8371083: FollowReferences reports non-class objects as JVMTI_HEAP_REFERENCE_SYSTEM_CLASS

Reviewed-by: lmesnik, sspitsyn

Author: Alex Menkov

src/hotspot/share/prims/jvmtiTagMap.cpp

@@ -2190,6 +2190,39 @@ class SimpleRootsClosure : public OopClosure {
   virtual void do_oop(narrowOop* obj_p) { ShouldNotReachHere(); }
 };
 
+// A supporting closure used to process ClassLoaderData roots.
+class CLDRootsClosure: public OopClosure {
+private:
+  bool _continue;
+public:
+  CLDRootsClosure(): _continue(true) {}
+
+  inline bool stopped() {
+    return !_continue;
+  }
+
+  void do_oop(oop* obj_p) {
+    if (stopped()) {
+      return;
+    }
+
+    oop o = NativeAccess<AS_NO_KEEPALIVE>::oop_load(obj_p);
+    // ignore null
+    if (o == nullptr) {
+      return;
+    }
+
+    jvmtiHeapReferenceKind kind = JVMTI_HEAP_REFERENCE_OTHER;
+    if (o->klass() == vmClasses::Class_klass()) {
+      kind = JVMTI_HEAP_REFERENCE_SYSTEM_CLASS;
+    }
+
+    // invoke the callback
+    _continue = CallbackInvoker::report_simple_root(kind, o);
+  }
+  virtual void do_oop(narrowOop* obj_p) { ShouldNotReachHere(); }
+};
+
 // A supporting closure used to process JNI locals
 class JNILocalRootsClosure : public OopClosure {
  private:
@@ -2776,10 +2809,10 @@ inline bool VM_HeapWalkOperation::collect_simple_roots() {
   }
 
   // Preloaded classes and loader from the system dictionary
-  blk.set_kind(JVMTI_HEAP_REFERENCE_SYSTEM_CLASS);
-  CLDToOopClosure cld_closure(&blk, ClassLoaderData::_claim_none);
+  CLDRootsClosure cld_roots_closure;
+  CLDToOopClosure cld_closure(&cld_roots_closure, ClassLoaderData::_claim_none);
   ClassLoaderDataGraph::always_strong_cld_do(&cld_closure);
-  if (blk.stopped()) {
+  if (cld_roots_closure.stopped()) {
     return false;
   }
 

test/hotspot/jtreg/serviceability/jvmti/FollowReferences/KindSystemClass/KindSystemClass.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8371083
+ * @summary Verify FollowReferences does not report non-classes roots as JVMTI_HEAP_REFERENCE_SYSTEM_CLASS
+ * @requires vm.jvmti
+ * @run main/othervm/native -agentlib:KindSystemClass
+ *                          KindSystemClass
+ */
+
+public class KindSystemClass {
+
+    static native int tagSysClasses();
+    static native Object[] getObjectsWithTags();
+
+    public static void main(String[] args) throws Exception {
+        System.loadLibrary("KindSystemClass");
+
+        int tagged = tagSysClasses();
+        System.out.println("Tagged " + tagged + " classes");
+
+        Object[] objs = getObjectsWithTags();
+        System.out.println("Tagged objects (total " + objs.length + "):");
+        int nonClassesCnt = 0;
+        for (int i = 0; i < objs.length; i++) {
+            Object obj = objs[i];
+            String s;
+            if (obj instanceof Class cls) {
+                s = "OK: " + cls;
+            } else {
+                nonClassesCnt++;
+                s = "ERROR, not a class: " + obj;
+            }
+            System.out.println("[" + i + "] " + s);
+        }
+        if (nonClassesCnt != 0) {
+            throw new RuntimeException("Found " + nonClassesCnt + " non-classes");
+        }
+    }
+}

test/hotspot/jtreg/serviceability/jvmti/FollowReferences/KindSystemClass/libKindSystemClass.cpp

@@ -0,0 +1,111 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+#include <jvmti.h>
+#include "jvmti_common.hpp"
+
+static jvmtiEnv *jvmti = nullptr;
+static int class_counter = 0;
+static int other_counter = 0;
+
+static jint JNICALL
+heap_reference_callback(jvmtiHeapReferenceKind reference_kind,
+                        const jvmtiHeapReferenceInfo* reference_info,
+                        jlong class_tag,
+                        jlong referrer_class_tag,
+                        jlong size,
+                        jlong* tag_ptr,
+                        jlong* referrer_tag_ptr,
+                        jint length,
+                        void* user_data) {
+  switch (reference_kind) {
+  case JVMTI_HEAP_REFERENCE_SYSTEM_CLASS:
+    *tag_ptr = ++class_counter;
+    break;
+  case JVMTI_HEAP_REFERENCE_OTHER:
+    ++other_counter;
+    break;
+  default:
+    break;
+  }
+  return JVMTI_VISIT_OBJECTS;
+}
+
+extern "C" JNIEXPORT jint JNICALL
+Java_KindSystemClass_tagSysClasses(JNIEnv* jni, jclass clazz) {
+  jvmtiHeapCallbacks callbacks = {};
+  callbacks.heap_reference_callback = heap_reference_callback;
+
+  jvmtiError err = jvmti->FollowReferences(0 /* filter nothing */,
+                                           nullptr /* no class filter */,
+                                           nullptr /* no initial object, follow roots */,
+                                           &callbacks,
+                                           nullptr);
+  check_jvmti_error(err, "FollowReferences failed");
+
+  LOG("JVMTI_HEAP_REFERENCE_SYSTEM_CLASS: %d, JVMTI_HEAP_REFERENCE_OTHER: %d\n", class_counter, other_counter);
+
+  return class_counter;
+}
+
+extern "C" JNIEXPORT jobjectArray JNICALL
+Java_KindSystemClass_getObjectsWithTags(JNIEnv* jni, jclass clazz) {
+  // request tagged objects with tags 1..class_counter
+  jlong* tags = nullptr;
+  jvmtiError err = jvmti->Allocate(class_counter * sizeof(jlong), (unsigned char**)&tags);
+  check_jvmti_error(err, "Allocate failed");
+
+  for (int i = 0; i < class_counter; i++) {
+    tags[i] = i + 1;
+  }
+
+  jint count = 0;
+  jobject* objects = nullptr;
+
+  err = jvmti->GetObjectsWithTags(class_counter, tags,
+                                  &count, &objects, nullptr);
+  check_jvmti_error(err, "GetObjectsWithTags failed");
+
+  jclass object_klass = jni->FindClass("java/lang/Object");
+  jobjectArray array = jni->NewObjectArray(count, object_klass, nullptr);
+
+  for (jint i = 0; i < count; i++) {
+    jni->SetObjectArrayElement(array, i, objects[i]);
+  }
+
+  deallocate(jvmti, jni, objects);
+
+  return array;
+}
+
+extern "C" JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *vm, char *options, void *reserved) {
+  if (vm->GetEnv(reinterpret_cast<void**>(&jvmti), JVMTI_VERSION) != JNI_OK || !jvmti) {
+    LOG("Could not initialize JVMTI\n");
+    abort();
+  }
+  jvmtiCapabilities capabilities;
+  memset(&capabilities, 0, sizeof(capabilities));
+  capabilities.can_tag_objects = 1;
+  check_jvmti_error(jvmti->AddCapabilities(&capabilities), "adding capabilities");
+  return JVMTI_ERROR_NONE;
+}