Implement JEP 451: Prepare to Disallow the Dynamic Loading of Agents

Print warning message when loading an agent via AttachAPI
loadAgentLibraryImpl() if -XX:+EnableDynamicAgentLoading is not
specified, and the agent wasn't loaded before;
Added isAgentLibraryLoaded() to determine if an agent has been loaded
already;
Implemented JVM_PrintWarningAtDynamicAgentLoad();
Added jcmd command JVMTI.agent_load;
Updated a few trace points within loadAgentLibraryGeneric().

Signed-off-by: Jason Feng <[email protected]>

jcl/src/java.base/share/classes/openj9/internal/tools/attach/target/DiagnosticUtils.java

@@ -46,7 +46,7 @@ public class DiagnosticUtils {
 	private static final String FORMAT_PREFIX = " Format: "; //$NON-NLS-1$
 
 	@SuppressWarnings("nls")
-	private static final String HEAP_DUMP_OPTION_HELP = " [request=<options>] [opts=<options] [<file path>]%n"
+	private static final String HEAP_DUMP_OPTION_HELP = " [request=<options>] [opts=<options>] [<file path>]%n"
 			+ " Set optional request= and opts= -Xdump options. The order of the parameters does not matter.%n";
 
 	@SuppressWarnings("nls")
@@ -100,7 +100,10 @@ public class DiagnosticUtils {
 	 * Get JVM statistics
 	 */
 	private static final String DIAGNOSTICS_STAT_CLASS = "jstat.class"; //$NON-NLS-1$
-
+
+	// load JVMTI agent
+	private static final String DIAGNOSTICS_LOAD_JVMTI_AGENT = "JVMTI.agent_load"; //$NON-NLS-1$
+
 	/**
 	 * Key for the command sent to executeDiagnosticCommand()
 	 */
@@ -354,7 +357,27 @@ private static DiagnosticProperties getJstatClass(String diagnosticCommand) {
 		bufferPrinter.flush();
 		return DiagnosticProperties.makeStringResult(buffer.toString());
 	}
-
+
+	@SuppressWarnings("nls")
+	private static DiagnosticProperties loadJVMTIAgent(String diagnosticCommand) {
+		DiagnosticProperties result;
+		String[] parts = diagnosticCommand.split(DIAGNOSTICS_OPTION_SEPARATOR);
+		// parts[0] is already verified as DIAGNOSTICS_LOAD_JVMTI_AGENT since we are here
+		if (parts.length < 2) {
+			result = DiagnosticProperties.makeErrorProperties("Too few arguments, the absolute path of the agent is required: " + diagnosticCommand);
+		} else if (parts.length > 3) {
+			result = DiagnosticProperties.makeErrorProperties("Command not recognized due to more than 3 arguments: " + diagnosticCommand);
+		} else {
+			String attachError = Attachment.loadAgentLibrary(parts[1], (parts.length == 3) ? parts[2] : "", false);
+			if (attachError == null) {
+				result = DiagnosticProperties.makeStringResult(DIAGNOSTICS_LOAD_JVMTI_AGENT + " succeeded");
+			} else {
+				result = DiagnosticProperties.makeStatusProperties(true, attachError);
+			}
+		}
+		return result;
+	}
+
 	private static DiagnosticProperties doHelp(String diagnosticCommand) {
 		String[] parts = diagnosticCommand.split(DIAGNOSTICS_OPTION_SEPARATOR);
 		/* print a list of the available commands */
@@ -416,7 +439,13 @@ private static DiagnosticProperties doHelp(String diagnosticCommand) {
 	private static final String DIAGNOSTICS_JSTAT_CLASS_HELP = "Show JVM classloader statistics.%n" //$NON-NLS-1$
 			+ FORMAT_PREFIX + DIAGNOSTICS_STAT_CLASS + "%n" //$NON-NLS-1$
 			+ "NOTE: this utility might significantly affect the performance of the target VM.%n"; //$NON-NLS-1$
-
+
+	@SuppressWarnings("nls")
+	private static final String DIAGNOSTICS_LOAD_JVMTI_AGENT_HELP = "Load JVMTI agent.%n"
+			+ FORMAT_PREFIX + DIAGNOSTICS_LOAD_JVMTI_AGENT + " <agentLibrary> [<agent option>]%n"
+			+ "          agentLibrary: the absolute path of the agent%n"
+			+ "          agent option: (Optional) the agent option string%n";
+
 	/* Initialize the command and help text tables */
 	static {
 		commandTable = new HashMap<>();
@@ -451,5 +480,8 @@ private static DiagnosticProperties doHelp(String diagnosticCommand) {
 
