Modifying Java Methods/Bytecode in Runtime with C++

Question

me and a friend of mine are working on a coding project and we ran into a little issue. We are essentially trying to modify Java Methods/Bytecode during runtime using C++. I am just curious if this is possible or how it is possible. We have already tried using JVMTI and JNI to accomplish this but it seems that you can only modify classes that are currently being loaded into the JVM. We are loading our code in as a DLL while the process is running so we can not do this. We also thought assembly might work by accomplishing this. A little demonstration of what we are trying to do can be seen below.

public boolean Method()
{

     boolean value = true;


     return value; //<- Value that we want to modify (in java)


}


jclass JVMUtilities::getClass(std::string clazzname) {
     jstring name = Wrapper::getJVMHook().getJNI()->NewStringUTF(clazzname.c_str());
     jobject classLoader = getclassloader();
     jmethodID mid = Wrapper::getJVMHook().getJNI()->GetMethodID(Wrapper::getJVMHook().getJNI()->GetObjectClass(classLoader), "findClass", "(Ljava/lang/String;)Ljava/lang/Class;");
     return (jclass)Wrapper::getJVMHook().getJNI()->CallObjectMethod(classLoader, mid, name);
     //This is the function we use for geting the class information in c++ including location
}

Basically going by above, we want to change the return value to what we want during run time by manipulating the byte code.

Out of curiosity, how would the C++ function know the location (address) of the Byte Code to modify? How would you pass a pointer to this location? — Thomas Matthews, Sep 29 '17 at 00:51
I added the code we use for getting the class information in C++. — BigLenny, Sep 29 '17 at 01:51

score 0 · Answer 1 · answered Sep 29 '17 at 06:32

0

There is a public standard API to do this - JVM TI RedefineClasses and RetransformClasses.

answered Sep 29 '17 at 06:32

apangin

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Thanks for the help, however something I noticed when I try to use the RetransformClasses method is it often results in random crashes and glitches in the program. Do you think this same thing would occur when using RedefineClasses? – BigLenny Sep 29 '17 at 21:27
@BigLenny This is not a problem of RetransformClasses, but the way it is used. Of course, incorrect usage of RedefineClasses may also lead do bugs. Use `-Xverify:all` option to make sure the transformed classes are valid. – apangin Sep 29 '17 at 23:42
Thanks for the quick response I actually used the RetransformClasses to trigger the ClassFileLoadHook event in C++. Now the only thing I need to figure out is how I can modify the raw bytecode. I am able to get it fine with ClassFileLoadHook but I need a way to modify it. – BigLenny Sep 30 '17 at 03:21
@BigLenny [ClassFileLoadHook](http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.html#ClassFileLoadHook) has `new_class_data` argument that serves exactly for replacing bytecode. – apangin Sep 30 '17 at 06:57

Modifying Java Methods/Bytecode in Runtime with C++

1 Answers1