Fix read-write of dylink section (#1648)

The 'dylink' user section must be emitted before all other sections, per the spec (to allow simple parsing by loaders)

This PR makes reading and writing of a dynamic library remain a valid dynamic library.

Author: kripken

check.py

@@ -239,6 +239,21 @@ def run_crash_tests():
       run_command(cmd, expected_err='parse exception:', err_contains=True, expected_status=1)
 
 
+def run_dylink_tests():
+  print "\n[ we emit dylink sections properly... ]\n"
+
+  for t in os.listdir(options.binaryen_test):
+    if t.startswith('dylib') and t.endswith('.wasm'):
+      print '..', t
+      t = os.path.join(options.binaryen_test, t)
+      cmd = WASM_OPT + [t, '-o', 'a.wasm']
+      run_command(cmd)
+      with open('a.wasm') as output:
+        index = output.read().find('dylink')
+        print '  ', index
+        assert index == 11, 'dylink section must be first, right after the magic number etc.'
+
+
 def run_ctor_eval_tests():
   print '\n[ checking wasm-ctor-eval... ]\n'
 
@@ -640,6 +655,7 @@ def main():
   run_wasm_dis_tests()
   run_wasm_merge_tests()
   run_crash_tests()
+  run_dylink_tests()
   run_ctor_eval_tests()
   run_wasm_metadce_tests()
   if has_shell_timeout():

src/tools/wasm-merge.cpp

@@ -106,7 +106,7 @@ struct Mergeable {
       }
     }
     for (auto& section : wasm.userSections) {
-      if (section.name == "dylink") {
+      if (section.name == BinaryConsts::UserSections::Dylink) {
         WasmBinaryBuilder builder(wasm, section.data, false);
         totalMemorySize = std::max(totalMemorySize, builder.getU32LEB());
         totalTableSize = std::max(totalTableSize, builder.getU32LEB());

src/wasm-binary.h

@@ -340,6 +340,8 @@ namespace UserSections {
 extern const char* Name;
 extern const char* SourceMapUrl;
 
+extern const char* Dylink;
+
 enum Subsection {
   NameFunction = 1,
   NameLocal = 2,
@@ -713,7 +715,9 @@ class WasmBinaryWriter {
   void writeNames();
   void writeSourceMapUrl();
   void writeSymbolMap();
-  void writeUserSections();
+  void writeEarlyUserSections();
+  void writeLateUserSections();
+  void writeUserSection(const UserSection& section);
 
   void writeSourceMapProlog();
   void writeSourceMapEpilog();

src/wasm/wasm-binary.cpp

@@ -39,6 +39,9 @@ void WasmBinaryWriter::prepare() {
 
 void WasmBinaryWriter::write() {
   writeHeader();
+
+  writeEarlyUserSections();
+
   if (sourceMap) {
     writeSourceMapProlog();
   }
@@ -62,7 +65,7 @@ void WasmBinaryWriter::write() {
     writeSourceMapEpilog();
   }
 
-  writeUserSections();
+  writeLateUserSections();
 
   finishUp();
 }
@@ -535,17 +538,33 @@ void WasmBinaryWriter::writeSourceMapEpilog() {
   *sourceMap << "\"}";
 }
 
-void WasmBinaryWriter::writeUserSections() {
+void WasmBinaryWriter::writeEarlyUserSections() {
+  // The dylink section must be the first in the module, per
+  // the spec, to allow simple parsing by loaders.
   for (auto& section : wasm->userSections) {
-    auto start = startSection(0);
-    writeInlineString(section.name.c_str());
-    for (size_t i = 0; i < section.data.size(); i++) {
-      o << uint8_t(section.data[i]);
+    if (section.name == BinaryConsts::UserSections::Dylink) {
+      writeUserSection(section);
     }
-    finishSection(start);
   }
 }
 
+void WasmBinaryWriter::writeLateUserSections() {
+  for (auto& section : wasm->userSections) {
+    if (section.name != BinaryConsts::UserSections::Dylink) {
+      writeUserSection(section);
+    }
+  }
+}
+
+void WasmBinaryWriter::writeUserSection(const UserSection& section) {
+  auto start = startSection(0);
+  writeInlineString(section.name.c_str());
+  for (size_t i = 0; i < section.data.size(); i++) {
+    o << uint8_t(section.data[i]);
+  }
+  finishSection(start);
+}
+
 void WasmBinaryWriter::writeDebugLocation(Expression* curr, Function* func) {
   auto& debugLocations = func->debugLocations;
   auto iter = debugLocations.find(curr);

src/wasm/wasm.cpp

@@ -29,6 +29,8 @@ namespace BinaryConsts {
 namespace UserSections {
 const char* Name = "name";
 const char* SourceMapUrl = "sourceMappingURL";
+
+const char* Dylink = "dylink";
 }
 }