twasm_utils/stack_height/

mod.rs

//! The pass that tries to make stack overflows deterministic, by introducing
//! an upper bound of the stack size.
//!
//! This pass introduces a global mutable variable to track stack height,
//! and instruments all calls with preamble and postamble.
//!
//! Stack height is increased prior the call. Otherwise, the check would
//! be made after the stack frame is allocated.
//!
//! The preamble is inserted before the call. It increments
//! the global stack height variable with statically determined "stack cost"
//! of the callee. If after the increment the stack height exceeds
//! the limit (specified by the `rules`) then execution traps.
//! Otherwise, the call is executed.
//!
//! The postamble is inserted after the call. The purpose of the postamble is to decrease
//! the stack height by the "stack cost" of the callee function.
//!
//! Note, that we can't instrument all possible ways to return from the function. The simplest
//! example would be a trap issued by the host function.
//! That means stack height global won't be equal to zero upon the next execution after such trap.
//!
//! # Thunks
//!
//! Because stack height is increased prior the call few problems arises:
//!
//! - Stack height isn't increased upon an entry to the first function, i.e. exported function.
//! - Start function is executed externally (similar to exported functions).
//! - It is statically unknown what function will be invoked in an indirect call.
//!
//! The solution for this problems is to generate a intermediate functions, called 'thunks', which
//! will increase before and decrease the stack height after the call to original function, and
//! then make exported function and table entries, start section to point to a corresponding thunks.
//!
//! # Stack cost
//!
//! Stack cost of the function is calculated as a sum of it's locals
//! and the maximal height of the value stack.
//!
//! All values are treated equally, as they have the same size.
//!
//! The rationale is that this makes it possible to use the following very naive wasm executor:
//!
//! - values are implemented by a union, so each value takes a size equal to
//!   the size of the largest possible value type this union can hold. (In MVP it is 8 bytes)
//! - each value from the value stack is placed on the native stack.
//! - each local variable and function argument is placed on the native stack.
//! - arguments pushed by the caller are copied into callee stack rather than shared
//!   between the frames.
//! - upon entry into the function entire stack frame is allocated.

use crate::std::string::String;
use crate::std::vec::Vec;

use tetsy_wasm::elements::{self, Type};
use tetsy_wasm::builder;

/// Macro to generate preamble and postamble.
macro_rules! instrument_call {
	($callee_idx: expr, $callee_stack_cost: expr, $stack_height_global_idx: expr, $stack_limit: expr) => {{
		use $crate::tetsy_wasm::elements::Instruction::*;
		[
			// stack_height += stack_cost(F)
			GetGlobal($stack_height_global_idx),
			I32Const($callee_stack_cost),
			I32Add,
			SetGlobal($stack_height_global_idx),
			// if stack_counter > LIMIT: unreachable
			GetGlobal($stack_height_global_idx),
			I32Const($stack_limit as i32),
			I32GtU,
			If(elements::BlockType::NoResult),
			Unreachable,
			End,
			// Original call
			Call($callee_idx),
			// stack_height -= stack_cost(F)
			GetGlobal($stack_height_global_idx),
			I32Const($callee_stack_cost),
			I32Sub,
			SetGlobal($stack_height_global_idx),
		]
	}};
}

mod max_height;
mod thunk;

/// Error that occured during processing the module.
///
/// This means that the module is invalid.
#[derive(Debug)]
pub struct Error(String);

pub(crate) struct Context {
	stack_height_global_idx: u32,
	func_stack_costs: Vec<u32>,
	stack_limit: u32,
}

impl Context {
	/// Returns index in a global index space of a stack_height global variable.
	fn stack_height_global_idx(&self) -> u32 {
		self.stack_height_global_idx
	}

	/// Returns `stack_cost` for `func_idx`.
	fn stack_cost(&self, func_idx: u32) -> Option<u32> {
		self.func_stack_costs.get(func_idx as usize).cloned()
	}

