It's not too late for access control

[David M. Lloyd]

On 07/12/2016 04:00 AM, Remi Forax wrote:
> ----- Mail original -----
>> De: "David M. Lloyd" <david.lloyd at redhat.com>
>> À: "jigsaw-dev" <jigsaw-dev at openjdk.java.net>
>> Envoyé: Lundi 11 Juillet 2016 16:21:46
>> Objet: It's not too late for access control
>>
>> The crux of this access control discussion is that, up until JDK 9,
>> "public" meant "public".  End of story.  If you did not want something
>> to be visible, you made it not public.  Very simple and very clear.  The
>> word "public" literally means "accessible to all" after all; that's why
>> the term was selected in the first place, and it is nearly certain that
>> this was the intent of the thing up to this date.
>>
>> What is being required in the JPMS spec, and, I dare to suggest, what
>> has been generally asked for by the public (and even required by the
>> JDK), is a way to provide an additional capability - the ability to
>> selectively share otherwise unshared code.
>>
>> The existing Java language accessibility model is (of course)
>> well-understood by experts.  For new developers, the concepts takes a
>> bit of time to explain but can generally be grasped.  In the end the
>> accessibility of a member is generally easily determined by examining
>> the qualifiers of that member.
>>
>> What has been proposed and implemented in Jigsaw is essentially a
>> completely new approach to access checking.  Because it is new, and
>> because it is essentially untried, I predicted that issues would arise
>> not unlike the ones being currently discussed.  I argue that this
>> approach is not optimal, for at least reasons discussed on this list and
>> in this email, but that even now, it's not too late to change the approach.
>>
>> I propose, once again, that rather than changing the meaning of "public"
>> to something unintuitive (and indeed counter to the definition of the
>> actual word), we instead allow the selective extension of
>> package-private.  Users would make public any type or member which is
>> *meant* to be public, i.e. accessible by all.  Rather than (at best)
>> changing their expectations as to the behavior of "public" only to
>> immediately betray that expectation by forcing them through a backdoor
>> in order to meet practical needs, we ensure that their expectations
>> remain: public members are public, and things that are secret are not
>> public.
>>
>> Conceptually (and, hopefully, technically) this should not be too far
>> away from where we've arrived at now in Jigsaw, at least as far as the
>> package inventory is shared between modules for the purpose of access
>> control.  Can anyone think of any good reason we should *not* do this,
>> or ways that this would be substantially weaker than restricting public?
>> --
>> - DML
>>
>
> Hi David,
> package private means package private :)
>
> One early design idea of jigsaw was to introduce a new modifier "module" with a visibility in between public and package private.
> It's a bad idea !

I am not suggesting that; we definitely should not do that for reasons 
discussed in the past.

> First, there is already in the Java ecosystem a notion of non-exported package, packages startings with com.sun or packages containing internal, it was just a convention and not something enforced by the VM. What the JPMS spec does is just to normalize how to declare an exported package and mandate that the VM check this new rule.
>
> Sure it means that public classes are not accessible/visible by everybody anymore, but a class like sun.misc.Unsafe was never really accessible by everybody despite being declared public.

The key advantage of using package-private is that you can drop the 
access of shared internal classes out of public, and just make that 
package accessible directly to its internal consumers.  From the JDK 
perspective: imagine getting rid of all those "shared secret" classes, 
for example.  In fact many JDK internal classes and APIs could just be 
hidden, closing innumerable security holes.

For many, many existing projects, this approach promises exact 
compatibility with the status quo in terms of accessibility, while also 
opening an avenue forward to improve module security.

> Furthermore, declaring if something is exported or not at class level instead of at package level seems wrong to me, usually, several classes works together for a purpose and you want these classes to be exported or not, so it's not something that should be decided at class level.

I did not propose deciding at a class level; I think that would be a bad 
idea.

> So i see the JPMS spec conept of non-exported package as a standardization of an existing practice not something new that people will have trouble to understand and reason about.

Except that there is no existing practice here.  Today, if I can access 
a Class or a ClassLoader, I can access all its public members, and this 
is the crux of the whole thing.  There is no valid compatibility-based 
argument for changing this behavior.  The idea is to take the status 
quo, and add a new capability that can optionally be utilized for better 
security, rather than to break the status quo, call it "compatible", and 
completely break many, many users instantly.

-- 
- DML