6 +47,141 @@ assert(ok and result == true)
 	}
 }
 
+func TestReadOnlySlicePusher(t *testing.T) {
+	L := glua.NewState()
+	defer L.Close()
+	values := []int{3, 5}
+	NewReadOnlySlicePusher[int](L)(L, values)
+	L.SetGlobal("values", L.Get(-1))
+	L.Pop(1)
+	if err := L.DoString(`
+assert(#values == 2 and values[1] == 3 and values[2] == 5)
+assert(not pcall(function() values[1] = 7 end))
+assert(not pcall(function() return values + 7 end))
+local sum = 0
+for _, value in values() do sum = sum + value end
+assert(sum == 8)
+`); err != nil {
+		t.Fatal(err)
+	}
+	got := L.GetGlobal("values").(*glua.LUserData).Value.([]int)
+	if &got[0] != &values[0] || values[0] != 3 || values[1] != 5 {
+		t.Fatal("read-only binding copied or modified the slice")
+	}
+}
+
+func TestReadOnlyIPSlicePusher(t *testing.T) {
+	L := glua.NewState()
+	defer L.Close()
+	ips := []net.IP{{192, 0, 2, 1}, net.ParseIP("2001:db8::1")}
+	NewReadOnlySlicePusher[net.IP](L)(L, ips)
+	L.SetGlobal("ips", L.Get(-1))
+	L.Pop(1)
+	readOnly := luar.MT(L, ips).RawGetString("__newindex")
+	NewReadOnlySlicePusher[net.IP](L)
+	if luar.MT(L, ips).RawGetString("__newindex") != readOnly {
+		t.Fatal("IP metatables were initialized more than once")
+	}
+	if err := L.DoString(`
+local function readOnly(value)
+    assert(not pcall(function() value[1] = value[1] end))
+    assert(not pcall(function() return value + value[1] end))
+end
+assert(#ips == 2 and ips[1]:String() == "192.0.2.1")
+readOnly(ips)
+for _, ip in ips() do
+    readOnly(ip)
+    readOnly(ip:To16())
+    if ip:To4() then
+        readOnly(ip:To4())
+        readOnly(ip:DefaultMask())
+    end
+    assert(ip:Equal(ip:To16()) and type(ip:String()) == "string")
+    assert(ip.AppendText == nil and ip.appendText == nil)
+    local text, err = ip:MarshalText()
+    assert(not err)
+    text[1] = 88 -- Independently owned byte buffers remain writable.
+    assert(not pcall(function() return ips + text end))
+end
+`); err != nil {
+		t.Fatal(err)
+	}
+	got := L.GetGlobal("ips").(*glua.LUserData).Value.([]net.IP)
+	if &got[0] != &ips[0] || &got[0][0] != &ips[0][0] || &got[1][0] != &ips[1][0] {
+		t.Fatal("read-only binding copied the IP data")
+	}
+	for i, want := range []string{"192.0.2.1", "2001:db8::1"} {
+		if got[i].String() != want {
+			t.Fatalf("IP %d changed to %s; want %s", i, got[i], want)
+		}
+	}
+}
+
+func TestReadOnlyNestedSlicePusher(t *testing.T) {
+	L := glua.NewState()
+	defer L.Close()
+	values := [][][]byte{{{3, 5}}}
+	NewReadOnlySlicePusher[[][]byte](L)(L, values)
+	L.SetGlobal("values", L.Get(-1))
+	L.Pop(1)
+	ips := [][]net.IP{{{192, 0, 2, 1}}}
+	NewReadOnlySlicePusher[[]net.IP](L)(L, ips)
+	L.SetGlobal("ips", L.Get(-1))
+	L.Pop(1)
+	readOnly := luar.MT(L, ips[0]).RawGetString("__newindex")
+	if err := L.DoString(`
+local function readOnly(value)
+    assert(not pcall(function() value[1] = value[1] end))
+    assert(not pcall(function() return value + value[1] end))
+end
+assert(values[1][1][1] == 3 and values[1][1][2] == 5)
+readOnly(values)
+for _, group in values() do
+    readOnly(group)
+    for _, bytes in group() do readOnly(bytes) end
+end
+readOnly(ips)
+readOnly(ips[1])
+local ip = ips[1][1]
+assert(ip:String() == "192.0.2.1")
+readOnly(ip)
+readOnly(ip:To4())
+readOnly(ip:To16())
+readOnly(ip:DefaultMask())
+assert(ip.AppendText == nil and ip.appendText == nil)
+`); err != nil {
+		t.Fatal(err)
+	}
+	NewReadOnlySlicePusher[net.IP](L)
+	if luar.MT(L, ips[0]).RawGetString("__newindex") != readOnly {
+		t.Fatal("nested slice metatable was initialized again")
+	}
+	got := L.GetGlobal("values").(*glua.LUserData).Value.([][][]byte)
+	if &got[0][0][0] != &values[0][0][0] || values[0][0][0] != 3 || values[0][0][1] != 5 {
+		t.Fatal("read-only binding copied or modified the nested slice")
+	}
+}
+
+type recursiveReadOnlySlice []recursiveReadOnlySlice
+
+func TestReadOnlyRecursiveSlicePusher(t *testing.T) {
+	L := glua.NewState()
+	defer L.Close()
+	values := make(recursiveReadOnlySlice, 1)
+	values[0] = values
+	NewReadOnlySlicePusher[recursiveReadOnlySlice](L)(L, values)
+	L.SetGlobal("values", L.Get(-1))
+	L.Pop(1)
+	if err := L.DoString(`
+local nested = values[1][1][1]
+assert(#nested == 1)
+assert(not pcall(function() nested[1] = nested end))
+assert(not pcall(function() return nested + nested end))
+`); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestSlicePusherMetatablePerState(t *testing.T) {
 	first := glua.NewState()
 	defer first.Close()