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

BUG=chromium:658426
R=ahaas@chromium.org,titzer@chromium.org,gdeepti@chromium.org

Review-Url: https://codereview.chromium.org/2447683004
Cr-Commit-Position: refs/heads/master@{#40572}

src/wasm/ast-decoder.cc

@@ -324,7 +324,7 @@ class WasmDecoder : public Decoder {
       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) {
@@ -341,7 +341,8 @@ class WasmDecoder : public Decoder {
             return 3;
           }
           default:
-            UNREACHABLE();
+            error("invalid SIMD opcode");
+            return 2;
         }
       }
       default:
@@ -1139,7 +1140,7 @@ class WasmFullDecoder : public WasmDecoder {
           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));

test/unittests/wasm/ast-decoder-unittest.cc

@@ -2500,6 +2500,19 @@ TEST_F(WasmOpcodeLengthTest, SimpleExpressions) {
   EXPECT_LENGTH(1, kExprI64ReinterpretF64);
 }
 
+TEST_F(WasmOpcodeLengthTest, SimdExpressions) {
+#define TEST_SIMD(name, opcode, sig) \
+  EXPECT_LENGTH_N(2, kSimdPrefix, static_cast<byte>(kExpr##name & 0xff));
+  FOREACH_SIMD_0_OPERAND_OPCODE(TEST_SIMD)
+#undef TEST_SIMD
+#define TEST_SIMD(name, opcode, sig) \
+  EXPECT_LENGTH_N(3, kSimdPrefix, static_cast<byte>(kExpr##name & 0xff));
+  FOREACH_SIMD_1_OPERAND_OPCODE(TEST_SIMD)
+#undef TEST_SIMD
+  // test for bad simd opcode
+  EXPECT_LENGTH_N(2, kSimdPrefix, 0xff);
+}
+
 typedef ZoneVector<LocalType> LocalTypeMap;
 
 class LocalDeclDecoderTest : public TestWithZone {