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https://github.com/kyren/piccolo
An experimental stackless Lua VM implemented in pure Rust
https://github.com/kyren/piccolo
Last synced: 26 days ago
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An experimental stackless Lua VM implemented in pure Rust
- Host: GitHub
- URL: https://github.com/kyren/piccolo
- Owner: kyren
- License: cc0-1.0
- Created: 2018-04-05T04:11:40.000Z (over 6 years ago)
- Default Branch: master
- Last Pushed: 2024-05-16T00:28:21.000Z (6 months ago)
- Last Synced: 2024-05-18T23:19:29.025Z (6 months ago)
- Language: Rust
- Homepage:
- Size: 2.44 MB
- Stars: 1,471
- Watchers: 45
- Forks: 53
- Open Issues: 15
-
Metadata Files:
- Readme: README.md
- Changelog: CHANGELOG.md
- License: LICENSE-CC0
Awesome Lists containing this project
README
[](https://crates.io/crates/piccolo)
[](https://docs.rs/piccolo)
[](https://circleci.com/gh/kyren/piccolo)
[](https://discord.gg/CSJCVTvgNB)
## piccolo - An experimental stackless Lua VM implemented in pure Rust
**(After *four* years, now UN-paused!)**
Project Goals, in roughly descending priority:
* Be an arguably working, useful Lua interpreter.
* Be an easy way to *confidently* sandbox untrusted Lua scripts.
* Be resilient against DoS from untrusted scripts (scripts should not be able
to cause the interpreter to panic or use an unbounded amount of memory and
should be guaranteed to return control to the caller in some bounded amount
of time).
* Be an easy way to bind Rust APIs to Lua safely, with a bindings system that is
resilient against weirdness and edge cases, and with user types that can
safely participate in runtime garbage collection.
* Be pragmatically compatible with some version(s) of PUC-Rio Lua.
* Don't be obnoxiously slow (for example, avoid abstractions that would make
the interpreter fundamentally slower than PUC-Rio Lua).
You read more about the design of `piccolo` (and try it out a live REPL!) in
[this blog post](https://kyju.org/blog/piccolo-a-stackless-lua-interpreter/).
## API Instability
Expect *frequent* pre-1.0 API breakage, this crate is still very experimental.
All API incompatible changes will be accompanied by minor version bumps, but
these will be very common.
## Safety
The goal with `piccolo` is to have the majority of it written in safe Rust.
Currently, there are a few sources of unsafety, but crucially these sources
of unsafety are *isolated*. `piccolo` will avoid at all costs relying on
abstractions which *leak* unsafety, it should always be possible to interact
with even low level details of `piccolo` without using `unsafe`.
The current primary sources of unsafety:
* The particularly weird requirements of Lua tables require using hashbrown's
low level RawTable API.
* Userdata requires unsafety to allow for downcasting non-'static userdata
with a safe interface.
* The implementation of async `Sequence`s require unsafety to "tunnel" the
normal `Sequence` method parameters into the future (this is completely
hidden from the user behind a safe interface).
* Unsafe code is required to avoid fat pointers in several Lua types, to keep
`Value` as small as possible and allow potential future smaller `Value`
representations.
*(`piccolo` makes no attempt yet to guard against side channel attacks like
spectre, so even if the VM is memory safe, running untrusted scripts may carry
additional risk. With no JIT or callback API to accurately measure time, this
might be practically impossible anwyay.)*
## A unique system for Rust <-> GC interaction
*The garbage collector system for `piccolo` is now in its [own repo](
https://github.com/kyren/gc-arena), and also on crates.io. See the README in the
linked repo for more detail about the GC design.*
`piccolo` has a real, cycle detecting, incremental garbage collector with
zero-cost `Gc` pointers (they are machine pointer sized and implement `Copy`)
that are usable from safe Rust. It achieves this by combining two things:
1) An unsafe `Collect` trait which allows tracing through garbage collected
types that, despite being unsafe, can be implemented safely using procedural
macros.
2) Branding `Gc` pointers by unique, invariant "generative" lifetimes to ensure
that such pointers are isolated to a single root object, and to guarantee
that, outside an active call to `mutate`, all such pointers are either
reachable from the root object or are safe to collect.
## Stackless VM
The `mutate` based GC API means that long running calls to `mutate` can be
problematic. No garbage collection can take place during a call to `mutate`, so
we have to make sure to regularly return from the `mutate` call to allow garbage
collection to take place.
The VM in `piccolo` is thus written in what is sometimes called "stackless"
or "trampoline" style. It does not rely on the rust stack for Lua -> Rust and
Rust -> Lua nesting, instead callbacks can either have some kind of immediate
result (return values, yield values from a coroutine, resume a thread, error),
or they can produce a `Sequence`. A `Sequence` is a bit like a `Future` in
that it is a multi-step operation that the parent `Executor` will drive to
completion. `Executor` will repeatedly call `Sequence::poll` until the sequence
is complete, and the `Sequence` can yield values and call arbitrary Lua
functions while it is being polled.
