Add option to set input expression size limit in type checker.

checker/checker_options.h

@@ -42,6 +42,12 @@ struct CheckerOptions {
   // Enabled by default, but can be disabled to preserve the original type name
   // as parsed.
   bool update_struct_type_names = true;
+
+  // Maximum number (inclusive) of expression nodes to check for an input
+  // expression.
+  //
+  // If exceeded, the checker should return a status with code InvalidArgument.
+  int max_expression_node_count = 100000;
 };
 
 }  // namespace cel

checker/internal/type_checker_impl.cc

@@ -458,9 +458,9 @@ class ResolveVisitor : public AstVisitorBase {
   // These are handled separately to disambiguate between namespaces and field
   // accesses
   absl::flat_hash_set<const Expr*> deferred_select_operations_;
-  absl::Status status_;
   std::vector<std::unique_ptr<VariableScope>> comprehension_vars_;
   std::vector<ComprehensionScope> comprehension_scopes_;
+  absl::Status status_;
 
   // References that were resolved and may require AST rewrites.
   absl::flat_hash_map<const Expr*, FunctionResolution> functions_;
@@ -1252,8 +1252,23 @@ absl::StatusOr<ValidationResult> TypeCheckerImpl::Check(
 
   TraversalOptions opts;
   opts.use_comprehension_callbacks = true;
-  AstTraverse(ast_impl.root_expr(), visitor, opts);
-  CEL_RETURN_IF_ERROR(visitor.status());
+
+  auto traversal = AstTraversal::Create(ast_impl.root_expr(), opts);
+  for (int step = 0; step < options_.max_expression_node_count * 2; ++step) {
+    bool has_next = traversal.Step(visitor);
+    if (!visitor.status().ok()) {
+      return visitor.status();
+    }
+    if (!has_next) {
+      break;
+    }
+  }
+
+  if (!traversal.IsDone()) {
+    return absl::InvalidArgumentError(
+        absl::StrCat("Max expression node count exceeded: ",
+                     options_.max_expression_node_count));
+  }
 
   if (env_.expected_type().has_value()) {
     visitor.AssertExpectedType(ast_impl.root_expr(), *env_.expected_type());

checker/internal/type_checker_impl_test.cc

@@ -57,6 +57,7 @@ namespace checker_internal {
 namespace {
 
 using ::absl_testing::IsOk;
+using ::absl_testing::StatusIs;
 using ::cel::ast_internal::AstImpl;
 using ::cel::ast_internal::Reference;
 using ::cel::expr::conformance::proto3::TestAllTypes;
@@ -65,6 +66,7 @@ using ::testing::_;
 using ::testing::Contains;
 using ::testing::ElementsAre;
 using ::testing::Eq;
+using ::testing::HasSubstr;
 using ::testing::IsEmpty;
 using ::testing::Pair;
 using ::testing::Property;
@@ -1013,6 +1015,22 @@ TEST(TypeCheckerImplTest, NullLiteral) {
   EXPECT_TRUE(ast_impl.type_map()[1].has_null());
 }
 
+TEST(TypeCheckerImplTest, ExpressionLimitInclusive) {
+  TypeCheckEnv env(GetSharedTestingDescriptorPool());
+  CheckerOptions options;
+  options.max_expression_node_count = 2;
+  TypeCheckerImpl impl(std::move(env), options);
+  ASSERT_OK_AND_ASSIGN(auto ast, MakeTestParsedAst("{}.foo"));
+  ASSERT_OK_AND_ASSIGN(ValidationResult result, impl.Check(std::move(ast)));
+
+  ASSERT_TRUE(result.IsValid());
+
+  ASSERT_OK_AND_ASSIGN(ast, MakeTestParsedAst("{}.foo.bar"));
+  EXPECT_THAT(impl.Check(std::move(ast)),
+              StatusIs(absl::StatusCode::kInvalidArgument,
+                       HasSubstr("expression node count exceeded: 2")));
+}
+
 TEST(TypeCheckerImplTest, ComprehensionUnsupportedRange) {
   TypeCheckEnv env(GetSharedTestingDescriptorPool());
   google::protobuf::Arena arena;