[wasm] fix simd opcode read and error case for bad simd opcodes

src/wasm/ast-decoder.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/signature.h"
 
 #include "src/bit-vector.h"
 #include "src/flags.h"
 #include "src/handles.h"
 #include "src/zone/zone-containers.h"
@@ skipping 306 matching lines @@
         ImmI64Operand operand(this, pc);
         return 1 + operand.length;
       }
       case kExprI8Const:
         return 2;
       case kExprF32Const:
         return 5;
       case kExprF64Const:
         return 9;
       case kSimdPrefix: {
-        byte simd_index = *(pc + 1);
+        byte simd_index = checked_read_u8(pc, 1, "simd_index");
         WasmOpcode opcode =
             static_cast<WasmOpcode>(kSimdPrefix << 8 | simd_index);
         switch (opcode) {
 #define DECLARE_OPCODE_CASE(name, opcode, sig) case kExpr##name:
           FOREACH_SIMD_0_OPERAND_OPCODE(DECLARE_OPCODE_CASE)
 #undef DECLARE_OPCODE_CASE
           {
             return 2;
           }
 #define DECLARE_OPCODE_CASE(name, opcode, sig) case kExpr##name:
           FOREACH_SIMD_1_OPERAND_OPCODE(DECLARE_OPCODE_CASE)
 #undef DECLARE_OPCODE_CASE
           {
             return 3;
           }
           default:
-            UNREACHABLE();
+            error("invalid SIMD opcode");
+            return 2;
         }
       }
       default:
         return 1;
     }
   }
 };
 
 static const int32_t kNullCatch = -1;
 
@@ skipping 777 matching lines @@
               TFNode** rets = nullptr;
               BUILD(CallIndirect, operand.index, buffer, &rets, position());
               PushReturns(operand.sig, rets);
             }
             len = 1 + operand.length;
             break;
           }
           case kSimdPrefix: {
             CHECK_PROTOTYPE_OPCODE(wasm_simd_prototype);
             len++;
-            byte simd_index = *(pc_ + 1);
+            byte simd_index = checked_read_u8(pc_, 1, "simd index");
             opcode = static_cast<WasmOpcode>(opcode << 8 | simd_index);
             TRACE("  @%-4d #%02x #%02x:%-20s|", startrel(pc_), kSimdPrefix,
                   simd_index, WasmOpcodes::ShortOpcodeName(opcode));
             len += DecodeSimdOpcode(opcode);
             break;
           }
           default: {
             // Deal with special asmjs opcodes.
             if (module_ && module_->origin == kAsmJsOrigin) {
               sig = WasmOpcodes::AsmjsSignature(opcode);
@@ skipping 845 matching lines @@
 BitVector* AnalyzeLoopAssignmentForTesting(Zone* zone, size_t num_locals,
                                            const byte* start, const byte* end) {
   FunctionBody body = {nullptr, nullptr, nullptr, start, end};
   WasmFullDecoder decoder(zone, nullptr, body);
   return decoder.AnalyzeLoopAssignmentForTesting(start, num_locals);
 }
 
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8