 		commandTable.put(DIAGNOSTICS_STAT_CLASS, DiagnosticUtils::getJstatClass);
 		helpTable.put(DIAGNOSTICS_STAT_CLASS, DIAGNOSTICS_JSTAT_CLASS_HELP);
+
+		commandTable.put(DIAGNOSTICS_LOAD_JVMTI_AGENT, DiagnosticUtils::loadJVMTIAgent);
+		helpTable.put(DIAGNOSTICS_LOAD_JVMTI_AGENT, DIAGNOSTICS_LOAD_JVMTI_AGENT_HELP);
 	}
 }

runtime/j9vm/javanextvmi.cpp

@@ -24,6 +24,7 @@
 #include "bcverify_api.h"
 #include "j9.h"
 #include "j9cfg.h"
+#include "jvminit.h"
 #include "rommeth.h"
 #include "ut_j9scar.h"
 #include "util_api.h"
@@ -548,10 +549,14 @@ JVM_VirtualThreadHideFrames(JNIEnv *env, jobject vthread, jboolean hide)
 JNIEXPORT jboolean JNICALL
 JVM_PrintWarningAtDynamicAgentLoad()
 {
-	/* A temporary implementation, an actual solution is provided via
-	 * https://github.com/eclipse-openj9/openj9/issues/17500
-	 */
-	return JNI_TRUE;
+	jboolean result = JNI_TRUE;
+	J9JavaVM *vm = BFUjavaVM;
+	if (J9_ARE_ANY_BITS_SET(vm->runtimeFlags, J9_RUNTIME_ALLOW_DYNAMIC_AGENT)
+		&& (0 <= FIND_ARG_IN_VMARGS(EXACT_MATCH, VMOPT_XXENABLEDYNAMICAGENTLOADING, NULL))
+	) {
+		result = JNI_FALSE;
+	}
+	return result;
 }
 
 JNIEXPORT void JNICALL

runtime/jvmti/j9jvmti.tdf

@@ -563,7 +563,7 @@ TraceExit=Trc_JVMTI_issueAgentOnLoadAttach_Exit Overhead=1 Level=1 Noenv Templat
 TraceEvent=Trc_JVMTI_issueAgentOnLoadAttach_close_shared_library Overhead=1 Level=1 Noenv Template="issueAgentOnLoadAttach unloading %s"
 
 TraceEntry=Trc_JVMTI_loadAgentLibraryGeneric_Entry Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric loading library %s"
-TraceExit=Trc_JVMTI_loadAgentLibraryGeneric_Exit Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric successfully loaded library %s"
+TraceExit=Trc_JVMTI_loadAgentLibraryGeneric_Exit Obsolete Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric successfully loaded library %s"
 
 TraceEntry=Trc_JVMTI_loadAgentLibraryOnAttach_Entry Overhead=1 Level=1 Noenv Template="loadAgentLibraryOnAttach loading library %s"
 TraceExit=Trc_JVMTI_loadAgentLibraryOnAttach_Exit Overhead=1 Level=1 Noenv Template="loadAgentLibraryOnAttach loaded library %s, status=%d"
@@ -602,11 +602,11 @@ TraceException=Trc_JVMTI_issueAgentOnLoadAttach_loadFunctionFailed Overhead=1 Le
 TraceEvent=Trc_JVMTI_issueAgentOnLoadAttach_loadFunctionSucceeded Overhead=1 Level=5 Noenv Template="issueAgentOnLoadAttach: Load function %s succeeded"
 
