[wasm] Add a magic word and a version number to the binary.

src/wasm/module-decoder.cc

Index: src/wasm/module-decoder.cc
diff --git a/src/wasm/module-decoder.cc b/src/wasm/module-decoder.cc
index 9ba0b3b663f5bdd5c8e6f53861272faeb83d05ab..777acdfb1c043f1f10a62579c986c9f944b2587f 100644
--- a/src/wasm/module-decoder.cc
+++ b/src/wasm/module-decoder.cc
@@ -60,6 +60,22 @@ class ModuleDecoder : public Decoder {
     bool sections[kMaxModuleSectionCode];
     memset(sections, 0, sizeof(sections));
 
+    const byte* pos = pc_;
+    uint32_t magic_number = consume_u32("wasm magic");
+    if (magic_number != kWasmMagic) {
+      error(pos, pos, "expected magic bytes 0 a s m");

[JF, 2016/02/27 19:43:04]

"0 a s m" will be incorrect if we change kWasmMagic. Can you %02x %02x %02x %02x
kWasmMagic in as below instead?

Also, can you print out magic_number?

[titzer, 2016/02/27 23:05:09]

Done.

+      return toResult(module);
+    }
+
+    pos = pc_;
+    uint32_t magic_version = consume_u32("wasm version");
+    if (magic_version != kWasmVersion) {
+      error(pos, pos, "expected version %02x %02x %02x %02x",
+            (kWasmVersion & 0xff), (kWasmVersion >> 8) & 0xff,
+            (kWasmVersion >> 16) & 0xff, (kWasmVersion >> 24) & 0xff);

[JF, 2016/02/27 19:43:04]

Same here, print magic_version.

[titzer, 2016/02/27 23:05:09]

Done.

+      return toResult(module);
+    }
+
     // Decode the module sections.
     while (pc_ < limit_) {
       TRACE("DecodeSection\n");