CDS and JVM-TI agent questions and crash

[Michael Rasmussen]

> I logged issue in JDK-8212200, 
> https://bugs.openjdk.java.net/browse/JDK-8212200
> Thanks,
> Jiangli

Hi Jiangli,

Thanks -- though regarding your comment there, I don't know if it only applies when changing Object's bytes?
I did a small test (attached below) with adding a default method to Serializable, and it crashes as well, though differently -- don't know if what Volker mentioned fixes this though.

Running the below with CDS off, I get:
$ ../jdk/bin/java -Xshare:off -agentpath:./libagent.so Test.java
public default java.lang.Object java.io.Serializable._$_$_blah()

With it on, I get the following crash:
$ ../jdk/bin/java -Xshare:on -agentpath:./libagent.so Test.java
#  Internal Error (.../src/hotspot/share/classfile/javaClasses.cpp:667), pid=277, tid=278
#  assert(java_string->klass() == SystemDictionary::String_klass()) failed: must be java_string

/Michael

Stack trace:
V  [libjvm.so+0xe80ea5]  java_lang_String::equals(oop, unsigned short*, int)+0x85
V  [libjvm.so+0x9e6e44]  CompactHashtable<oop, char>::decode_entry(CompactHashtable<oop, char>*, unsigned int, char const*, int)+0x54
V  [libjvm.so+0x167fae6]  StringTable::lookup_shared(unsigned short*, int, unsigned int)+0x166
V  [libjvm.so+0x16801df]  StringTable::intern(Handle, unsigned short*, int, Thread*)+0x6f
V  [libjvm.so+0x16802e3]  StringTable::intern(Symbol*, Thread*)+0x53
V  [libjvm.so+0xae9921]  ConstantPool::uncached_string_at(int, Thread*)+0x31
V  [libjvm.so+0xc4426c]  fieldDescriptor::string_initial_value(Thread*) const+0x9c
V  [libjvm.so+0xe88056]  initialize_static_field(fieldDescriptor*, Handle, Thread*)+0x1c6
V  [libjvm.so+0xe23819]  InstanceKlass::do_local_static_fields(void (*)(fieldDescriptor*, Handle, Thread*), Handle, Thread*)+0xd9
V  [libjvm.so+0xe70157]  java_lang_Class::initialize_mirror_fields(Klass*, Handle, Handle, Thread*)+0x117
V  [libjvm.so+0xe723c3]  java_lang_Class::create_mirror(Klass*, Handle, Handle, Handle, Thread*)+0x723
V  [libjvm.so+0x95b8fd]  ClassFileParser::fill_instance_klass(InstanceKlass*, bool, Thread*)+0x1c8d
V  [libjvm.so+0x95c12f]  ClassFileParser::create_instance_klass(bool, Thread*)+0x5f
V  [libjvm.so+0x118e5f5]  KlassFactory::create_from_stream(ClassFileStream*, Symbol*, ClassLoaderData*, Handle, InstanceKlass const*, GrowableArray<Handle>*, Thread*)+0x615
V  [libjvm.so+0x96b76c]  ClassLoader::load_class(Symbol*, bool, Thread*)+0x1bc
V  [libjvm.so+0x171a3e6]  SystemDictionary::load_instance_class(Symbol*, Handle, Thread*)+0x5f6
V  [libjvm.so+0x1718139]  SystemDictionary::resolve_instance_class_or_null(Symbol*, Handle, Handle, Thread*)+0xa69
V  [libjvm.so+0x1718662]  SystemDictionary::resolve_or_null(Symbol*, Handle, Handle, Thread*)+0x42
V  [libjvm.so+0x17187de]  SystemDictionary::resolve_or_fail(Symbol*, Handle, Handle, bool, Thread*)+0x1e
V  [libjvm.so+0x1718991]  SystemDictionary::initialize_wk_klass(SystemDictionary::WKID, int, Thread*)+0x151
V  [libjvm.so+0x1718af1]  SystemDictionary::initialize_wk_klasses_until(SystemDictionary::WKID, SystemDictionary::WKID&, Thread*)+0x61
V  [libjvm.so+0x1718d33]  SystemDictionary::initialize_preloaded_classes(Thread*)+0x153
V  [libjvm.so+0x17190ea]  SystemDictionary::initialize(Thread*)+0x21a
V  [libjvm.so+0x17a8140]  Universe::genesis(Thread*)+0x5f0
V  [libjvm.so+0x17a8b5c]  universe2_init()+0x2c
V  [libjvm.so+0xe1e558]  init_globals()+0xa8
V  [libjvm.so+0x1766d17]  Threads::create_vm(JavaVMInitArgs*, bool*)+0x2d7


