Implement a FNV1-a style checksum to the binary decoded Wasm data, and display a help notice about needing to serve the HTML/JS file with UTF-8 encoding. This helps developers identify if their own shell files might not have the necessary encoding.

juj · juj · commit f198db6523f3 · 2025-12-06T17:12:16.000+02:00
diff --git a/src/postamble_minimal.js b/src/postamble_minimal.js
@@ -151,6 +151,9 @@ function initRuntime(wasmExports) {
 
 #if SINGLE_FILE && SINGLE_FILE_BINARY_ENCODE && !WASM2JS
 Module['wasm'] = binaryDecode("<<< WASM_BINARY_DATA >>>");
+#if ASSERTIONS
+assert(Module['wasm'].reduce((x,y) => ((x^y)*65599) >>> 0, 2166136261) == "<<< WASM_BINARY_DATA_CHECKSUM >>>", "The checksum of the binary decoded WebAssembly Module code does not match the original data. Double check that this .html/.js file is interpreted with UTF-8 encoding, e.g. by setting <meta charset='utf-8'> inside <head> of the HTML file, or by passing 'Content-Type: application/javascript; charset=utf-8' HTTP header.");
+#endif
 #elif SINGLE_FILE && WASM == 1 && !WASM2JS
 Module['wasm'] = base64Decode('<<< WASM_BINARY_DATA >>>');
 #endif
diff --git a/src/preamble.js b/src/preamble.js
@@ -876,6 +876,10 @@ function getWasmImports() {
   return exports;
 #else
   wasmBinaryFile ??= findWasmBinary();
+#if SINGLE_FILE && SINGLE_FILE_BINARY_ENCODE && !WASM2JS && ASSERTIONS
+  assert(wasmBinaryFile.reduce((x,y) => ((x^y)*65599) >>> 0, 2166136261) == "<<< WASM_BINARY_DATA_CHECKSUM >>>", "The checksum of the binary decoded WebAssembly Module code does not match the original data. Double check that this .html/.js file is interpreted with UTF-8 encoding, e.g. by setting <meta charset='utf-8'> inside <head> of the HTML file, or by passing 'Content-Type: application/javascript; charset=utf-8' HTTP header.");
+#endif
+
 #if WASM_ASYNC_COMPILATION
 #if RUNTIME_DEBUG
   dbg('asynchronously preparing wasm');
diff --git a/tools/link.py b/tools/link.py
@@ -2490,6 +2490,12 @@ def phase_binaryen(target, options, wasm_target):
 
     if settings.SINGLE_FILE_BINARY_ENCODE:
       js = do_replace(js, '"<<< WASM_BINARY_DATA >>>"', binary_encode(wasm_target))
+      if settings.ASSERTIONS:
+        def checksum(a):
+          h = 2166136261
+          for b in a: h = (h ^ b) * 65599 & 0xffffffff
+          return h
+        js = do_replace(js, '"<<< WASM_BINARY_DATA_CHECKSUM >>>"', str(checksum(utils.read_binary(wasm_target))))
     else:
       js = do_replace(js, '<<< WASM_BINARY_DATA >>>', base64_encode(wasm_target))
     delete_file(wasm_target)