 TraceEvent=Trc_JVMTI_loadAgentLibraryGeneric_loadingAgentAs Overhead=1 Level=5 Noenv Template="loadAgentLibraryGeneric: Loading agent as %s"
-TraceException=Trc_JVMTI_loadAgentLibraryGeneric_execNameNotFound Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric: failed obtaining executable name; error code=%d"
-TraceException=Trc_JVMTI_loadAgentLibraryGeneric_failedOpeningAgentLibrary Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric: Failed opening agent library %s"
+TraceException=Trc_JVMTI_loadAgentLibraryGeneric_execNameNotFound Obsolete Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric: failed obtaining executable name; error code=%d"
+TraceException=Trc_JVMTI_loadAgentLibraryGeneric_failedOpeningAgentLibrary Obsolete Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric: Failed opening agent library %s"
 TraceEvent=Trc_JVMTI_loadAgentLibraryGeneric_openedAgentLibrary Overhead=1 Level=5 Noenv Template="loadAgentLibraryGeneric: Opened agent %s (address=%p) %s"
-TraceException=Trc_JVMTI_loadAgentLibraryGeneric_agentAttachFailed1 Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric: Failed attaching agent library; load function %s not found"
-TraceException=Trc_JVMTI_loadAgentLibraryGeneric_agentAttachFailed2 Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric: Failed attaching agent library; load function failed with error code %zd"
+TraceException=Trc_JVMTI_loadAgentLibraryGeneric_agentAttachFailed1 Obsolete Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric: Failed attaching agent library; load function %s not found"
+TraceException=Trc_JVMTI_loadAgentLibraryGeneric_agentAttachFailed2 Obsolete Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric: Failed attaching agent library; load function failed with error code %zd"
 TraceEvent=Trc_JVMTI_loadAgentLibraryGeneric_agentAttachedSuccessfully Overhead=1 Level=5 Noenv Template="loadAgentLibraryGeneric: Attached agent %s successfully"
 
 TraceEvent=Trc_JVMTI_loadAgentLibraryOnAttach_attachingAgentDynamically Overhead=1 Level=5 Noenv Template="loadAgentLibraryOnAttach: Attached agent %s dynamically"
@@ -660,3 +660,8 @@ TraceEntry=Trc_JVMTI_jvmtiHookVirtualThreadMount_Entry Overhead=1 Level=5 Noenv
 TraceExit=Trc_JVMTI_jvmtiHookVirtualThreadMount_Exit Overhead=1 Level=5 Noenv Template="HookVirtualThreadMount"
 
 TraceEvent=Trc_JVMTI_loadAgentLibraryOnAttach_agentLoadingDisabled Overhead=1 Level=5 Noenv Template="loadAgentLibraryOnAttach: Dynamic agent loading is disabled"
+TraceExit-Exception=Trc_JVMTI_loadAgentLibraryGeneric_execNameNotFound_Exit Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric failed to obtain executable name with error code=%d while loading library %s"
+TraceExit-Exception=Trc_JVMTI_loadAgentLibraryGeneric_failedOpeningAgentLibrary_Exit Overhead==1 Level=1 Noenv Template="loadAgentLibraryGeneric failed to open agent library %s with error message %s"
+TraceExit-Exception=Trc_JVMTI_loadAgentLibraryGeneric_agentAttachFailed1_Exit Overhead==1 Level=1 Noenv Template="loadAgentLibraryGeneric failed attaching agent library %s; load function %s not found"
+TraceExit-Exception=Trc_JVMTI_loadAgentLibraryGeneric_agentAttachFailed2_Exit Overhead==1 Level=1 Noenv Template="loadAgentLibraryGeneric failed attaching agent library %s; load function %s failed with error code=%d"
+TraceExit=Trc_JVMTI_loadAgentLibraryGeneric_succeed_Exit Overhead=1 Level=1 Noenv Template="loadAgentLibraryGeneric successfully loaded library %s and function %s"

runtime/jvmti/jvmtiStartup.c

@@ -52,8 +52,8 @@ static jint loadAgentLibrary (J9JavaVM * vm, J9JVMTIAgentLibrary * agentLibrary)
 static jint createXrunLibraries (J9JavaVM * vm);
 static J9JVMTIAgentLibrary * findAgentLibrary(J9JavaVM * vm, const char *libraryAndOptions, UDATA libraryLength);
 static jint issueAgentOnLoadAttach(J9JavaVM * vm, J9JVMTIAgentLibrary * agentLibrary, const char* options, char *loadFunctionName, BOOLEAN * foundLoadFn);
-I_32 JNICALL loadAgentLibraryOnAttach(struct J9JavaVM * vm, const char * library, const char *options, UDATA decorate) ;
-
+I_32 JNICALL loadAgentLibraryOnAttach(struct J9JavaVM *vm, const char *library, const char *options, UDATA decorate);
+static BOOLEAN isAgentLibraryLoaded(J9JavaVM *vm, const char *library);
 
