Shared memory doesn't break the sandbox. Sharing all memory by default would, but controlled sharing of specific memory doesn't. Think of it like inter-process shared memory, rather than threads sharing an entire address space.
I was wondering why the focus on copying memory between the processes. It should be possible to make capabilities which represent a pointer and a length (and maybe an access mode), which could be used to give a process direct access to shared memory safely. I don't know if that could be done with low enough overhead for small objects, it's possible that there would need to be a threshold below which you would just copy values to be efficient, but it seems like exploring fat pointer capabilities would be worthwhile.
Memory mapping files or inter-process shared memory is the very coarse-grained version of this, using hardware protection. I feel like it should be possible to do something more efficient and finer grained with pointer capabilities and the static type checking that is done to verify WASM during compilation, but it may be a significant research project of its own.
For languages that can express unforgeable pointers as first-class concept, that is indeed a very attractive, fine-grained approach. Unfortunately bringing that to languages like C/C++/Rust is a different matter altogether.
Since we want to support those languages as first-class citizens, we can't require GC support as a base concept, so we have to treat a nanoprocess as the unit of isolation from the outside.
Once we have GC support, nothing will prevent languages that can use it from expressing finer-grained capabilities even within a nanoprocess, and that seems highly desirable indeed.
(full disclosure: I'm a Mozilla employee and one of the people who set up the Bytecode Alliance.)
Forging a pointer is UB in all of these languages as far as I know.
It seems like you should be able to have opaque types that represent these unforgeable pointers which you can't do arithmetic on or cast to raw pointers, but can access values in type safe ways, or provide a view to a byte slice which does bounds check on access.
Is there a good place for discussion of this design? I seem to be having this conversation with you and Josh both here and on Reddit, and it seems like a lot of the discussion is spread out in a lot of places.
Given that most code in Rust is safe code and includes bounds checks before access, you should be able to have the verifier rely on those when they exist, and add in bounds checks in cases in which the access is not protected by a bounds check.
Maybe that would be intractable, or to inefficient to be worth it with all of the extra bounds checks. I'm not sure. I'm asking because it's something that I feel should be possible, but I haven't been involved in the research or development, so I'm wondering if those who have been more involved have references to discussion about the topic.
The semantics of these languages aren’t incompatible with unforgeable references, though: it generally works in practice, but it’s technically undefined to create pointers out of thin air. Why can’t we take advantage of the standard here to disallow illegally created references? (Which, as I understand it, many other vendors are already beginning to do with e.g. pointer authentication and memory tagging.)
I feel like you're overthinking it. Can't you just have a table that holds GCable objects and only hand out indexes to C and co?
It doesn't help with making capabilities more fine-grained, though: we have to treat all code that has access to that table as having the same level of trust.
Anyway, that's great to hear! Any suggestions for discussions and such that one could follow to see how it develops?