Assembly Language for Beginners

4.1. JAVA

Thenewinstruction creates an object, but doesn’t call the constructor (it is called at offset 4).

Theset_a()method is called at offset 16.

Theafield is accessed using thegetstaticinstruction at offset 21.

4.1.17 Simple patching

First example

Let’s proceed with a simple code patching task.

public class nag
{
public static void nag_screen()
{
System.out.println("This program is not registered");
};
public static void main(String[] args)
{
System.out.println("Greetings from the mega-software");
nag_screen();
}
}

How would we remove the printing of “This program is not registered” string?

Let’s load the .class file into IDA:

Figure 4.1:IDA

Let’s patch the first byte of the function to 177 (which is thereturninstruction’s opcode):