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//! A tiny and incomplete wasm interpreter //! //! This module contains a tiny and incomplete wasm interpreter built on top of //! `parity-wasm`'s module structure. Each `Interpreter` contains some state //! about the execution of a wasm instance. The "incomplete" part here is //! related to the fact that this is *only* used to execute the various //! descriptor functions for wasm-bindgen. //! //! As a recap, the wasm-bindgen macro generate "descriptor functions" which //! basically as a mapping of rustc's trait resolution in executable code. This //! allows us to detect, after the macro is invoke, what trait selection did and //! what types of functions look like. By executing descriptor functions they'll //! each invoke a known import (with only one argument) some number of times, //! which gives us a list of `u32` values to then decode. //! //! The interpreter here is only geared towards this one exact use case, so it's //! quite small and likely not extra-efficient. #![deny(missing_docs)] extern crate parity_wasm; use std::collections::HashMap; use parity_wasm::elements::*; /// A ready-to-go interpreter of a wasm module. /// /// An interpreter currently represents effectively cached state. It is reused /// between calls to `interpret` and is precomputed from a `Module`. It houses /// state like the wasm stack, wasm memory, etc. #[derive(Default)] pub struct Interpreter { // Number of imported functions in the wasm module (used in index // calculations) imports: usize, // Function index of the `__wbindgen_describe` imported function. We special // case this to know when the environment's imported function is called. describe_idx: Option<u32>, // A mapping of string names to the function index, filled with all exported // functions. name_map: HashMap<String, u32>, // The numerical index of the code section in the wasm module, indexed into // the module's list of sections. code_idx: Option<usize>, // The current stack pointer (global 0) and wasm memory (the stack). Only // used in a limited capacity. sp: i32, mem: Vec<i32>, // The wasm stack. Note how it's just `i32` which is intentional, we don't // support other types. stack: Vec<i32>, // The descriptor which we're assembling, a list of `u32` entries. This is // very specific to wasm-bindgen and is the purpose for the existence of // this module. descriptor: Vec<u32>, } impl Interpreter { /// Creates a new interpreter from a provided `Module`, precomputing all /// information necessary to interpret further. /// /// Note that the `module` passed in to this function must be the same as /// the `module` passed to `interpret` below. pub fn new(module: &Module) -> Interpreter { let mut ret = Interpreter::default(); // The descriptor functions shouldn't really use all that much memory // (the LLVM call stack, now the wasm stack). To handle that let's give // our selves a little bit of memory and set the stack pointer (global // 0) to the top. ret.mem = vec![0; 0x100]; ret.sp = ret.mem.len() as i32; // Figure out where our code section, if any, is. for (i, s) in module.sections().iter().enumerate() { match s { Section::Code(_) => ret.code_idx = Some(i), _ => {} } } // Figure out where the `__wbindgen_describe` imported function is, if // it exists. We'll special case calls to this function as our // interpretation should only invoke this function as an imported // function. if let Some(i) = module.import_section() { ret.imports = i.functions(); let mut idx = 0; for entry in i.entries() { match entry.external() { External::Function(_) => idx += 1, _ => continue, } if entry.module() != "__wbindgen_placeholder__" { continue } if entry.field() != "__wbindgen_describe" { continue } ret.describe_idx = Some(idx - 1 as u32); } } // Build up the mapping of exported functions to function indices. if let Some(e) = module.export_section() { for e in e.entries() { let i = match e.internal() { Internal::Function(i) => i, _ => continue, }; ret.name_map.insert(e.field().to_string(), *i); } } return ret } /// Interprets the execution of the descriptor function `func`. /// /// This function will execute `func` in the `module` provided. Note that /// the `module` provided here must be the same as the one passed to `new` /// when this `Interpreter` was constructed. /// /// The `func` must be a wasm-bindgen descriptor function meaning that it /// doesn't do anything like use floats or i64. Instead all it should do is /// call other functions, sometimes some stack pointer manipulation, and /// then call the one imported `__wbindgen_describe` function. Anything else /// will cause this interpreter to panic. /// /// When the descriptor has finished running the assembled descriptor list /// is returned. The descriptor returned can then be re-parsed into an /// actual `Descriptor` in the cli-support crate. /// /// # Return value /// /// Returns `Some` if `func` was found in the `module` and `None` if it was /// not found in the `module`. pub fn interpret(&mut self, func: &str, module: &Module) -> Option<&[u32]> { self.descriptor.truncate(0); let idx = *self.name_map.get(func)?; let code = match &module.sections()[self.code_idx.unwrap()] { Section::Code(s) => s, _ => panic!(), }; // We should have a blank wasm and LLVM stack at both the start and end // of the call. assert_eq!(self.sp, self.mem.len() as i32); assert_eq!(self.stack.len(), 0); self.call(idx, code); assert_eq!(self.stack.len(), 0); assert_eq!(self.sp, self.mem.len() as i32); Some(&self.descriptor) } fn call(&mut self, idx: u32, code: &CodeSection) { use parity_wasm::elements::Instruction::*; let idx = idx as usize; assert!(idx >= self.imports); // can't call imported functions let body = &code.bodies()[idx - self.imports]; // Allocate space for our call frame's local variables. All local // variables should be of the `i32` type. assert!(body.locals().len() <= 1, "too many local types"); let locals = body.locals() .get(0) .map(|i| { assert_eq!(i.value_type(), ValueType::I32); i.count() }) .unwrap_or(0); let mut locals = vec![0; locals as usize]; // Actual interpretation loop! We keep track of our stack's length to // recover it as part of the `Return` instruction, and otherwise this is // a pretty straightforward interpretation loop. let before = self.stack.len(); for instr in body.code().elements() { match instr { I32Const(x) => self.stack.push(*x), SetLocal(i) => locals[*i as usize] = self.stack.pop().unwrap(), GetLocal(i) => self.stack.push(locals[*i as usize]), Call(idx) => { // If this function is calling the `__wbindgen_describe` // function, which we've precomputed the index for, then // it's telling us about the next `u32` element in the // descriptor to return. We "call" the imported function // here by directly inlining it. // // Otherwise this is a normal call so we recurse. if Some(*idx) == self.describe_idx { self.descriptor.push(self.stack.pop().unwrap() as u32); } else { self.call(*idx, code); } } GetGlobal(0) => self.stack.push(self.sp), SetGlobal(0) => self.sp = self.stack.pop().unwrap(), I32Sub => { let b = self.stack.pop().unwrap(); let a = self.stack.pop().unwrap(); self.stack.push(a - b); } I32Add => { let a = self.stack.pop().unwrap(); let b = self.stack.pop().unwrap(); self.stack.push(a + b); } I32Store(/* align = */ 2, offset) => { let val = self.stack.pop().unwrap(); let addr = self.stack.pop().unwrap() as u32; self.mem[((addr + *offset) as usize) / 4] = val; } I32Load(/* align = */ 2, offset) => { let addr = self.stack.pop().unwrap() as u32; self.stack.push(self.mem[((addr + *offset) as usize) / 4]); } Return => self.stack.truncate(before), End => break, // All other instructions shouldn't be used by our various // descriptor functions. LLVM optimizations may mean that some // of the above instructions aren't actually needed either, but // the above instructions have empirically been required when // executing our own test suite in wasm-bindgen. // // Note that LLVM may change over time to generate new // instructions in debug mode, and we'll have to react to those // sorts of changes as they arise. s => panic!("unknown instruction {:?}", s), } } assert_eq!(self.stack.len(), before); } }