 #define INSTRUMENT_LIBRARY "instrument"
 
@@ -212,6 +212,7 @@ IDATA J9VMDllMain(J9JavaVM* vm, IDATA stage, void* reserved)
 			}
 
 			vm->loadAgentLibraryOnAttach = &loadAgentLibraryOnAttach;
+			vm->isAgentLibraryLoaded = &isAgentLibraryLoaded;
 
 			break;
 		}
@@ -446,45 +447,50 @@ issueAgentOnLoadAttach(J9JavaVM * vm, J9JVMTIAgentLibrary * agentLibrary, const
 }
 
 /**
- * Load a JVMTI agent and call the agent's initialization function
- * @param vm Java VM
- * @param agentLibrary environment for the agent
- * @param name of the initialization function
- * @return JNI_ERR, JNI_OK
+ * Load a JVMTI agent and call the agent's initialization function.
+ *
+ * @param[in] vm Java VM
+ * @param[in] agentLibrary environment for the agent
+ * @param[in] loadFunctionName name of the initialization function
+ * @param[in] loadStatically a boolean indicating if the library is to be loaded statically
+ * @param[in/out] found a pointer to a boolean indicating whether the load function was found
+ * @param[in/out] errorMessage a pointer to the error message when opening the shared library
+ *
+ * @return JNI_ERR if failed, otherwise JNI_OK
  */
 static jint
-loadAgentLibraryGeneric(J9JavaVM * vm, J9JVMTIAgentLibrary * agentLibrary, char *loadFunctionName, BOOLEAN loadStatically, BOOLEAN * found, const char **errorMessage)
+loadAgentLibraryGeneric(J9JavaVM *vm, J9JVMTIAgentLibrary *agentLibrary, char *loadFunctionName, BOOLEAN loadStatically, BOOLEAN *found, const char **errorMessage)
 {
 	PORT_ACCESS_FROM_JAVAVM(vm);
-	jint rc;
-	J9JVMTIData * jvmtiData = J9JVMTI_DATA_FROM_VM(vm);
-	J9NativeLibrary * nativeLib = &(agentLibrary->nativeLib);
+	jint rc = JNI_OK;
+	J9JVMTIData *jvmtiData = J9JVMTI_DATA_FROM_VM(vm);
+	J9NativeLibrary *nativeLib = &(agentLibrary->nativeLib);
 
 	Trc_JVMTI_loadAgentLibraryGeneric_Entry(agentLibrary->nativeLib.name);
 	if (NULL == agentLibrary->xRunLibrary) {
 		jint i = 0;
-		char * fullLibName = NULL;
-		const char * systemAgentName;
+		char *fullLibName = NULL;
+		const char *systemAgentName = NULL;
 		UDATA openFlags = agentLibrary->decorate ? J9PORT_SLOPEN_DECORATE | J9PORT_SLOPEN_LAZY : J9PORT_SLOPEN_LAZY;
-		char * agentPath = NULL; /* Don't free agentPath; may point at persistent, system areas. */
+		char *agentPath = NULL; /* Don't free agentPath; may point at persistent, system areas. */
 
 		if (loadStatically) {
 			/* If flag is set, the agent library ought to be opened statically, that is, the executable itself. */
 			if (0 == j9sysinfo_get_executable_name(NULL, &agentPath)) {
 				openFlags |= J9PORT_SLOPEN_OPEN_EXECUTABLE;
 			} else {
 				/* Report failure to obtain executable name; caller will proceed with dynamic linking. */
-				Trc_JVMTI_loadAgentLibraryGeneric_execNameNotFound(j9error_last_error_number());
-				Trc_JVMTI_loadAgentLibraryGeneric_Exit(agentLibrary->nativeLib.name);
+				I_32 errorno = j9error_last_error_number();
+				Trc_JVMTI_loadAgentLibraryGeneric_execNameNotFound_Exit(errorno, agentLibrary->nativeLib.name);
 				return JNI_ERR;
 			}
 		} else {
 			/* If the user did not specify an explicit agent path, then ensure that the library
 			 * is loaded from our current jre tree. This avoids picking up stray agents that might
 			 * be found on the library path. See CMVC 144382.
 			 */
-			while ((systemAgentName = systemAgentNames[i++]) != NULL) {
-				if (strcmp(nativeLib->name, systemAgentName) == 0) {
+			while (NULL != (systemAgentName = systemAgentNames[i++])) {
+				if (0 == strcmp(nativeLib->name, systemAgentName)) {
 					fullLibName = prependSystemAgentPath(vm, agentLibrary, nativeLib->name);
 					Trc_JVMTI_loadAgentLibraryGeneric_loadingAgentAs(fullLibName);
 					break;
@@ -493,17 +499,15 @@ loadAgentLibraryGeneric(J9JavaVM * vm, J9JVMTIAgentLibrary * agentLibrary, char
 			agentPath = (NULL != fullLibName) ? fullLibName : nativeLib->name;
 		}
 