As an example, it is of course possible for Lua to call a Rust callback, which
then in turn creates a new Lua coroutine and runs it. In order to do so, a
callback would take a Lua function as a parameter, then create a new coroutine
`Thread` from it and return `SequencePoll:Resume` to run it. The outer main
`Executor` will run the created `Thread`, and when it is finished it will
"return" via `Sequence::poll` (or `Sequence::error`). This is exactly how the
`coroutine.resume` Lua stdlib function is implemented.
As another example, `pcall` is easy to implement here, a callback can call the
provided function with a `Sequence` underneath it, and the sequence can catch
the error and return the error status.
Yet another example, imagine Rust code calling a Lua coroutine thread which
calls a Rust `Sequence` which calls yet more Lua code which then yields. Our
stack will look something like this:
```
[Rust] -> [Lua Coroutine] -> [Rust Sequence] -> [Lua code that yields]
```
This is no problem with this VM style, the inner Rust callback is paused as a
`Sequence`, and the inner yield will return the value all the way to the top
level Rust code. When the coroutine thread is resumed and eventually returns,
the Rust `Sequence` will be resumed.
With any number of nested Lua threads and `Sequence`s, control will always
continuously return outside the GC arena and to the outer Rust code driving
everything. This is the "trampoline" here, when using this interpreter,
somewhere there is a loop that is continuously calling `Arena::mutate` and
`Executor::step`, and it can stop or pause or change tasks at any time, not
requiring unwinding the Rust stack.
This "stackless" style has many benefits, it allows for concurrency patterns
that are difficult in some other VMs (like tasklets), and makes the VM much more
resilient against untrusted script DoS.
## Async `Sequence`s
The downside of the "stackless" style is that writing things as a `Sequence`
implementation is much more difficult than writing in normal, straight control
flow. This is identical to the problem Rust had before proper `async` support,
where it required implementing `Future` manually or using difficult to use
combinators. Ideally, if we could somehow implement `Collect` for the generated
state machine for a rust `async` block, then we could use rust `async` (or more
directly, unstable Rust coroutines) to implement our `Sequence` state machines.
Unfortunately, implementing a trait like this for a Rust async (coroutine) state
machine is not currently possible. HOWEVER, `piccolo` is currently still able to
provide a safe way to implement `Sequence` using async blocks by using a clever
trick: a shadow stack.
The `async_sequence` function can create a `Sequence` impl from an `async`
block, and the generated `Future` tells the outer sequence what actions to
take on its behalf. Since the Rust future cannot (safely) hold GC pointers
(since it cannot possibly implement `Collect` in today's Rust), we instead
allow it to hold proxy "stashed" values, and these "stashed" values point to
a "shadow stack" held inside the outer sequence which allows them to be traced
and collected properly! We provide a `Locals` object inside async sequences
and this is the future's "shadow stack"; it can be used to stash / fetch any
GC value and any values stashed using this object are treated as owned by the
outer `Sequence`. In this way, we end up with a Rust future that can store GC
values safely, both in the sense of being sound and not leading to dangling
`Gc` pointers, but also in a way that cannot possibly lead to things like
uncollectable cycles. It is slightly more inconvenient than if Rust async blocks
could implement `Collect` directly (it requires entering and exiting the GC
context manually and stashing / unstashing GC values), but it is MUCH easier
than manually implementing a custom `Sequence` state machine!
Using this, it is easy to write very complex Rust callbacks that can themselves
call into Lua or resume threads or yield values back to Lua (or simply return
control to the outermost Rust code), while also maintaining complex internal
state. In addition, these running callbacks are themselves *proper* garbage
collected values, and all of the GC values they hold will be collected if they
are (for example) forgotten as part of a suspended Lua coroutine. Without async
sequences, this would require writing complex state machines by hand, so this is
*critical* for very complex uses of `piccolo`.
## Executor "fuel" and VM memory tracking
The stackless VM style "periodically" returns control to the outer Rust code
driving everything, and how often this happens can be controlled using the
"fuel" system.
Lua and Lua driven callback code *always* happens within some call to
`Executor::step`. This method takes a `fuel` parameter which controls how long
the VM should run before pausing, with fuel measured (roughly) in units of VM
instructions.
Different amounts of fuel provided to `Executor::step` bound the amount of Lua
execution that can occur, bounding both the CPU time used and also the amount of
memory allocation that can occur within a single `Executor::step` call (assuming
certain rules are followed w.r.t. provided callbacks).
