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|
use super::*;
use std::mem;
use wasm_encoder::BlockType;
impl Module {
/// Ensure that all of this Wasm module's functions will terminate when
/// executed.
///
/// This adds a new mutable, exported global to the module to keep track of
/// how much "fuel" is left. Fuel is decremented at the head of each loop
/// and function. When fuel reaches zero, a trap is raised.
///
/// The index of the fuel global is returned, so that you may control how
/// much fuel the module is given.
pub fn ensure_termination(&mut self, default_fuel: u32) -> u32 {
let fuel_global = self.globals.len() as u32;
self.globals.push(GlobalType {
val_type: ValType::I32,
mutable: true,
});
self.defined_globals.push((
fuel_global,
GlobalInitExpr::ConstExpr(ConstExpr::i32_const(default_fuel as i32)),
));
for code in &mut self.code {
let check_fuel = |insts: &mut Vec<Instruction>| {
// if fuel == 0 { trap }
insts.push(Instruction::GlobalGet(fuel_global));
insts.push(Instruction::I32Eqz);
insts.push(Instruction::If(BlockType::Empty));
insts.push(Instruction::Unreachable);
insts.push(Instruction::End);
// fuel -= 1
insts.push(Instruction::GlobalGet(fuel_global));
insts.push(Instruction::I32Const(1));
insts.push(Instruction::I32Sub);
insts.push(Instruction::GlobalSet(fuel_global));
};
let instrs = match &mut code.instructions {
Instructions::Generated(list) => list,
// only present on modules contained within
// `MaybeInvalidModule`, which doesn't expose its internal
// `Module`.
Instructions::Arbitrary(_) => unreachable!(),
};
let mut new_insts = Vec::with_capacity(instrs.len() * 2);
// Check fuel at the start of functions to deal with infinite
// recursion.
check_fuel(&mut new_insts);
for inst in mem::replace(instrs, vec![]) {
let is_loop = matches!(&inst, Instruction::Loop(_));
new_insts.push(inst);
// Check fuel at loop heads to deal with infinite loops.
if is_loop {
check_fuel(&mut new_insts);
}
}
*instrs = new_insts;
}
fuel_global
}
}
|
