[wasm] improve handling of malformed input

src/wasm/ast-decoder.cc

Index: src/wasm/ast-decoder.cc
diff --git a/src/wasm/ast-decoder.cc b/src/wasm/ast-decoder.cc
index 390a67300a34a22640952dc5bc92339d341217f7..b45596eb59c473e75ff6a8d895e2baf9f7c87bf2 100644
--- a/src/wasm/ast-decoder.cc
+++ b/src/wasm/ast-decoder.cc
@@ -238,7 +238,7 @@ class WasmDecoder : public Decoder {
     return true;
   }
 
-  int OpcodeArity(const byte* pc) {
+  unsigned OpcodeArity(const byte* pc) {
 #define DECLARE_ARITY(name, ...)                          \
   static const LocalType kTypes_##name[] = {__VA_ARGS__}; \
   static const int kArity_##name =                        \
@@ -319,7 +319,7 @@ class WasmDecoder : public Decoder {
     }
   }
 
-  int OpcodeLength(const byte* pc) {
+  unsigned OpcodeLength(const byte* pc) {
     switch (static_cast<WasmOpcode>(*pc)) {
 #define DECLARE_OPCODE_CASE(name, opcode, sig) case kExpr##name:
       FOREACH_LOAD_MEM_OPCODE(DECLARE_OPCODE_CASE)
@@ -563,7 +563,7 @@ class SR_WasmDecoder : public WasmDecoder {
       }
     }
     // Decode local declarations, if any.
-    int length;
+    unsigned length;
     uint32_t entries = consume_u32v(&length, "local decls count");
     while (entries-- > 0 && pc_ < limit_) {
       uint32_t count = consume_u32v(&length, "local count");
@@ -601,7 +601,7 @@ class SR_WasmDecoder : public WasmDecoder {
     if (pc_ >= limit_) return;  // Nothing to do.
 
     while (true) {  // decoding loop.
-      int len = 1;
+      unsigned len = 1;
       WasmOpcode opcode = static_cast<WasmOpcode>(*pc_);
       TRACE("  @%-6d #%02x:%-20s|", startrel(pc_), opcode,
             WasmOpcodes::ShortOpcodeName(opcode));
@@ -1435,9 +1435,9 @@ class SR_WasmDecoder : public WasmDecoder {
         new (zone_) BitVector(static_cast<int>(local_type_vec_.size()), zone_);
     int depth = 0;
     // Iteratively process all AST nodes nested inside the loop.
-    while (pc < limit_) {
+    while (pc < limit_ && limit_ != start_) {
       WasmOpcode opcode = static_cast<WasmOpcode>(*pc);
-      int length = 1;
+      unsigned length = 1;
       switch (opcode) {
         case kExprLoop:
         case kExprIf:
@@ -1465,7 +1465,7 @@ class SR_WasmDecoder : public WasmDecoder {
       if (depth <= 0) break;
       pc += length;
     }
-    return assigned;
+    return (limit_ != start_) ? assigned : NULL;

[titzer, 2016/06/10 07:40:00]

You can just use ok().

   }
 
   inline wasm::WasmCodePosition position() {
@@ -1515,12 +1515,12 @@ std::ostream& operator<<(std::ostream& os, const Tree& tree) {
   return os;
 }
 
-int OpcodeLength(const byte* pc, const byte* end) {
+unsigned OpcodeLength(const byte* pc, const byte* end) {
   WasmDecoder decoder(nullptr, nullptr, pc, end);
   return decoder.OpcodeLength(pc);
 }
 
-int OpcodeArity(const byte* pc, const byte* end) {
+unsigned OpcodeArity(const byte* pc, const byte* end) {
   WasmDecoder decoder(nullptr, nullptr, pc, end);
   return decoder.OpcodeArity(pc);
 }
@@ -1564,7 +1564,7 @@ void PrintAst(base::AccountingAllocator* allocator, FunctionBody& body) {
   os << "// body: \n";
   int control_depth = 0;
   while (pc < body.end) {
-    size_t length = decoder.OpcodeLength(pc);
+    unsigned length = decoder.OpcodeLength(pc);
 
     WasmOpcode opcode = static_cast<WasmOpcode>(*pc);
     if (opcode == kExprElse) control_depth--;