Sample code:
/* --- snip ---- */
#include <string.h>
#include <jni.h>
#include <jvmti.h>

static char *serializable_class_name = "java/io/Serializable";
/*
  public interface java.io.Serializable { 
    default Object _$_$_blah() { return this; } 
  }
*/
static jbyte serializable_class_bytes[] = {
	202, 254, 186, 190,   0,   0,   0,  55,   0,  10,   1,   0,  20, 106,  97, 118,
	 97,  47, 105, 111,  47,  83, 101, 114, 105,  97, 108, 105, 122,  97,  98, 108,
	101,   7,   0,   1,   1,   0,  16, 106,  97, 118,  97,  47, 108,  97, 110, 103,
	 47,  79,  98, 106, 101,  99, 116,   7,   0,   3,   1,   0,  17,  83, 101, 114,
	105,  97, 108, 105, 122,  97,  98, 108, 101,  46, 106,  97, 118,  97,   1,   0,
	  9,  95,  36,  95,  36,  95,  98, 108,  97, 104,   1,   0,  20,  40,  41,  76,
	106,  97, 118,  97,  47, 108,  97, 110, 103,  47,  79,  98, 106, 101,  99, 116,
	 59,   1,   0,   4,  67, 111, 100, 101,   1,   0,  10,  83, 111, 117, 114,  99,
	101,  70, 105, 108, 101,   6,   1,   0,   2,   0,   4,   0,   0,   0,   0,   0,
	  1,   0,   1,   0,   6,   0,   7,   0,   1,   0,   8,   0,   0,   0,  14,   0,
	  1,   0,   1,   0,   0,   0,   2,  42, 176,   0,   0,   0,   0,   0,   1,   0,
	  9,   0,   0,   0,   2,   0,   5
};
static jsize serializable_class_bytes_len = 183;

void JNICALL callback_ClassFileLoadHook(jvmtiEnv *jvmti, JNIEnv* env,
	jclass class_being_redefined, jobject loader, const char* name, jobject protection_domain,
	jint class_data_len, const unsigned char* class_data,
	jint* new_class_data_len, unsigned char** new_class_data) {

	if (strcmp(serializable_class_name, name) == 0) {
		*new_class_data_len = serializable_class_bytes_len;
		(*jvmti)->Allocate(jvmti, *new_class_data_len, new_class_data);
		memcpy(*new_class_data, serializable_class_bytes, *new_class_data_len);
	}
}

JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *vm, char *options, void *reserved) {
	jvmtiEnv *jvmti;
	jint rc = (*vm)->GetEnv(vm, (void **)&jvmti, JVMTI_VERSION_9);
	
	jvmtiCapabilities caps;
	memset(&caps, 0, sizeof(caps));

	caps.can_redefine_classes = 1;
	caps.can_generate_all_class_hook_events = 1;
	caps.can_generate_early_vmstart = 1;
	caps.can_generate_early_class_hook_events = 1;

	(*jvmti)->AddCapabilities(jvmti, &caps);

	jvmtiEventCallbacks callbacks;
	memset(&callbacks, 0, sizeof(callbacks));
	callbacks.ClassFileLoadHook = &callback_ClassFileLoadHook;
	(*jvmti)->SetEventCallbacks(jvmti, &callbacks, sizeof(callbacks));
	(*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE, JVMTI_EVENT_CLASS_FILE_LOAD_HOOK, NULL);

	return 0;
}
/* ---- snap --- */
import java.io.Serializable;
import java.util.stream.Stream;

public class Test {
  public static void main(String[] args) {
    Stream.of(Serializable.class.getDeclaredMethods()).forEach(System.out::println);
  }
}
/* ---- snude ---- */