	/// Returns stack limit specified by the rules.
	fn stack_limit(&self) -> u32 {
		self.stack_limit
	}
}

/// Instrument a module with stack height limiter.
///
/// See module-level documentation for more details.
///
/// # Errors
///
/// Returns `Err` if module is invalid and can't be
pub fn inject_limiter(
	mut module: elements::Module,
	stack_limit: u32,
) -> Result<elements::Module, Error> {
	let mut ctx = Context {
		stack_height_global_idx: generate_stack_height_global(&mut module),
		func_stack_costs: compute_stack_costs(&module)?,
		stack_limit,
	};

	instrument_functions(&mut ctx, &mut module)?;
	let module = thunk::generate_thunks(&mut ctx, module)?;

	Ok(module)
}

/// Generate a new global that will be used for tracking current stack height.
fn generate_stack_height_global(module: &mut elements::Module) -> u32 {
	let global_entry = builder::global()
		.value_type()
		.i32()
		.mutable()
		.init_expr(elements::Instruction::I32Const(0))
		.build();

	// Try to find an existing global section.
	for section in module.sections_mut() {
		if let elements::Section::Global(gs) = section {
			gs.entries_mut().push(global_entry);
			return (gs.entries().len() as u32) - 1;
		}
	}

	// Existing section not found, create one!
	module.sections_mut().push(elements::Section::Global(
		elements::GlobalSection::with_entries(vec![global_entry]),
	));
	0
}

/// Calculate stack costs for all functions.
///
/// Returns a vector with a stack cost for each function, including imports.
fn compute_stack_costs(module: &elements::Module) -> Result<Vec<u32>, Error> {
	let func_imports = module.import_count(elements::ImportCountType::Function);

	// TODO: optimize!
	(0..module.functions_space())
		.map(|func_idx| {
			if func_idx < func_imports {
				// We can't calculate stack_cost of the import functions.
				Ok(0)
			} else {
				compute_stack_cost(func_idx as u32, &module)
			}
		})
		.collect()
}

/// Stack cost of the given *defined* function is the sum of it's locals count (that is,
/// number of arguments plus number of local variables) and the maximal stack
/// height.
fn compute_stack_cost(func_idx: u32, module: &elements::Module) -> Result<u32, Error> {
	// To calculate the cost of a function we need to convert index from
	// function index space to defined function spaces.
	let func_imports = module.import_count(elements::ImportCountType::Function) as u32;
	let defined_func_idx = func_idx.checked_sub(func_imports).ok_or_else(|| {
		Error("This should be a index of a defined function".into())
	})?;

	let code_section = module.code_section().ok_or_else(|| {
		Error("Due to validation code section should exists".into())
	})?;
	let body = &code_section
		.bodies()
		.get(defined_func_idx as usize)
		.ok_or_else(|| Error("Function body is out of bounds".into()))?;
	let locals_count = body.locals().len() as u32;

	let max_stack_height =
		max_height::compute(
			defined_func_idx,
			module
		)?;

	Ok(locals_count + max_stack_height)
}

fn instrument_functions(ctx: &mut Context, module: &mut elements::Module) -> Result<(), Error> {
	for section in module.sections_mut() {
		if let elements::Section::Code(code_section) = section {
			for func_body in code_section.bodies_mut() {
				let opcodes = func_body.code_mut();
				instrument_function(ctx, opcodes)?;
			}
		}
	}
	Ok(())
}

/// This function searches `call` instructions and wrap each call
/// with preamble and postamble.
///
/// Before:
///
/// ```text
/// get_local 0
/// get_local 1
/// call 228
/// drop
/// ```
///
/// After:
///
/// ```text
/// get_local 0
/// get_local 1
///
/// < ... preamble ... >
///
/// call 228
///
/// < .. postamble ... >
///
/// drop
/// ```
fn instrument_function(
	ctx: &mut Context,
	instructions: &mut elements::Instructions,
) -> Result<(), Error> {
	use tetsy_wasm::elements::Instruction::*;

	let mut cursor = 0;
	loop {
		if cursor >= instructions.elements().len() {
			break;
		}

		enum Action {
			InstrumentCall {
				callee_idx: u32,
				callee_stack_cost: u32,
			},
			Nop,
		}

		let action: Action = {
			let instruction = &instructions.elements()[cursor];
			match instruction {
				Call(callee_idx) => {
					let callee_stack_cost = ctx
						.stack_cost(*callee_idx)
						.ok_or_else(||
							Error(
								format!("Call to function that out-of-bounds: {}", callee_idx)
							)
						)?;

