Few cents about my commits

Fix: java.lang.IllegalAccessError: Attempt to access protected method from Lambda

18 Apr 2022 | fix debugger ios15 dyld

Issue mentioned in report #641 Code is quite simple to reproduce:

package a;
public class A {
    protected void foo(){};
}

package b;
import a.A;
public class B extends A {
    public void test() {
        Runnable r = this::foo;
        r.run();
    }
}

Root case

Is a way how RoboVM generate lambda class for this case, it looks as bellow:

package b;

public class B$lambda$1 implements Runnable{
    private final B $arg1;

    public B$lambda$1(B $arg1) {
        this.$arg1 = $arg1;
    }

    @Override
    public void run() {
        $arg1.foo();
    }
}

// and caller class 
public class B extends A {
    public void test() {
       Runnable r = new B$lambda$1(this); // was Runnable r = this::foo;
       r.run();
    }
}

and generated $arg1.foo(); will produce IllegalAccessError as foo() has protected access in A.

The fix

To work around this – lets make same trick as its done to provide access to private fields from lambdas. Lets create a trampoline method in B that will call method handle for this lambda and generate lambda code to call this trampoline. Method handle has signature:

implMethod = “InvokeVirtual(<a.A: void foo()>)”

This to be turned into following trampoline:

package b;

public class B$lambda$1 implements Runnable{
    private final B $arg1;

    public B$lambda$1(B $arg1) {
        this.$arg1 = $arg1;
    }


    @Override
    public void run() {
        B.trampoline$lambda$1($arg);
    }
}

// and caller class 
public class B extends A {
    public void test() {
        Runnable r = new B_lambda_1(this); // was Runnable r = this::foo;
        r.run();
    }

    private static void trampoline$lambda$1(A arg) {
        arg.foo();
    }
}

But trampoline will not work this was as protected foo method of receiver type A is not accessible from package B.

Lambda is created by LambdaMetafactory and receiver object is passed as first parameter of invokeType Callsite’s signature. And it will match calling class, not actual class where member is located. The hack is: when building trampoline, use first argument type from invokeType parameter list and rest from MethodHandle provided implementation. This sample trampoline with all fixes:

  private static void trampoline$lambda$1(B arg) {
    arg.foo();
  }

Code

The fix was proposed as PR645

Comments