Can Code be Protected From Rogue Callers In Ada?

Question

I'm a fairly new Ada programmer. I have read the book by Barnes (twice I might add) and even managed to write a fair terminal program in Ada. My main language is C++ though.

I am currently wondering if there is a way to "protect" subroutine calls in Ada, perhaps in Ada 2012 (of which I know basically nothing). Let me explain what I mean (although in C++ terms).

Suppose you have a class Secret like this:

class Secret
{
    private:
    int secret_int;

    public:
    Set_Secret_Value( int i );
}

Now this is the usual stuff, dont expose secret_int, manipulate it only through access functions. However, the problem is that anybody with access to an object of type Secret can manipulate the value, whether that particular code section is supposed to do it or not. So the danger of rogue altering of secret_int has been reduced to anybody altering secret_int through the permitted functions, even if it happens in a code section that's not supposed to manipulate it.

To remedy that I came up with the following construct

class Secret
{
    friend class Secret_Interface;

    private:
    int secret_int;
    Set_Secret_Value( int i );
    Super_Secret_Function();
};

class Secret_Interface
{
    friend class Client;
    private:

    static Set_Secret_Value( Secret &rc_secret_object, int i )
    {
         rc_secret_object.Set_Secret( i );
    }
};


class Client
{
     Some_Function()
     {
         ...
         Secret_Interface::Set_Secret_Value( c_object, some-value );
         ...
     }
}

Now the class Secret_Interface can determine which other classes can use it's private functions and by doing so, indirectly, the functions of class Secret that are exposed to Secret_Interface. This way class Secret still has private functions that can not be called by anybody outside the class, for instance function Super_Secret_Function().

Well I was wondering if anything of this sort is possible in Ada. Basically my desire is to be able to say:

    Code A may only be executed by code B but not by anybody else

Thanks for any help.

Edit: I add a diagram here with a program structure like I have in mind that shows that what I mean here is a transport of a data structure across a wide area of the software, definition, creation and use of a record should happen in code sections that are otherwise unrleated

Answer 1

You could do this by using child packages:

package Hidden is

private

   A : Integer;
   B : Integer;

end Hidden;

and then

package Hidden.Client_A_View is

   function Get_A return Integer;
   procedure Set_A (To : Integer);

end Hidden.Client_A_View;

Then, Client_A can write

with Hidden.Client_A_View;
procedure Client_A is
   Tmp : Integer;
begin
   Tmp := Hidden.Client_A_View.Get_A;
   Hidden.Client_A_View.Set_A (Tmp + 1);
end Client_A;

Answer 2

I think the key is to realize that, unlike C++ and other languages, Ada's primary top-level unit is the package, and visibility control (i.e. public vs. private) is on a per-package basis, not a per-type (or per-class) basis. I'm not sure I'm saying that correctly, but hopefully things will be explained below.

One of the main purposes of friend in C++ is so that you can write two (or more) closely related classes that both take part in implementing one concept. In that case, it makes sense that the code in one class would be able to have more direct access to the code in another class, since they're working together. I assume that in your C++ example, Secret and Client have that kind of close relationship. If I understand C++ correctly, they do all have to be defined in the same source file; if you say friend class Client, then the Client class has to be defined somewhere later in the same source file (and it can't be defined earlier, because at that point the methods in Secret or Secret_Interface haven't yet been declared).

In Ada, you can simply define the types in the same package.

package P is

    type Secret is tagged private;
    type Client is tagged private;

    -- define public operations for both types 

private

    type Secret is tagged record ... end record;
    type Client is tagged record ... end record;

    -- define private operations for either or both types

end P;

Now, the body of P will contain the actual code for the public and private operations of both types. All code in the package body of P has access to those things defined in P's private part, regardless of which type they operate on. And, in fact, all code has access to the full definitions of both types. This means that a procedure that operates on a Client can call a private operation that operates on a Secret, and in fact it can read and write a Secret's record components directly. (And vice versa.) This may seem bizarre to programmers used to the class paradigm used by most other OOP languages, but it works fine in Ada. (In fact, if you don't need Secret to be accessible to anything else besides the implementation of Client, the type and its operations can be defined in the private part of P, or the package body.) This arrangement doesn't violate the principles behind OOP (encapsulation, information hiding), as long as the two types are truly two pieces of the implementation of one coherent concept.

If that isn't what you want, i.e. if Secret and Client aren't that closely related, then I would need to see a larger example to find out just what kind of use case you're trying to implement.

