wasm: use strings for section names

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 4653e1d44bd7b53b145afb4bc8fbf8843231f6af..086788f4e8ccf8a271fe180ead88119b35bc62b5 100644
--- a/src/wasm/module-decoder.cc
+++ b/src/wasm/module-decoder.cc
@@ -78,18 +78,17 @@ class ModuleDecoder : public Decoder {
     // Decode the module sections.
     while (pc_ < limit_) {
       TRACE("DecodeSection\n");
-      uint8_t section_u8 = consume_u8("section");
+      int length;
+      uint32_t section_bytes = consume_u32v(&length, "section size");

[titzer, 2016/03/07 08:45:33]

Should the section size also include the string size? (I think yes). In that
case the below code needs check the string is contained in the section size and
only skip the remaining bytes.

[JF, 2016/03/09 02:38:57]

Agreed, fixed.

 
-      if (section_u8 >= kMaxModuleSectionCode) {
+      WasmSectionDeclCode section = consume_section_name();
+      if (section == kMaxModuleSectionCode) {
         // Skip unknown section.
-        int length;
-        uint32_t section_bytes = consume_u32v(&length, "section size");
         consume_bytes(section_bytes);
         continue;
       }
 
       // Each section should appear at most once.
-      auto section = static_cast<WasmSectionDeclCode>(section_u8);
       CheckForPreviousSection(sections, section, false);
       sections[section] = true;
 
@@ -521,6 +520,35 @@ class ModuleDecoder : public Decoder {
     return consume_offset(name ? name : "string");
   }
 
+  // Reads a section name.
+  WasmSectionDeclCode consume_section_name() {
+    static const char* sections[] = {
+#define F(enumerator, string) string,
+        FOR_EACH_WASM_SECTION_TYPE(F)
+#undef F
+    };
+    static uint8_t sizes[] {
+#define F(enumerator, string) sizeof(string),
+        FOR_EACH_WASM_SECTION_TYPE(F)
+#undef F
+    };
+    static_assert(
+        sizeof(sections) / sizeof(sections[0]) == kMaxModuleSectionCode,
+        "expected enum WasmSectionDeclCode to be monotonic from 0");
+    int length;
+    uint32_t string_bytes = consume_u32v(&length, "section module name");
+    const byte* start = pc_;
+    consume_bytes(string_bytes);
+    if (failed()) return kMaxModuleSectionCode;
+    for (size_t i = 0; i != kMaxModuleSectionCode; ++i) {
+      if (sizes[i] == string_bytes &&
+          0 == memcmp(sections[i], start, string_bytes)) {
+        return (WasmSectionDeclCode)i;
+      }
+    }
+    return kMaxModuleSectionCode;
+  }
+
   // Reads a single 8-bit integer, interpreting it as a local type.
   LocalType consume_local_type() {
     byte val = consume_u8("local type");