runtime/openjdkjvmti/ti_redefine.cc

/* Copyright (C) 2016 The Android Open Source Project
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This file implements interfaces from the file jvmti.h. This implementation
 * is licensed under the same terms as the file jvmti.h.  The
 * copyright and license information for the file jvmti.h follows.
 *
 * Copyright (c) 2003, 2011, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

#include "ti_redefine.h"

#include <limits>

#include "android-base/stringprintf.h"

#include "art_jvmti.h"
#include "base/logging.h"
#include "events-inl.h"
#include "gc/allocation_listener.h"
#include "instrumentation.h"
#include "jni_env_ext-inl.h"
#include "jvmti_allocator.h"
#include "mirror/class.h"
#include "mirror/class_ext.h"
#include "mirror/object.h"
#include "object_lock.h"
#include "runtime.h"
#include "ScopedLocalRef.h"

namespace openjdkjvmti {

using android::base::StringPrintf;

// Moves dex data to an anonymous, read-only mmap'd region.
std::unique_ptr<art::MemMap> Redefiner::MoveDataToMemMap(const std::string& original_location,
                                                         jint data_len,
                                                         unsigned char* dex_data,
                                                         std::string* error_msg) {
  std::unique_ptr<art::MemMap> map(art::MemMap::MapAnonymous(
      StringPrintf("%s-transformed", original_location.c_str()).c_str(),
      nullptr,
      data_len,
      PROT_READ|PROT_WRITE,
      /*low_4gb*/false,
      /*reuse*/false,
      error_msg));
  if (map == nullptr) {
    return map;
  }
  memcpy(map->Begin(), dex_data, data_len);
  // Make the dex files mmap read only. This matches how other DexFiles are mmaped and prevents
  // programs from corrupting it.
  map->Protect(PROT_READ);
  return map;
}

jvmtiError Redefiner::RedefineClass(ArtJvmTiEnv* env,
                                    art::Runtime* runtime,
                                    art::Thread* self,
                                    jclass klass,
                                    const std::string& original_dex_location,
                                    jint data_len,
                                    unsigned char* dex_data,
                                    std::string* error_msg) {
  std::unique_ptr<art::MemMap> map(MoveDataToMemMap(original_dex_location,
                                                    data_len,
                                                    dex_data,
                                                    error_msg));
  std::ostringstream os;
  char* generic_ptr_unused = nullptr;
  char* signature_ptr = nullptr;
  if (env->GetClassSignature(klass, &signature_ptr, &generic_ptr_unused) != OK) {
    signature_ptr = const_cast<char*>("<UNKNOWN CLASS>");
  }
  if (map.get() == nullptr) {
    os << "Failed to create anonymous mmap for modified dex file of class " << signature_ptr
       << "in dex file " << original_dex_location << " because: " << *error_msg;
    *error_msg = os.str();
    return ERR(OUT_OF_MEMORY);
  }
  if (map->Size() < sizeof(art::DexFile::Header)) {
    *error_msg = "Could not read dex file header because dex_data was too short";
    return ERR(INVALID_CLASS_FORMAT);
  }
  uint32_t checksum = reinterpret_cast<const art::DexFile::Header*>(map->Begin())->checksum_;
  std::unique_ptr<const art::DexFile> dex_file(art::DexFile::Open(map->GetName(),
                                                                  checksum,
                                                                  std::move(map),
                                                                  /*verify*/true,
                                                                  /*verify_checksum*/true,
                                                                  error_msg));
  if (dex_file.get() == nullptr) {
    os << "Unable to load modified dex file for " << signature_ptr << ": " << *error_msg;
    *error_msg = os.str();
    return ERR(INVALID_CLASS_FORMAT);
  }
  // Get shared mutator lock.
  art::ScopedObjectAccess soa(self);
  art::StackHandleScope<1> hs(self);
  Redefiner r(runtime, self, klass, signature_ptr, dex_file, error_msg);
  // Lock around this class to avoid races.
  art::ObjectLock<art::mirror::Class> lock(self, hs.NewHandle(r.GetMirrorClass()));
  return r.Run();
}

// TODO *MAJOR* This should return the actual source java.lang.DexFile object for the klass.
// TODO Make mirror of DexFile and associated types to make this less hellish.
// TODO Make mirror of BaseDexClassLoader and associated types to make this less hellish.
art::mirror::Object* Redefiner::FindSourceDexFileObject(
    art::Handle<art::mirror::ClassLoader> loader) {
  const char* dex_path_list_element_array_name = "[Ldalvik/system/DexPathList$Element;";
  const char* dex_path_list_element_name = "Ldalvik/system/DexPathList$Element;";
  const char* dex_file_name = "Ldalvik/system/DexFile;";
  const char* dex_path_list_name = "Ldalvik/system/DexPathList;";
  const char* dex_class_loader_name = "Ldalvik/system/BaseDexClassLoader;";

  CHECK(!self_->IsExceptionPending());
  art::StackHandleScope<11> hs(self_);
  art::ClassLinker* class_linker = runtime_->GetClassLinker();

  art::Handle<art::mirror::ClassLoader> null_loader(hs.NewHandle<art::mirror::ClassLoader>(
      nullptr));
  art::Handle<art::mirror::Class> base_dex_loader_class(hs.NewHandle(class_linker->FindClass(
      self_, dex_class_loader_name, null_loader)));