MORE THOUGHTS: After looking over your diagram, I think that the way you're trying to solve the problem is inferior design--an anti-pattern, if you will. When you write a "module" (whatever that means--a class or package, or in some cases two or more closely related classes or packages cooperating with each other), the module defines how other modules may use it--what public operations it provides on its objects, and what those operations do.

But the module (let's call it M1) should work the same way, according to its contract, regardless of what other module calls it, and how. M1 will get a sequence of "messages" instructing it to perform certain tasks or return certain information; M1 should not care where those messages are coming from. In particular, M1 should not be making decisions about the structure of the clients that use it. By having M1 decree that "procedure XYZ can only be called from package ABC", M1 is imposing structural requirements on the clients that use it. This, I believe, causes M1 to be too tightly coupled to the rest of the program. It is not good design.

However, it may make sense for the module that uses M1 to exercise some sort of control like that, internally. Suppose we have a "module" M2 that actually uses a number of packages as part of its implementation. The "main" package in M2 (the one that clients of M2 use to get M2 to perform its task) uses M1 to create a new object, and then passes that object to several other packages that do the work. It seems like a reasonable design goal to find a way that M2 could pass that object to some packages or subprograms without giving them the ability to, say, update the object, but pass it to other packages or subprograms that would have that ability.

There are some solutions that would protect against most accidents. For example:

package M1 is

    type Secret is tagged private;
    procedure Harmless_Operation (X : in out Secret);

    type Secret_With_Updater is new Secret with null record;
    procedure Dangerous_Operation (X : in out Secret_With_Updater);

end M1;

Now, the packages that could take a "Secret" object but should not have the ability to update it would have procedures defined with Secret'Class parameters. M2 would create a Secret_With_Updater object; since this object type is in Secret'Class, it could be passed as a parameter to procedures with Secret'Class parameters. However, those procedures would not be able to call Dangerous_Operation on their parameters; that would not compile.

A package with a Secret'Class parameter could still call the dangerous operation with a type conversion:

procedure P (X : in out Secret'Class) is
begin
    -- ...
    M1.Secret_With_Updater(X).Dangerous_Operation;
    -- ...
end P;

The language can't prevent this, because it can't make Secret_With_Updater visible to some packages but not others (without using a child package hierarchy). But it would be harder to do this accidentally. If you really wish to go further and prevent even this (if you think there will be a programmer whose understanding of good design principles is so poor that they'd be willing to write code like this), then you could go a little further:

package M1 is

    type Secret is tagged private;
    procedure Harmless_Operation (X : in out Secret);
    type Secret_Acc is access all Secret;

    type Secret_With_Updater is tagged private;
    function Get_Secret (X : Secret_With_Updater) return Secret_Acc;
        -- this will be "return X.S"
    procedure Dangerous_Operation (X : in out Secret_With_Updater);

private
    -- ...
    type Secret_With_Updater is tagged record
        S : Secret_Acc;
    end record;
    -- ...
end M1;

Then, to create a Secret, M2 would call something that creates a Secret_With_Updater that returns a record with an access to a Secret. It would then pass X.Get_Secret to those procedures which would not be allowed to call Dangerous_Operation, but X itself to those that would be allowed. (You might also be able to declare S : aliased Secret, declare Get_Secret to return access Secret, and implement it with return X.S'access. This may avoid a potential memory leak, but it may also run into accessibility-check issues. I haven't tried this.)

Anyway, perhaps some of these ideas could help accomplish what you want to accomplish without introducing unnecessary coupling by forcing M1 to know about the structure of the application that uses it. It's hard to tell because your description of the problem, even with the diagram, is still at too abstract a level for me to see what you really want to do.

Answer 3

Your question is extremely unclear (and all the C++ code doesn't help explaining what you need), but if your point is that you want a type to have some publicly accessible operations, and some private operations, then it is easily done:

package Example is
   type Instance is private;

   procedure Public_Operation (Item : in out Instance);
private
   procedure Private_Operation (Item : in out Instance);

   type Instance is ... -- whatever you need it to be
end Example;

The procedure Example.Private_Operation is accessible to children of Example. If you want an operation to be purely internal, you declare it only in the package body:

package body Example is
   procedure Internal_Operation (Item : in out Instance);

   ...
end Example;

Answer 4

Well I was wondering if anything of this sort is possible in Ada. Basically my desire is to be able to say:

    Code A may only be executed by code B but not by anybody else

If limited to language features, no.

Programmatically, code execution can be protected if the provider must be provided an approved "key" to allow execution of its services, and only authorized clients are supplied with such keys.

Devising the nature, generation, and security of such keys is left as an exercise for the reader.