invokespecial-super-init

[John Rose]

The invokespecial-super-init dance is the thing MH's can't quite do, the "super" call every constructor (except Object.<init>).

It very hard to secure this pattern; just ask anybody who has worked on (de-)serialization security.

But, we can look at it from a more limited point of view which might improve your use case, Jochen.

A method handle is supposed to be a fully competent replacement for hardwired bytecodes, and it is, except for invokespecial-super from a constructor.  The reason this is hard is that there is no way to constrain such a method handle, once constructed, to operate inside a constructor.  And superclasses have a right to expect that you are running their constructor as a unique, non-repeatable part of creating a subclass object.  (By "have a right" I really mean "it would be wrong to do the unexpected" by which I also mean "attack surfaces are likely to open up if we do this.)

So, is there a way to package up a method handle so that it can only be used as as unique, non-repeatable part of creating a subclass object?  Yes, it can:  Wire in an unconditional "new instance" operation, and immediately run the "invokespecial super" on the new thing.

Now the problem reduces to:  Your class (just like its super) has a right to expect that constructor code will be run on every newly-created instance (after the super constructor), before the new object is made available to other code.  Can we package up the previous new-invokespecial-super method handle so it can only be used in this way?  Well, no, since every constructor *also* has a hardwired call to invokespecial; we are back to the pre-existing new-invokespecial type of MH.

There are several possible ways out, but the problem is delicate.  The purpose of constructors is to statically mark code that must be executed before any (normally published) reference to an object is reachable by non-class code.  If there were a way to statically mark code as "post-super-init" ("<postsuperinit>"?), we could make an agreement with a class that such a method would serve as the equivalent of a constructor, but it would be the caller's responsibility to allocate the new instance *and* call the super init.  Allowing bytecode to call this stuff would require a bunch of new verifier rules, in a place where the verifier is already hard to understand.  Perhaps a method handle could be allowed to operate where normal bytecode cannot, but you see the problem:  Method handles are designed to give a dynamic alternative to things you can already do in bytecode.

The "post-super-init" convention can be a private convention within a class, in the special case of Groovy, since Groovy is responsible for generating the whole class, and can trust itself to invoke all necessary initialization code on each new instance.  So if you had an new-invokespecial-super MH in a private context within a Groovy-generated class, you could use it to create a "mostly blank" instance, and then fill it in before sharing it with anybody else.  Such an invokespecial-super MH could be adequately protected from other users by requiring that "Lookup.findSpecialConstructor" can only work on full-powered lookups, regardless of the accessibility of the super constructor.

There are two further problems with this, though.  First, constructors have a unique ability and obligation to initialize blank final variables (the non-static ones).  So the Lookup.findSpecialConstructor MH has to take an argument, not just for its super-constructor, but also for *each* final variable in the *current* class.  (Note that Lookup.findSetter will *not* allow finals to be set, because it cannot prove that the caller is somehow "inside" a constructor, and, even if inside it, is trustably acting on behalf of it.)  There are other ways to go, but you can see this problem too:  The new-invokespecial operator has to take responsibility for working with the caller to fill in the blank finals.

The second further problem is even more delicate.  The JVM enforces rules of calling <init> even (sometimes) against the wishes of people who generate class files.  We don't fully understand the practical effects of relaxing these rules.  Proofs of assertions (such as type correctness and security) require strong premises, and the rigid rules about <init> help provide such premises.  An example of a proof-failure would be somebody looking at a class, ensuring that all instances are secure based on the execution of <init> methods, but then fail to notice that the class *also* runs some instances through an alternate path, using new-invokespecial-super, which invalidates the proof by failing to run some crucial setup code.

With all that said, there is still wiggle room.  For example, one *possible* solution that might help Groovy, while being restrictive enough to avoid the problems above, would be to split <init> methods and sew them together again with method handles.

Suppose there were a reliable way to "split" an <init> method into two parts:  Everything up to the invokespecial-super-<init> call, and everything afterwards.  (Perhaps it must be preceded *only* by load-from-local opcodes.)  Call such <init> methods "splittable".  Not all will be splittable.  Then we could consider allowing a class to replace one of its splittable constructors by a new hybrid consisting of a differently-selected super-constructor, followed by the tail of the splittable constructor.  (Note that this neatly handles blank finals.)  It would not be valid for any party other than the sub-class itself to perform such a split, but it might, arguably, be reasonable for a class to do such a thing.

There are always many defects with such schemes.  In this case, there is no robust way to detect that a splittable constructor has in fact been split.  (I keep wanting to invent new bytecodes or verifier rules here!)  Any rule for splittability is going to be a little hacky, hence hard to understand and use correctly.  Specific constructors might be "coupled" strongly to matching super-constructors, in such a way that a mix-and-match will cause surprises, even to the author of the subclass.  (Having stuff happen by invisible magic gets old, as soon as you realize you have to vouch for the behavior of code which you can really only see in source form.)  Finally (as noted above) MHs are quite robustly understanable from the principle that they are "just another way" to do what bytecodes have already done; violating this principle pushes uncertainties into equivalence proofs about MHs and bytecodes.

In the end, I think Groovy may be better off using its ugly <init> bytecode sequence, where every subclass constructor calls (via a switch) every superclass constructor.

I hope this helps, although it's kind of disappointing.  We ran into same dangerous dance, in the Valhalla bytecode interpreter, and had to fake it from random bits of the MH runtime.

— John

On Feb 26, 2015, at 2:27 AM, Jochen Theodorou <blackdrag at gmx.org> wrote:
> 
> Am 26.02.2015 01:02, schrieb Charles Oliver Nutter:
>> After talking with folks at the Jfokus VM Summit, it seems like
>> there's a number of nice-to-have and a few need-to-have features we'd
>> like to see get into java.lang.invoke. Vladimir suggested I start a
>> thread on these features.
> 
> my biggest request: allow the call of a super constructor (like super(foo,bar)) using MethodHandles an have it understood by the JVM like a normal super constructor call... same for this(...)
> 
> Because what we currently do is annoying and a major pita, plus it bloats the bytecode we have to produce. And let us better not talk about speed or the that small verifier change that made our hack unusable in several java update versions for 7 and 8.
> 
> This has been denied in the past because of security reasons... And given that we need dynamic argument types to determine the constructor to be called, and since that we have to do a call from the runtime in the uncached case, I fully understand why this is not done... just... it would be nice to have a solution that does not require us doing basically a big switch table with several invokespecial calls
> 
> bye Jochen
> 
> -- 
> Jochen "blackdrag" Theodorou - Groovy Project Tech Lead
> blog: http://blackdragsview.blogspot.com/
> german groovy discussion newsgroup: de.comp.lang.misc
> For Groovy programming sources visit http://groovy-lang.org
> 
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