					// Instrument only calls to a functions which stack_cost is
					// non-zero.
					if callee_stack_cost > 0 {
						Action::InstrumentCall {
							callee_idx: *callee_idx,
							callee_stack_cost,
						}
					} else {
						Action::Nop
					}
				},
				_ => Action::Nop,
			}
		};

		match action {
			// We need to wrap a `call idx` instruction
			// with a code that adjusts stack height counter
			// and then restores it.
			Action::InstrumentCall { callee_idx, callee_stack_cost } => {
				let new_seq = instrument_call!(
					callee_idx,
					callee_stack_cost as i32,
					ctx.stack_height_global_idx(),
					ctx.stack_limit()
				);

				// Replace the original `call idx` instruction with
				// a wrapped call sequence.
				//
				// To splice actually take a place, we need to consume iterator
				// splice returns. So we just `count()` it.
				let _ = instructions
					.elements_mut()
					.splice(cursor..(cursor + 1), new_seq.iter().cloned())
					.count();

				// Advance cursor to be after the inserted sequence.
				cursor += new_seq.len();
			}
			// Do nothing for other instructions.
			_ => {
				cursor += 1;
			}
		}
	}

	Ok(())
}

fn resolve_func_type(
	func_idx: u32,
	module: &elements::Module,
) -> Result<&elements::FunctionType, Error> {
	let types = module.type_section().map(|ts| ts.types()).unwrap_or(&[]);
	let functions = module
		.function_section()
		.map(|fs| fs.entries())
		.unwrap_or(&[]);

	let func_imports = module.import_count(elements::ImportCountType::Function);
	let sig_idx = if func_idx < func_imports as u32 {
		module
			.import_section()
			.expect("function import count is not zero; import section must exists; qed")
			.entries()
			.iter()
			.filter_map(|entry| match entry.external() {
				elements::External::Function(idx) => Some(*idx),
				_ => None,
			})
			.nth(func_idx as usize)
			.expect(
				"func_idx is less than function imports count;
				nth function import must be `Some`;
				qed",
			)
	} else {
		functions
			.get(func_idx as usize - func_imports)
			.ok_or_else(|| Error(format!("Function at index {} is not defined", func_idx)))?
			.type_ref()
	};
	let Type::Function(ty) = types.get(sig_idx as usize).ok_or_else(|| {
		Error(format!(
			"Signature {} (specified by func {}) isn't defined",
			sig_idx, func_idx
		))
	})?;
	Ok(ty)
}

#[cfg(test)]
mod tests {
	extern crate wabt;
	use tetsy_wasm::elements;
	use super::*;

	fn parse_wat(source: &str) -> elements::Module {
		elements::deserialize_buffer(&wabt::wat2wasm(source).expect("Failed to wat2wasm"))
			.expect("Failed to deserialize the module")
	}

	fn validate_module(module: elements::Module) {
		let binary = elements::serialize(module).expect("Failed to serialize");
		wabt::Module::read_binary(&binary, &Default::default())
			.expect("Wabt failed to read final binary")
			.validate()
			.expect("Invalid module");
	}

	#[test]
	fn test_with_params_and_result() {
		let module = parse_wat(
			r#"
(module
	(func (export "i32.add") (param i32 i32) (result i32)
		get_local 0
	get_local 1
	i32.add
	)
)
"#,
		);

		let module = inject_limiter(module, 1024)
			.expect("Failed to inject stack counter");
		validate_module(module);
	}
}