Question

I am using ASM in order to do bytecode instrumentation for Java programs. What I’m doing is simple – When instrumenting a method, if the instruction is a PUTFIELD, simply do a DUP_X1 right before the instruction, then visit the PUTFIELD and inject a function call with an argument that includes the DUP’ed stack entry.

                public void visitFieldInsn(
                                int opcode,
                                String owner,  // owner of this field...
                                String name,
                                String desc) {

                    boolean did_dup = false;

                    if(opcode == Opcodes.PUTFIELD) {
                        if(!owner.equals("java/lang/System")) {
                            if (desc.startsWith("L")) {
                                mv.visitInsn(Opcodes.DUP_X1);                                       
                                did_dup = true;
                            }
                        }

                    } 

                    mv.visitFieldInsn(opcode, owner, name, desc);

                    if (did_dup) {
                        mv.visitVarInsn(Opcodes.ALOAD, 0);
                        mv.visitMethodInsn(Opcodes.INVOKESTATIC, "com/amir/ASide", "updateG", "(Ljava/lang/Object;Ljava/lang/Object;)V");
                    }

                 }

The code looks like this BEFORE and AFTER instrumentation, respectively :

public void setA(ClassA classa)
{
    refA = classa;
    eyeColor = classa.eyeColor;
}


public void setA(ClassA classa)
{
    ASide.updateG(refA = classa, this);
    ASide.updateG(eyeColor = classa.eyeColor, this);
}

But when I run the instrumented code, I get :

java.lang.VerifyError: Expecting to find object/array on stack

Can you offer has any help on this?

Answer 1

There is some holes in your instrumentation. For instance, it doesn’t check if you are inside static method, so this variable is present.