The VM also now accurately tracks all memory allocated within its inner
`gc-arena::Arena` using `gc-arena` memory tracking features. This can extend
to userdata and userdata APIs, and assuming the correct rules are follwed in
exposed userdata and callbacks, allows for accurate memory reporting and memory
limits.
*Assuming* that both of these mechanisms work correctly, and *assuming* that all
callback / userdata APIs also follow the same rules, this allows for completely
sandboxing untrusted scripts not only in memory safety and API access but also
in CPU and RAM usage. These are big assumptions though, and `piccolo` is still
very much WIP, so ensuring this is done correctly is an ongoing effort.
## What currently works
* An actual cycle detecting, incremental GC similar to the incremental collector
in PUC-Rio Lua 5.3 / 5.4
* Lua source code is compiled to a VM bytecode similar to PUC-Rio Lua's, and
there are a complete set of VM instructions implemented
* Almost all of the core Lua language works. Some tricky Lua features that
currently actually work:
* Real closures with proper upvalue handling
* Proper tail calls
* Variable arguments and returns and generally proper vararg (`...`) handling
* Coroutines, including yielding that is transparent to Rust callbacks
* Gotos with label handling that matches Lua 5.3 / 5.4
* Proper _ENV handling
* Metatables and metamethods, including fully recursive metamethods that
trigger other metamethods (Not every metamethod is implemented yet,
particularly `__gc` finalizers).
* A robust Rust callback system with sequencing callbacks that don't block
the interpreter and allow calling into and returning from Lua without using
the Rust stack, and a way to integrate Rust async so that implementing these
callbacks is not wildly painful.
* Garbage collected "userdata" with safe downcasting.
* Some of the stdlib (almost all of the core, fundamental parts of the stdlib
are implemented, e.g. things like the `coroutine` library, `pcall`, `error`,
most everything that exposes some fundamental runtime feature is implemented).
* A simple REPL (try it with `cargo run --example interpreter`)
## What currently doesn't work
* A large amount of the stdlib is not implemented yet. Most "peripheral" parts
of the stdlib are this way, the `io`, `file`, `os`, `package`, `string`,
`table`, and `utf8` libs are either missing or very sparsely implemented.
* There is no support yet for finalization. `gc-arena` supports finalization in
such a way now that it should be possible to implement `__gc` metamethods with
resurrection and tables with weak keys / values and ephemeron tables fully,
but it has not been done yet. Currently, the `__gc` metamethod has no effect.
* The compiled VM code is in a couple of ways worse than what PUC-Rio Lua will
generate. Notably, there is a JMP chaining optimization that is not yet
implemented that makes most loops much slower than in PUC-Rio Lua.
* Error messages that don't make you want to cry
* Stack traces
* Debugger
* Aggressive optimization and *real* effort towards matching or beating (or
even just being within a respectable distance of) PUC-Rio Lua's performance in
all cases.
* Probably much more I've forgotten about
## What will probably never be implemented
This is not an exhaustive list, but these are some things which I currently
consider *almost definite* non-goals.
* An API compatible with the PUC-Rio Lua C API. It would be amazingly difficult
to implement and would be very slow, and some of it would be basically
impossible (longjmp error handling and adjacent behavior).
* Perfect compatibility with certain classes of behavior in PUC-Rio Lua:
* PUC-Rio Lua behaves differently on systems depending on the OS, environment,
compilation settings, system locale, etc. (In certain versions of PUC-Rio Lua,
even the behavior of the *lexer* changes depending on the system locale!)
`piccolo` is more or less aiming to emulate PUC-Rio Lua behavior with the
"C" locale set with the default settings in `luaconf.h` on 64-bit Linux.
* The specific format of error messages.
* The specific iteration order of tables, and the specific behavior of the
length operator (the length operator currently functions correctly and will
always return a table "border", but for tables that are not sequences,
the choice of border that is returned may differ).
* The `debug` library is unimplemented and much of it will probably never be
implemented due to fundamental VM differences.
* Compatibility with PUC-Rio Lua bytecode
* `os.setlocale` and other weirdness inherited from C
* `package.loadlib` and all functionality which allows loading C libraries.
* Perfectly matching all of the (sometimes quite exotic) garbage collector
corner case behavior in PUC-Rio Lua.
## Why is it called 'piccolo'?
It's a cute little "pico" Lua, get it?
It's not really all that "pico" anymore, but it's still a cute little instrument
you can safely carry with you anywhere!
## Wasn't this project called something else? Luster? Deimos?
There was an embarassing naming kerfluffle where I somehow ended up with other
people's project names *twice*. They're all the same project. I promise I'm done
renaming it.
## License
`piccolo` is licensed under either of:
* MIT license [LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT
* Creative Commons CC0 1.0 Universal Public Domain Dedication
[LICENSE-CC0](LICENSE-CC0) or
https://creativecommons.org/publicdomain/zero/1.0/
at your option.