wasm-instrument/src/stack_limiter/thunk.rs

use alloc::{collections::BTreeMap as Map, vec::Vec};
use parity_wasm::{
	builder,
	elements::{self, FunctionType, Internal},
};

use super::{resolve_func_type, Context};

struct Thunk {
	signature: FunctionType,
	// Index in function space of this thunk.
	idx: Option<u32>,
	callee_stack_cost: u32,
}

pub fn generate_thunks(
	ctx: &mut Context,
	module: elements::Module,
) -> Result<elements::Module, &'static str> {
	// First, we need to collect all function indices that should be replaced by thunks
	let mut replacement_map: Map<u32, Thunk> = {
		let exports = module.export_section().map(|es| es.entries()).unwrap_or(&[]);
		let elem_segments = module.elements_section().map(|es| es.entries()).unwrap_or(&[]);
		let start_func_idx = module.start_section();

		let exported_func_indices = exports.iter().filter_map(|entry| match entry.internal() {
			Internal::Function(function_idx) => Some(*function_idx),
			_ => None,
		});
		let table_func_indices =
			elem_segments.iter().flat_map(|segment| segment.members()).cloned();

		// Replacement map is at least export section size.
		let mut replacement_map: Map<u32, Thunk> = Map::new();

		for func_idx in exported_func_indices
			.chain(table_func_indices)
			.chain(start_func_idx.into_iter())
		{
			let callee_stack_cost = ctx.stack_cost(func_idx).ok_or("function index isn't found")?;

			// Don't generate a thunk if stack_cost of a callee is zero.
			if callee_stack_cost != 0 {
				replacement_map.insert(
					func_idx,
					Thunk {
						signature: resolve_func_type(func_idx, &module)?.clone(),
						idx: None,
						callee_stack_cost,
					},
				);
			}
		}

		replacement_map
	};

	// Then, we generate a thunk for each original function.

	// Save current func_idx
	let mut next_func_idx = module.functions_space() as u32;

	let mut mbuilder = builder::from_module(module);
	for (func_idx, thunk) in replacement_map.iter_mut() {
		let instrumented_call = instrument_call!(
			*func_idx,
			thunk.callee_stack_cost as i32,
			ctx.stack_height_global_idx(),
			ctx.stack_limit()
		);
		// Thunk body consist of:
		//  - argument pushing
		//  - instrumented call
		//  - end
		let mut thunk_body: Vec<elements::Instruction> =
			Vec::with_capacity(thunk.signature.params().len() + instrumented_call.len() + 1);

		for (arg_idx, _) in thunk.signature.params().iter().enumerate() {
			thunk_body.push(elements::Instruction::GetLocal(arg_idx as u32));
		}
		thunk_body.extend_from_slice(&instrumented_call);
		thunk_body.push(elements::Instruction::End);

		// TODO: Don't generate a signature, but find an existing one.

		mbuilder = mbuilder
			.function()
			// Signature of the thunk should match the original function signature.
			.signature()
			.with_params(thunk.signature.params().to_vec())
			.with_results(thunk.signature.results().to_vec())
			.build()
			.body()
			.with_instructions(elements::Instructions::new(thunk_body))
			.build()
			.build();

		thunk.idx = Some(next_func_idx);
		next_func_idx += 1;
	}
	let mut module = mbuilder.build();

	// And finally, fixup thunks in export and table sections.

	// Fixup original function index to a index of a thunk generated earlier.
	let fixup = |function_idx: &mut u32| {
		// Check whether this function is in replacement_map, since
		// we can skip thunk generation (e.g. if stack_cost of function is 0).
		if let Some(thunk) = replacement_map.get(function_idx) {
			*function_idx =
				thunk.idx.expect("At this point an index must be assigned to each thunk");
		}
	};

	for section in module.sections_mut() {
		match section {
			elements::Section::Export(export_section) =>
				for entry in export_section.entries_mut() {
					if let Internal::Function(function_idx) = entry.internal_mut() {
						fixup(function_idx)
					}
				},
			elements::Section::Element(elem_section) =>
				for segment in elem_section.entries_mut() {
					for function_idx in segment.members_mut() {
						fixup(function_idx)
					}
				},
			elements::Section::Start(start_idx) => fixup(start_idx),
			_ => {},
		}
	}

	Ok(module)
}