Memory unsafety in Stak Scheme virtual machines

@raviqqe

June 7, 2025

Contents

• Stak Scheme
• Progress
  • Library system in eval procedure
  • case-lambda syntax
• Memory unsafety in virtual machines
• Future work

Stak Scheme

• A bytecode compiler and virtual machine (VM) for Scheme
  • The compiler is written in Scheme.
  • The VM is written in Rust.
• It aims to support the R7RS-small standard.
• Forked from Ribbit Scheme

References

• GitHub
• Website

Progress

• Library system in eval procedure
• case-lambda syntax

Library system in eval procedure

• Stak Scheme now supports the define-library syntax in the eval procedure.
• The eval procedure creates a new library environment for a given library definition.
• It also allows later calls to the eval procedure to import the defined libraries.
• It is implemented by sharing logic of the library system in the bytecode compiler.
  • Compiler inception again :)

VM memory unsafety

What is missing in Stak Scheme virtual machine?

• Ribbit Scheme's design achieves simplicity, performance, portability, and extensibility at the same time.
  • A Compact and Extensible Portable Scheme VM
  • Stak Scheme adopts the same design and architecture of the language processor.
• What is missing??
  • Security!

VM memory unsafety in Stak Scheme

• The host language of Rust is memory safe.
• So the virtual machine of Stak Scheme is memory safe.
• However, Scheme programs are not memory safe in terms of memory on the virtual machine.
  • You can violate invariants of the VM memory relatively easily.
  • Today, we call this VM memory unsafety.
• It is intentionally VM memory unsafe.
  • Performance gets higher without type checks.
  • The unsafety enables primitive operations in bytecodes.
    • e.g. direct manipulation of stacks

Current status

• For both host and VM memory safety
• Checks in the current implementation
  1. Language-level type checks
  2. Primitive type checks
     • i.e. cons or number
  3. Index bound check on memory read/write
  4. Index bound check on pointer construction

Upcoming plans

• 1 in Rust?
• 4 instead of 3?

Future work

• Unicode support
• include syntax
• Tree shaking
• Synchronous and asynchronous APIs in the same crate

Summary

• Building Scheme is fun!