Why is Ldvirtftn unverifiable?

Question

Can anyone explain when using an anonymously hosted dynamic method why I get an unverifiable exception by ldvirtftn for a public virtual method on a public class? I set the following assembly level attributes as well:

[assembly: SecurityTransparent]
[assembly: SecurityRules(SecurityRuleSet.Level2,SkipVerificationInFullTrust=true)]

Here is the example code:

public class Program
{
    public virtual void Foo() {}
    public static void Main(string[] args)
    {
        Action<ILGenerator> genfunc = il => il
            .newobj<Program>()
            .ldvirtftn(typeof(Program).GetMethod("Foo"))
            .ret();
        try
        {
            Console.WriteLine(CodeGen.CreateDelegate<Func<IntPtr>>(genfunc).Invoke());

        }
        catch (System.Security.VerificationException) { }
        Console.WriteLine(CodeGen.CreateDelegate<Func<IntPtr>>(genfunc,owner:typeof(Program)).Invoke());
    }
}

If the method is owned, then it does not throw an exception.

Even more curious is that if I change the code like so then both methods compile and run without issue:

public class Program
{
    public virtual void Foo() {}
    public static void Main(string[] args)
    {
        Action<ILGenerator> genfunc = il => il
            .newobj<Program>()
            .dup()
            .ldvirtftn(typeof(Program).GetMethod("Foo"))
            .newobj<Action>(typeof(object),typeof(IntPtr))
            .ret();
        try
        {
            Console.WriteLine(CodeGen.CreateDelegate<Func<Action>>(genfunc).Invoke());
        }
        catch (System.Security.VerificationException) { }
        Console.WriteLine(CodeGen.CreateDelegate<Func<Action>>(genfunc,owner:typeof(Program)).Invoke());
    }
}

This code was written with the reflective library.

CodeGen.CreateDelegate simply uses the type parameter to determine the signature of the dynamic method. Here is the method::

    public static TDelegate CreateDelegate<TDelegate>(
        Action<ILGenerator> genfunc, string name = "", object target = null, Type owner = null, bool skipVisibility = false)
        where TDelegate : class
    {
        var invokeMethod = typeof(TDelegate).GetMethod("Invoke");
        var parameters = invokeMethod.GetParameters();
        var paramTypes = new Type[parameters.Length + 1];
        paramTypes[0] = typeof(object);
        parameters.Select(p => p.ParameterType).ToArray().CopyTo(paramTypes, 1);
        var method = owner != null ?
            new DynamicMethod(name, invokeMethod.ReturnType, paramTypes, owner, skipVisibility) :
            new DynamicMethod(name, invokeMethod.ReturnType, paramTypes, skipVisibility);
        genfunc(method.GetILGenerator());
        return method.CreateDelegate(typeof(TDelegate), target) as TDelegate;
    }

Answer 1

Short answer

The code that you're trying to emit is unverifiable, and anonymously hosted dynamic methods can never contain unverifiable IL. Since dynamic methods associated with a type or module can contain unverifiable IL (subject to appropriate security checks), your code is usable from those dynamic methods.

Mode details

Despite MSDN's documentation, the ldvirtftn does not load a native int onto the stack; it loads a method pointer. Just as treating an object reference as a native int is valid but unverifiable, treating a method pointer as a native int is also valid but unverifiable. The easiest way to see this is to create an assembly on disk with the same IL instructions (e.g. by using System.Reflection.Emit or ilasm) and run PEVerify on it.

I believe that the only verifiable uses of method pointers are:

• Constructing a delegate using the dup; ldvirtftn; newobj or ldftn; newobj patterns to create a new delegate of a compatible delegate type
• Using calli with compatible arguments to make an indirect call through the method pointer

This explains why your other code can be called through an anonymously hosted dynamic method: the delegate creation pattern you are using is one of the few verifiable uses of a method pointer.

Answer 2

ldvirtfn loads a native int onto the stack. I think you need to convert that to IntPtr first before returning.

Answer 3

Strange behaviour (IntPtr != IntPtr):

//work normal
public static void F_Ldvirtftn_Action()
{
  Action<ILGenerator> genfunc = il =>
  {
    il.Emit(OpCodes.Newobj, typeof(Program).GetConstructor(Type.EmptyTypes));
    il.Emit(OpCodes.Dup);
    il.Emit(OpCodes.Ldvirtftn, typeof(Program).GetMethod("Foo2"));
    il.Emit(OpCodes.Newobj, typeof(Action).GetConstructor(new[] { typeof(object), typeof(IntPtr) }));
    il.Emit(OpCodes.Ret);
  };
  Console.WriteLine(CreateDelegate<Func<object>>(genfunc).Invoke());
}
// failed: VerificationException: Operation could destabilize the runtime
public static void F_IntPtr_Action()
{
  Action<ILGenerator> genfunc = il =>
  {
    il.Emit(OpCodes.Newobj, typeof(Program).GetConstructor(Type.EmptyTypes));
    il.Emit(OpCodes.Dup);
    il.Emit(OpCodes.Call, typeof(Program).GetMethod("Ptr"));
    il.Emit(OpCodes.Newobj, typeof(Action).GetConstructor(new[] { typeof(object), typeof(IntPtr) }));
    il.Emit(OpCodes.Ret);
  };
  Console.WriteLine(CreateDelegate<Func<object>>(genfunc).Invoke());
}
// failed: VerificationException: Operation could destabilize the runtime 
public static void F_Ldvirtftn_MyAction()
{
  Action<ILGenerator> genfunc = il =>
  {
    il.Emit(OpCodes.Newobj, typeof(Program).GetConstructor(Type.EmptyTypes));
    il.Emit(OpCodes.Dup);
    il.Emit(OpCodes.Ldvirtftn, typeof(Program).GetMethod("Foo2"));
    il.Emit(OpCodes.Newobj, typeof(MyAction).GetConstructor(new[] { typeof(object), typeof(IntPtr) }));
    il.Emit(OpCodes.Ret);
  };
  Console.WriteLine(CreateDelegate<Func<object>>(genfunc).Invoke());
}
//work normal
public static void F_IntPtr_MyAction()
{
  Action<ILGenerator> genfunc = il =>
  {
    il.Emit(OpCodes.Newobj, typeof(Program).GetConstructor(Type.EmptyTypes));
    il.Emit(OpCodes.Dup);
    il.Emit(OpCodes.Call, typeof(Program).GetMethod("Ptr"));
    il.Emit(OpCodes.Newobj, typeof(MyAction).GetConstructor(new[] { typeof(object), typeof(IntPtr) }));
    il.Emit(OpCodes.Ret);
  };
  Console.WriteLine(CreateDelegate<Func<object>>(genfunc).Invoke());
}

public static IntPtr Ptr(object z)
{
  return IntPtr.Zero;
}
public class MyAction
{
  public MyAction(object z, IntPtr adr) { }
}