Add support for -XX:[+/-]EnableDynamicAgentLoading flag

runtime/jvmti/j9jvmti.tdf

@@ -658,3 +658,5 @@ TraceExit=Trc_JVMTI_jvmtiHookVirtualThreadUnmount_Exit Overhead=1 Level=5 Noenv
 
 TraceEntry=Trc_JVMTI_jvmtiHookVirtualThreadMount_Entry Overhead=1 Level=5 Noenv Template="HookVirtualThreadMount"
 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"

runtime/jvmti/jvmtiStartup.c

@@ -563,9 +563,9 @@ I_32 JNICALL
 loadAgentLibraryOnAttach(struct J9JavaVM * vm, const char * library, const char *options, UDATA decorate)
 {
 	PORT_ACCESS_FROM_JAVAVM(vm);
-	UDATA rc;
+	UDATA rc = JNI_OK;
 	UDATA optionsLength = 0;
-	UDATA libraryLength;
+	UDATA libraryLength = 0;
 	J9JVMTIAgentLibrary *agentLibrary = NULL;
 	J9JVMTIData * jvmtiData = J9JVMTI_DATA_FROM_VM(vm);
 	char loadFunctionName[J9JVMTI_BUFFER_LENGTH + 1] = {0};
@@ -575,6 +575,12 @@ loadAgentLibraryOnAttach(struct J9JavaVM * vm, const char * library, const char
 
 	Trc_JVMTI_loadAgentLibraryOnAttach_Entry(library);
 
+	if (J9_ARE_NO_BITS_SET(vm->runtimeFlags, J9_RUNTIME_ALLOW_DYNAMIC_AGENT)) {
+		Trc_JVMTI_loadAgentLibraryOnAttach_agentLoadingDisabled();
+		rc = JNI_ERR;
+		goto exit;
+	}
+
 	Assert_JVMTI_true(NULL != library); /* Library name must be non-null. */
 
 	if (NULL != options) {
@@ -701,6 +707,16 @@ loadAgentLibrary(J9JavaVM * vm, J9JVMTIAgentLibrary * agentLibrary)
 
 	Trc_JVMTI_loadAgentLibrary_Entry(agentLibrary->nativeLib.name);
 
+	/* If dynamic agent loading is disabled, the HCR/OSR flags default to off.
+	 * However, adding agents at launch is allowed. In this case we need to
+	 * enable the flags for proper functionality.
+	 */
+	if (J9_ARE_NO_BITS_SET(vm->runtimeFlags, J9_RUNTIME_ALLOW_DYNAMIC_AGENT)) {
+		omrthread_monitor_enter(vm->runtimeFlagsMutex);
+		vm->extendedRuntimeFlags |= J9_EXTENDED_RUNTIME_OSR_SAFE_POINT | J9_EXTENDED_RUNTIME_ENABLE_HCR;
+		omrthread_monitor_exit(vm->runtimeFlagsMutex);
+	}
+
 	/* Attempt linking the agent statically, looking for Agent_OnLoad_L.
 	 * If this is not found, fall back on the regular, dynamic linking way.
 	 */

runtime/oti/j9consts.h

@@ -286,7 +286,7 @@ extern "C" {
 #define J9_RUNTIME_ARGENCODING_UNICODE 0x2000
 #define J9_RUNTIME_ARGENCODING_LATIN 0x4000
 #define J9_RUNTIME_ARGENCODING_UTF8 0x8000
-#define J9_RUNTIME_UNUSED_0x10000 0x10000
+#define J9_RUNTIME_ALLOW_DYNAMIC_AGENT 0x10000
 #define J9_RUNTIME_UNUSED_0x20000 0x20000
 #define J9_RUNTIME_UNUSED_0x40000 0x40000
 #define J9_RUNTIME_UNUSED_0x80000 0x80000

runtime/oti/jvminit.h

@@ -618,6 +618,9 @@ enum INIT_STAGE {
 #define VMOPT_XSYSLOG_OPT "-Xsyslog"
 #define MAPOPT_XSYSLOG_OPT_COLON "-Xsyslog:"
 
+#define VMOPT_XXENABLEDYNAMICAGENTLOADING "-XX:+EnableDynamicAgentLoading"
+#define VMOPT_XXNOENABLEDYNAMICAGENTLOADING "-XX:-EnableDynamicAgentLoading"
+
 /* Modularity command line options */
 #define VMOPT_MODULE_UPGRADE_PATH "--upgrade-module-path"
 #define VMOPT_MODULE_PATH "--module-path"

runtime/vm/jvminit.c

@@ -3569,10 +3569,20 @@ processVMArgsFromFirstToLast(J9JavaVM * vm)
 	if (J2SE_VERSION(vm) >= J2SE_V11) {
 		vm->extendedRuntimeFlags |= (UDATA)J9_EXTENDED_RUNTIME_RESTRICT_IFA; /* Enable zAAP switching for Registered Natives and JVMTI callbacks by default in Java 9 and later. */
 	}
-	vm->extendedRuntimeFlags |= J9_EXTENDED_RUNTIME_OSR_SAFE_POINT; /* Enable OSR safe point by default */
-	vm->extendedRuntimeFlags |= (UDATA)J9_EXTENDED_RUNTIME_ENABLE_HCR; /* Enable HCR by default */
 	vm->extendedRuntimeFlags2 |= J9_EXTENDED_RUNTIME2_ENABLE_UTF_CACHE; /* Enable UTF cache by default */
 
+	{
+		IDATA noDynamicAgentLoading = FIND_AND_CONSUME_VMARG(EXACT_MATCH, VMOPT_XXNOENABLEDYNAMICAGENTLOADING, NULL);
+		IDATA dynamicAgentLoading = FIND_AND_CONSUME_VMARG(EXACT_MATCH, VMOPT_XXENABLEDYNAMICAGENTLOADING, NULL);
+		if (noDynamicAgentLoading > dynamicAgentLoading) {
+			vm->runtimeFlags &= ~J9_RUNTIME_ALLOW_DYNAMIC_AGENT;
+			vm->extendedRuntimeFlags &= ~(J9_EXTENDED_RUNTIME_OSR_SAFE_POINT | J9_EXTENDED_RUNTIME_ENABLE_HCR);
+		} else {
+			vm->runtimeFlags |= J9_RUNTIME_ALLOW_DYNAMIC_AGENT;
+			vm->extendedRuntimeFlags |= (J9_EXTENDED_RUNTIME_OSR_SAFE_POINT | J9_EXTENDED_RUNTIME_ENABLE_HCR);
+		}
+	}
+
 #if defined(J9VM_ARCH_X86) || defined(J9VM_ARCH_POWER) || defined(J9VM_ARCH_S390)
 	/* Enabled field watch by default on x86, Power, and S390 platforms */
 	vm->extendedRuntimeFlags |= J9_EXTENDED_RUNTIME_JIT_INLINE_WATCHES;