[dl] Implement RTLD_NOLOAD

Implement RTLD_NOLOAD for libdl and add a basic test.

Bug: 392191799
Fixed: 348727901
Change-Id: Icc9f48f14c4bbffbdc21969e451c5d30161b0caf
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1193402
Commit-Queue: Caslyn Tonelli <[email protected]>
Fuchsia-Auto-Submit: Caslyn Tonelli <[email protected]>
Reviewed-by: Roland McGrath <[email protected]>

Author: Caslyn

sdk/lib/dl/runtime-dynamic-linker.h

@@ -105,6 +105,10 @@ class RuntimeDynamicLinker {
       return diag.ok(found);
     }
 
+    if (mode & OpenFlags::kNoload) {
+      return diag.ok(nullptr);
+    }
+
     // A Module for `file` does not yet exist; create a new LinkingSession
     // to perform the loading and linking of the file and all its dependencies.
     LinkingSession<Loader> linking_session{modules_, max_static_tls_modid_};

sdk/lib/dl/test/BUILD.gn

@@ -40,6 +40,7 @@ test("dl-unittests") {
       "dl-load-tests-fail.cc",
       "dl-load-tests-global.cc",
       "dl-load-tests-initfini.cc",
+      "dl-load-tests-open-flags.cc",
       "dl-load-tests-tls.cc",
       "dl-load-tests.h",
       "dl-system-tests.cc",

sdk/lib/dl/test/dl-impl-tests.h

@@ -28,8 +28,6 @@ class DlImplTests : public Base {
   // since the error message returned from the libdl implementation will be the
   // same regardless of the OS.
   static constexpr bool kCanMatchExactError = true;
-  // TODO(https://fxbug.dev/348727901): Implement RTLD_NOLOAD
-  static constexpr bool kSupportsNoLoadMode = false;
   // TODO(https://fxbug.dev/342480690): Support Dynamic TLS
   static constexpr bool kSupportsDynamicTls = false;
   // TODO(https://fxbug.dev/382529434): Have dlclose() run finalizers
@@ -50,6 +48,11 @@ class DlImplTests : public Base {
     auto result = dynamic_linker_->Open<typename Base::Loader>(
         file, mode, std::bind_front(&Base::RetrieveFile, this));
     if (result.is_ok()) {
+      // If RTLD_NOLOAD was passed and we have a NULL return value, there is no
+      // module to track.
+      if ((mode & RTLD_NOLOAD) && !result.value()) {
+        return result;
+      }
       // TODO(https://fxbug.dev/382527519): RuntimeDynamicLinker should have a
       // `RunInitializers` method that will run this with proper synchronization.
       static_cast<RuntimeModule*>(result.value())->InitializeModuleTree();

sdk/lib/dl/test/dl-load-tests-open-flags.cc

@@ -0,0 +1,30 @@
+// Copyright 2025 The Fuchsia Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "dl-load-tests.h"
+
+namespace {
+
+using dl::testing::DlTests;
+using dl::testing::TestModule;
+
+TYPED_TEST_SUITE(DlTests, dl::testing::TestTypes);
+
+// If RTLD_NOLOAD is passed and the module is not found, NULL is returned but an
+// error is not expected.
+TYPED_TEST(DlTests, RtldNoLoadBasic) {
+  const std::string kRet17File = TestModule("ret17");
+
+  auto open = this->DlOpen(kRet17File.c_str(), RTLD_NOW | RTLD_LOCAL | RTLD_NOLOAD);
+  ASSERT_TRUE(open.is_ok()) << open.error_value();
+  EXPECT_EQ(open.value(), nullptr);
+
+  // TODO(https://fxbug.dev/325494556): check that dlerror() returns NULL.
+}
+
+// TODO(https://fxbug.dev/348727901): test how RTLD_NOLOAD can be used in
+// combination with other flags, for example with RTLD_GLOBAL to promote a
+// module.
+
+}  // namespace

sdk/lib/dl/test/dl-tests-base.h

@@ -34,9 +34,6 @@ class DlTestsBase : public ::testing::Test {
   // Whether the dlopen implementation validates the mode argument.
   static constexpr bool kCanValidateMode = true;
 
-  // Whether the test fixture's implementation supports `RTLD_NOLOAD`
-  static constexpr bool kSupportsNoLoadMode = true;
-
   // Whether the test fixture will always prioritize a loaded module in symbol
   // resolution, regardless of whether it is a global module.
   static constexpr bool kStrictLoadOrderPriority = false;

sdk/lib/ld/test/modules/BUILD.gn

@@ -75,6 +75,7 @@ per_test_suffices = [
   "PrecedenceInDepResolution",
   "StartupModulesBasic",
   "StartupModulesPriorityOverGlobal",
+  "RtldNoLoadBasic",
 ]
 
 # This reaches all the runtime deps of //sdk/lib/dl/test:dl-unittests.