-		if (j9sl_open_shared_library(agentPath, &(nativeLib->handle), openFlags) != 0) {
-			Trc_JVMTI_loadAgentLibraryGeneric_failedOpeningAgentLibrary(agentPath);
-
+		if (0 != j9sl_open_shared_library(agentPath, &(nativeLib->handle), openFlags)) {
 			/* We may attempt to open the shared library again so save the error message
 			 * and print it once we know it is the final attempt */
 			*errorMessage = j9error_last_error_message();
 
 			if (NULL != fullLibName) {
 				j9mem_free_memory(fullLibName);
 			}
-			Trc_JVMTI_loadAgentLibraryGeneric_Exit(agentPath);
+			Trc_JVMTI_loadAgentLibraryGeneric_failedOpeningAgentLibrary_Exit(agentPath, *errorMessage);
 			return JNI_ERR;
 		}
 		Trc_JVMTI_loadAgentLibraryGeneric_openedAgentLibrary(agentPath,
@@ -519,15 +523,13 @@ loadAgentLibraryGeneric(J9JavaVM * vm, J9JVMTIAgentLibrary * agentLibrary, char
 	rc = issueAgentOnLoadAttach(vm, agentLibrary, agentLibrary->options, loadFunctionName, found);
 	if (!(*found)) {
 		/* If the load function wasn't found, issueAgentOnLoadAttach already closed the library. */
-		Trc_JVMTI_loadAgentLibraryGeneric_agentAttachFailed1(loadFunctionName);
-		Trc_JVMTI_loadAgentLibraryGeneric_Exit(agentLibrary->nativeLib.name);
+		Trc_JVMTI_loadAgentLibraryGeneric_agentAttachFailed1_Exit(agentLibrary->nativeLib.name, loadFunctionName);
 		return rc;
 	}
 
 	/* For errors other than load function not found ... */
 	if (JNI_OK != rc) {
-		Trc_JVMTI_loadAgentLibraryGeneric_agentAttachFailed2(rc);
-		Trc_JVMTI_loadAgentLibraryGeneric_Exit(agentLibrary->nativeLib.name);
+		Trc_JVMTI_loadAgentLibraryGeneric_agentAttachFailed2_Exit(agentLibrary->nativeLib.name, loadFunctionName, rc);
 		return rc;
 	}
 
@@ -540,14 +542,14 @@ loadAgentLibraryGeneric(J9JavaVM * vm, J9JVMTIAgentLibrary * agentLibrary, char
 	/* Add the library to the linked list */
 	issueWriteBarrier();
 	omrthread_monitor_enter(jvmtiData->mutex);
-	if (jvmtiData->agentLibrariesTail == NULL) {
+	if (NULL == jvmtiData->agentLibrariesTail) {
 		jvmtiData->agentLibrariesHead = jvmtiData->agentLibrariesTail = nativeLib;
 	} else {
 		jvmtiData->agentLibrariesTail->next = nativeLib;
 		jvmtiData->agentLibrariesTail = nativeLib;
 	}
 	omrthread_monitor_exit(jvmtiData->mutex);
-	Trc_JVMTI_loadAgentLibraryGeneric_Exit(agentLibrary->nativeLib.name);
+	Trc_JVMTI_loadAgentLibraryGeneric_succeed_Exit(agentLibrary->nativeLib.name, loadFunctionName);
 
 	return JNI_OK;
 }
@@ -1060,6 +1062,11 @@ createXrunLibraries(J9JavaVM * vm)
 	return JNI_OK;
 }
 
