From 25e5b89f97d652dc1bb497105988ba4adfecdd18 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alexander=20Thei=C3=9Fen?= <alex.theissen@me.com>
Date: Tue, 7 Sep 2021 02:00:22 +0200
Subject: [PATCH] Use linear time algorithm to inject stack height metering

---
 src/stack_height/mod.rs | 113 +++++++++++++++++++---------------------
 1 file changed, 54 insertions(+), 59 deletions(-)

diff --git a/src/stack_height/mod.rs b/src/stack_height/mod.rs
index d855cba..03c9da0 100644
--- a/src/stack_height/mod.rs
+++ b/src/stack_height/mod.rs
@@ -49,11 +49,11 @@
 //!   between the frames.
 //! - upon entry into the function entire stack frame is allocated.
 
-use crate::std::{string::String, vec::Vec};
+use crate::std::{mem, string::String, vec::Vec};
 
 use parity_wasm::{
 	builder,
-	elements::{self, Type},
+	elements::{self, Instruction, Instructions, Type},
 };
 
 /// Macro to generate preamble and postamble.
@@ -145,7 +145,7 @@ fn generate_stack_height_global(module: &mut elements::Module) -> u32 {
 		.value_type()
 		.i32()
 		.mutable()
-		.init_expr(elements::Instruction::I32Const(0))
+		.init_expr(Instruction::I32Const(0))
 		.build();
 
 	// Try to find an existing global section.
@@ -253,75 +253,70 @@ fn instrument_functions(ctx: &mut Context, module: &mut elements::Module) -> Res
 ///
 /// drop
 /// ```
-fn instrument_function(
-	ctx: &mut Context,
-	instructions: &mut elements::Instructions,
-) -> Result<(), Error> {
-	use parity_wasm::elements::Instruction::*;
-
-	let mut cursor = 0;
-	loop {
-		if cursor >= instructions.elements().len() {
-			break
-		}
+fn instrument_function(ctx: &mut Context, func: &mut Instructions) -> Result<(), Error> {
+	use Instruction::*;
 
-		enum Action {
-			InstrumentCall { callee_idx: u32, callee_stack_cost: u32 },
-			Nop,
-		}
+	struct InstrumentCall {
+		offset: usize,
+		callee: u32,
+		cost: u32,
+	}
 
-		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 }
+	let calls: Vec<_> = func
+		.elements()
+		.iter()
+		.enumerate()
+		.filter_map(|(offset, instruction)| {
+			if let Call(callee) = instruction {
+				ctx.stack_cost(*callee).and_then(|cost| {
+					if cost > 0 {
+						Some(InstrumentCall { callee: *callee, offset, cost })
 					} else {
-						Action::Nop
+						None
 					}
-				},
-				_ => Action::Nop,
+				})
+			} else {
+				None
 			}
-		};
-
-		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 } => {
+		})
+		.collect();
+
+	// The `instrumented_call!` contains the call itself. This is why we need to subtract one.
+	let len = func.elements().len() + calls.len() * (instrument_call!(0, 0, 0, 0).len() - 1);
+	let original_instrs = mem::replace(func.elements_mut(), Vec::with_capacity(len));
+	let new_instrs = func.elements_mut();
+
+	let mut calls = calls.into_iter().peekable();
+	for (original_pos, instr) in original_instrs.into_iter().enumerate() {
+		// whether there is some call instruction at this position that needs to be instrumented
+		let did_instrument = if let Some(call) = calls.peek() {
+			if call.offset == original_pos {
 				let new_seq = instrument_call!(
-					callee_idx,
-					callee_stack_cost as i32,
+					call.callee,
+					call.cost as i32,
 					ctx.stack_height_global_idx(),
 					ctx.stack_limit()
 				);
+				new_instrs.extend(new_seq);
+				true
+			} else {
+				false
+			}
+		} else {
+			false
+		};
 
-				// 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;
-			},
+		if did_instrument {
+			calls.next();
+		} else {
+			new_instrs.push(instr);
 		}
 	}
 
+	if calls.next().is_some() {
+		return Err(Error("Not all calls were used".into()))
+	}
+
 	Ok(())
 }