[wasm] Implement importing of WebAssembly.Memory.

src/wasm/module-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/wasm/module-decoder.h"
 
 #include "src/base/functional.h"
 #include "src/base/platform/platform.h"
 #include "src/flags.h"
 #include "src/macro-assembler.h"
@@ skipping 292 matching lines @@
             module->function_tables.push_back(
                 {0, 0, std::vector<int32_t>(), true, false});
             expect_u8("element type", 0x20);
             WasmIndirectFunctionTable* table = &module->function_tables.back();
             consume_resizable_limits("element count", "elements", kMaxUInt32,
                                      &table->size, &table->max_size);
             break;
           }
           case kExternalMemory: {
             // ===== Imported memory =========================================
-            //            import->index =
-            //            static_cast<uint32_t>(module->memories.size());
-            // TODO(titzer): imported memories
+            consume_resizable_limits(
+                "memory", "pages", WasmModule::kMaxLegalPages,
+                &module->min_mem_pages, &module->max_mem_pages);
             break;
           }
           case kExternalGlobal: {
             // ===== Imported global =========================================
             import->index = static_cast<uint32_t>(module->globals.size());
             module->globals.push_back(
                 {kAstStmt, false, WasmInitExpr(), 0, true, false});
             WasmGlobal* global = &module->globals.back();
             global->type = consume_value_type();
             global->mutability = consume_u8("mutability") != 0;
@@ skipping 441 matching lines @@
     }
   }
 
   // Reads a length-prefixed string, checking that it is within bounds. Returns
   // the offset of the string, and the length as an out parameter.
   uint32_t consume_string(uint32_t* length, bool validate_utf8) {
     *length = consume_u32v("string length");
     uint32_t offset = pc_offset();
     const byte* string_start = pc_;
     // Consume bytes before validation to guarantee that the string is not oob.
-    consume_bytes(*length, "string");
+    if (*length > 0) consume_bytes(*length, "string");
     if (ok() && validate_utf8 &&
         !unibrow::Utf8::Validate(string_start, *length)) {
       error(string_start, "no valid UTF-8 string");
     }
     return offset;
   }
 
   uint32_t consume_sig_index(WasmModule* module, FunctionSig** sig) {
     const byte* pos = pc_;
     uint32_t sig_index = consume_u32v("signature index");
@@ skipping 333 matching lines @@
     decoder.consume_bytes(size);
   }
   if (decoder.more()) decoder.error("unexpected additional bytes");
 
   return decoder.toResult(std::move(table));
 }
 
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8