+static BOOLEAN
+isAgentLibraryLoaded(J9JavaVM *vm, const char *library)
+{
+	return NULL != findAgentLibrary(vm, library, strlen(library));
+}
 
 static J9JVMTIAgentLibrary *
 findAgentLibrary(J9JavaVM * vm, const char *libraryAndOptions, UDATA libraryLength)
@@ -1087,4 +1094,3 @@ findAgentLibrary(J9JavaVM * vm, const char *libraryAndOptions, UDATA libraryLeng
 
 	return NULL;
 }
-

runtime/oti/j9nonbuilder.h

@@ -5802,6 +5802,7 @@ typedef struct J9JavaVM {
 	omrthread_monitor_t statisticsMutex;
 	struct J9Statistic* nextStatistic;
 	I_32  (JNICALL *loadAgentLibraryOnAttach)(struct J9JavaVM * vm, const char * library, const char *options, UDATA decorate) ;
+	BOOLEAN (*isAgentLibraryLoaded)(struct J9JavaVM *vm, const char *library);
 	struct J9AttachContext attachContext;
 	UDATA hotSwapCount;
 	UDATA zombieThreadCount;

runtime/vm/BytecodeInterpreter.hpp

@@ -27,26 +27,27 @@
 #include <errno.h>
 #endif /* JAVA_SPEC_VERSION >= 20 */
 
+#include "bcnames.h"
+#define FFI_BUILDING /* Needed on Windows to link libffi statically */
+#include "ffi.h"
+#include "j2sever.h"
 #include "j9.h"
+#include "j9bcvnls.h"
 #include "j9cfg.h"
-#include "j9protos.h"
 #include "j9consts.h"
-#include "j9vmnls.h"
 #include "j9jclnls.h"
-#include "j9bcvnls.h"
-#include "bcnames.h"
+#include "j9protos.h"
+#include "j9vmnls.h"
+#include "jitregmap.h"
+#include "jni.h"
+#include "jvminit.h"
+#include "objhelp.h"
 #include "rommeth.h"
 #include "stackwalk.h"
 #include "ut_j9vm.h"
 #include "util_api.h"
 #include "vm_internal.h"
-#include "jni.h"
-#define FFI_BUILDING /* Needed on Windows to link libffi statically */
-#include "ffi.h"
-#include "jitregmap.h"
-#include "j2sever.h"
 #include "vmaccess.h"
-#include "objhelp.h"
 
 #include "ArrayCopyHelpers.hpp"
 #include "AtomicSupport.hpp"
@@ -4863,8 +4864,24 @@ class INTERPRETER_CLASS
 //			Trc_JCL_attach_loadAgentLibrary(env, agentLibraryUTF, agentOptions, decorate);
 			const char *agentOptionsUTF = env->GetStringUTFChars(agentOptions, NULL);
 			if (NULL != agentOptionsUTF) {
+				if (J9_ARE_ANY_BITS_SET(_vm->runtimeFlags, J9_RUNTIME_ALLOW_DYNAMIC_AGENT)) {
+#if JAVA_SPEC_VERSION >= 21
+					/* The name "vm" is used by FIND_ARG_IN_VMARGS(). */
+					J9JavaVM *vm = _vm;
+					/* No warning if -XX:+EnableDynamicAgentLoading is specified. */
+					if (0 > FIND_ARG_IN_VMARGS(EXACT_MATCH, VMOPT_XXENABLEDYNAMICAGENTLOADING, NULL)) {
+						/* No warning if the same agent is already loaded. */
+						if (!vm->isAgentLibraryLoaded(vm, agentLibraryUTF)) {
+							fprintf(stderr, "WARNING: A JVM TI agent has been loaded dynamically (%s)\n", agentOptionsUTF);
+							fprintf(stderr, "WARNING: If a serviceability tool is in use, please run with -XX:+EnableDynamicAgentLoading to hide this warning\n");
+							fprintf(stderr, "WARNING: If a serviceability tool is not in use, please run with -Djdk.instrument.traceUsage for more information\n");
+							fprintf(stderr, "WARNING: Dynamic loading of agents will be disallowed by default in a future release\n");
+						}
+					}
+#endif /* JAVA_SPEC_VERSION >= 21 */
 					status = (_vm->loadAgentLibraryOnAttach)(_vm, agentLibraryUTF, agentOptionsUTF, decorate);
-					env->ReleaseStringUTFChars(agentOptions, agentOptionsUTF);
+				}
+				env->ReleaseStringUTFChars(agentOptions, agentOptionsUTF);
 			}
 			env->ReleaseStringUTFChars(agentLibrary, agentLibraryUTF);
 		}