[Jamvm-general] Illegal classes which should cause verification errors do not in JamVM.

[Steven G. <st...@mu...>]

Hello,

JamVM is able to use classes which are malformed and violate the
specification of the JVM.

Take the given class:

    public class Bar
    {
        public static void main(String... __args)
        {
            XclinitX();
        }
        
        static void XclinitX()
        {
            System.err.println(new Bar());
        }
    }

Compile it and modify it with a hex editor so that `XclinitX` becomes
`<clinit>`. Zero prints the following:

    Exception in thread "main" java.lang.ClassFormatError: Bad method
    name at constant pool index 14 in class file Bar

While JamVM prints this:

    Bar@97a2aa08
    Bar@97a2b290

According to the virtual machine specification, one cannot
`invokestatic` a static initializer.

> Class and interface initialization methods are invoked
> implicitly by the Java Virtual Machine; they are never invoked
> directly from any Java Virtual Machine instruction, but are invoked
> only indirectly as part of the class initialization process.

Another issue with the following class:

    public class Bar
    {
        public static void main(String... __args)
        {
        }
        
        public synchronized void XinitX(int __i)
        {
            System.err.println(__i);
        }
    }

Modify the compiled class in the hex editor so that `XinitX` becomes
`<init>`. Zero prints:

    Exception in thread "main" java.lang.ClassFormatError: Method <init>
    in class Bar has illegal modifiers: 0x21

JamVM prints nothing. However the class should not load because the
initializer of an instance cannot be synchronized (along with some other
flags).

This can also be seen with:

    public class Bar
    {
        public Bar()
        {
            System.err.println("Created");
        }
        
        public void XinitX(int __v)
        {
            System.err.println(__v);
        }
        
        public void hello()
        {
            System.err.println("Hello");
        }
        
        public static void main(String... __args)
        {
            Bar b = new Bar();
            b.XinitX(2);
            b.hello();
        }
    }

Hex edit `XinitX` to become `<init>`. JamVM prints:

    Created
    Hello

while Zero fails to load the class with:

    Exception in thread "main" java.lang.VerifyError: Constructor must
    call super() or this() before return