4 files changed, 29 insertions, 4 deletions
diff --git a/src/passes/Print.cpp b/src/passes/Print.cpp
index 9ead2aee7..c46a0621d 100644
--- a/src/passes/Print.cpp
+++ b/src/passes/Print.cpp
@@ -735,6 +735,11 @@ struct PrintSExpression : public Visitor<PrintSExpression> {
       visitFunction(child.get());
       o << maybeNewLine;
     }
+    for (auto& section : curr->userSections) {
+      doIndent(o, indent);
+      o << ";; custom section \"" << section.name << "\", size " << section.data.size();
+      o << maybeNewLine;
+    }
     decIndent();
     o << maybeNewLine;
     currModule = nullptr;
diff --git a/src/wasm-binary.h b/src/wasm-binary.h
index ecdde571e..f57d351c3 100644
--- a/src/wasm-binary.h
+++ b/src/wasm-binary.h
@@ -638,7 +638,7 @@ public:
   WasmBinaryBuilder(Module& wasm, std::vector<char>& input, bool debug) : wasm(wasm), allocator(wasm.allocator), input(input), debug(debug) {}
 
   void read();
-  void readUserSection();
+  void readUserSection(size_t payloadLen);
   bool more() { return pos < input.size();}
 
   uint8_t getInt8();
diff --git a/src/wasm.h b/src/wasm.h
index 9807ee63c..4ad0870cc 100644
--- a/src/wasm.h
+++ b/src/wasm.h
@@ -1528,6 +1528,14 @@ public:
   bool mutable_;
 };
 
+// "Opaque" data, not part of the core wasm spec, that is held in binaries.
+// May be parsed/handled by utility code elsewhere, but not in wasm.h
+class UserSection {
+public:
+  std::string name;
+  std::vector<char> data;
+};
+
 class Module {
 public:
   // wasm contents (generally you shouldn't access these from outside, except maybe for iterating; use add*() and the get() functions)
@@ -1541,6 +1549,8 @@ public:
   Memory memory;
   Name start;
 
+  std::vector<UserSection> userSections;
+
   MixedArena allocator;
 
 private:
diff --git a/src/wasm/wasm-binary.cpp b/src/wasm/wasm-binary.cpp
index f7fa821ea..095a0942f 100644
--- a/src/wasm/wasm-binary.cpp
+++ b/src/wasm/wasm-binary.cpp
@@ -951,7 +951,8 @@ void WasmBinaryBuilder::read() {
       case BinaryConsts::Section::Data: readDataSegments(); break;
       case BinaryConsts::Section::Table: readFunctionTableDeclaration(); break;
       default: {
-        readUserSection();
+        readUserSection(payloadLen);
+        assert(pos <= oldPos + payloadLen);
         pos = oldPos + payloadLen;
       }
     }
@@ -963,12 +964,21 @@ void WasmBinaryBuilder::read() {
   processFunctions();
 }
 
-void WasmBinaryBuilder::readUserSection() {
+void WasmBinaryBuilder::readUserSection(size_t payloadLen) {
+  auto oldPos = pos;
   Name sectionName = getInlineString();
   if (sectionName.equals(BinaryConsts::UserSections::Name)) {
     readNames();
   } else {
-    std::cerr << "unfamiliar section: " << sectionName << std::endl;
+    // an unfamiliar custom section
+    wasm.userSections.resize(wasm.userSections.size() + 1);
+    auto& section = wasm.userSections.back();
+    section.name = sectionName.str;
+    auto sectionSize = payloadLen - (pos - oldPos);
+    section.data.resize(sectionSize);
+    for (size_t i = 0; i < sectionSize; i++) {
+      section.data[i] = getInt8();
+    }
   }
 }