Merge pull request #1660 from michel-iwaniec/bugfix_script_engine_init

Fix bug with out-of-bounds memory writes on 8-bit values in init script

CHANGELOG.md

@@ -59,6 +59,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix issue where changing the dimensions of a sprite png image could cause the sprite editor's tile palette to display incorrectly
 - Fix line numbers in code editor for GBVM scripts with more than 999 lines
 - Fix issue preventing projects being created with the name "project"
+- Fix issue without-of-bounds memory writes on 8-bit values in engine field initialisation [@michel-iwaniec](https://github.com/michel-iwaniec)
 - Fix issue preventing checkbox type working in engine fields [@pau-tomas](https://github.com/pau-tomas)
 - Fix UI palette text control code. Palette indices now go from 1 to 8, because zero byte is a string terminator [@untoxa](https://github.com/untoxa)
 - Fix issue where migrating old projects could cause gbvm symbols to become empty, preventing build from completing (opening a broken project will now automatically fix this issue)

src/lib/compiler/compileBootstrap.ts

@@ -10,6 +10,7 @@ import {
 } from "./generateGBVMData";
 import { dirEnum } from "./helpers";
 import { PrecompiledAvatarData } from "./compileAvatars";
+import { gbvmSetConstForCType } from "shared/lib/engineFields/engineFieldToCType";
 
 interface InitialState {
   startX: number;
@@ -88,12 +89,16 @@ ${usedEngineFields
 _script_engine_init::
 ${usedEngineFields
   .map((engineField) => {
+    if (engineField.cType === "define") {
+      return "";
+    }
     const engineValue = engineFieldValues.find((v) => v.id === engineField.key);
     const value =
       engineValue && engineValue.value !== undefined
         ? engineValue.value
         : engineField.defaultValue;
-    return `        VM_SET_CONST_INT16      _${engineField.key}, ${value}`;
+    const gbvmSetConstInstruction = gbvmSetConstForCType(engineField.cType);
+    return `        ${gbvmSetConstInstruction}      _${engineField.key}, ${value}`;
   })
   .join("\n")}
 

src/shared/lib/engineFields/engineFieldToCType.ts

@@ -1,4 +1,5 @@
 import type { EngineFieldCType } from "store/features/engine/engineState";
+import { assertUnreachable } from "shared/lib/helpers/assert";
 
 export const is16BitCType = (cType: EngineFieldCType): boolean => {
   return cType === "WORD" || cType === "UWORD";
@@ -35,3 +36,16 @@ export const clampToCType = (
   const max = maxForCType(cType);
   return Math.max(min, Math.min(max, value));
 };
+
+export const gbvmSetConstForCType = (
+  cType: Exclude<EngineFieldCType, "define">
+): string => {
+  if (cType === "WORD" || cType === "UWORD") {
+    return "VM_SET_CONST_INT16";
+  }
+  if (cType === "BYTE" || cType === "UBYTE") {
+    return "VM_SET_CONST_INT8";
+  }
+  assertUnreachable(cType);
+  return "UNREACHABLE";
+};