  // Get all the ArtFields so we can look in the BaseDexClassLoader
  art::ArtField* path_list_field = base_dex_loader_class->FindDeclaredInstanceField(
      "pathList", dex_path_list_name);
  CHECK(path_list_field != nullptr);

  art::ArtField* dex_path_list_element_field =
      class_linker->FindClass(self_, dex_path_list_name, null_loader)
        ->FindDeclaredInstanceField("dexElements", dex_path_list_element_array_name);
  CHECK(dex_path_list_element_field != nullptr);

  art::ArtField* element_dex_file_field =
      class_linker->FindClass(self_, dex_path_list_element_name, null_loader)
        ->FindDeclaredInstanceField("dexFile", dex_file_name);
  CHECK(element_dex_file_field != nullptr);

  // Check if loader is a BaseDexClassLoader
  art::Handle<art::mirror::Class> loader_class(hs.NewHandle(loader->GetClass()));
  if (!loader_class->IsSubClass(base_dex_loader_class.Get())) {
    LOG(ERROR) << "The classloader is not a BaseDexClassLoader which is currently the only "
               << "supported class loader type!";
    return nullptr;
  }
  // Start navigating the fields of the loader (now known to be a BaseDexClassLoader derivative)
  art::Handle<art::mirror::Object> path_list(
      hs.NewHandle(path_list_field->GetObject(loader.Get())));
  CHECK(path_list.Get() != nullptr);
  CHECK(!self_->IsExceptionPending());
  art::Handle<art::mirror::ObjectArray<art::mirror::Object>> dex_elements_list(hs.NewHandle(
      dex_path_list_element_field->GetObject(path_list.Get())->
      AsObjectArray<art::mirror::Object>()));
  CHECK(!self_->IsExceptionPending());
  CHECK(dex_elements_list.Get() != nullptr);
  size_t num_elements = dex_elements_list->GetLength();
  art::MutableHandle<art::mirror::Object> current_element(
      hs.NewHandle<art::mirror::Object>(nullptr));
  art::MutableHandle<art::mirror::Object> first_dex_file(
      hs.NewHandle<art::mirror::Object>(nullptr));
  // Iterate over the DexPathList$Element to find the right one
  // TODO Or not ATM just return the first one.
  for (size_t i = 0; i < num_elements; i++) {
    current_element.Assign(dex_elements_list->Get(i));
    CHECK(current_element.Get() != nullptr);
    CHECK(!self_->IsExceptionPending());
    CHECK(dex_elements_list.Get() != nullptr);
    CHECK_EQ(current_element->GetClass(), class_linker->FindClass(self_,
                                                                  dex_path_list_element_name,
                                                                  null_loader));
    // TODO It would be cleaner to put the art::DexFile into the dalvik.system.DexFile the class
    // comes from but it is more annoying because we would need to find this class. It is not
    // necessary for proper function since we just need to be in front of the classes old dex file
    // in the path.
    first_dex_file.Assign(element_dex_file_field->GetObject(current_element.Get()));
    if (first_dex_file.Get() != nullptr) {
      return first_dex_file.Get();
    }
  }
  return nullptr;
}

art::mirror::Class* Redefiner::GetMirrorClass() {
  return self_->DecodeJObject(klass_)->AsClass();
}

art::mirror::ClassLoader* Redefiner::GetClassLoader() {
  return GetMirrorClass()->GetClassLoader();
}

art::mirror::DexCache* Redefiner::CreateNewDexCache(art::Handle<art::mirror::ClassLoader> loader) {
  return runtime_->GetClassLinker()->RegisterDexFile(*dex_file_, loader.Get());
}

// TODO Really wishing I had that mirror of java.lang.DexFile now.
art::mirror::LongArray* Redefiner::AllocateDexFileCookie(
    art::Handle<art::mirror::Object> java_dex_file_obj) {
  art::StackHandleScope<2> hs(self_);
  // mCookie is nulled out if the DexFile has been closed but mInternalCookie sticks around until
  // the object is finalized. Since they always point to the same array if mCookie is not null we
  // just use the mInternalCookie field. We will update one or both of these fields later.
  // TODO Should I get the class from the classloader or directly?
  art::ArtField* internal_cookie_field = java_dex_file_obj->GetClass()->FindDeclaredInstanceField(
      "mInternalCookie", "Ljava/lang/Object;");
  // TODO Add check that mCookie is either null or same as mInternalCookie
  CHECK(internal_cookie_field != nullptr);
  art::Handle<art::mirror::LongArray> cookie(
      hs.NewHandle(internal_cookie_field->GetObject(java_dex_file_obj.Get())->AsLongArray()));
  // TODO Maybe make these non-fatal.
  CHECK(cookie.Get() != nullptr);
  CHECK_GE(cookie->GetLength(), 1);
  art::Handle<art::mirror::LongArray> new_cookie(
      hs.NewHandle(art::mirror::LongArray::Alloc(self_, cookie->GetLength() + 1)));
  if (new_cookie.Get() == nullptr) {
    self_->AssertPendingOOMException();
    return nullptr;
  }
  // Copy the oat-dex field at the start.
  // TODO Should I clear this field?
  // TODO This is a really crappy thing here with the first element being different.
  new_cookie->SetWithoutChecks<false>(0, cookie->GetWithoutChecks(0));
  new_cookie->SetWithoutChecks<false>(
      1, static_cast<int64_t>(reinterpret_cast<intptr_t>(dex_file_.get())));
  new_cookie->Memcpy(2, cookie.Get(), 1, cookie->GetLength() - 1);
  return new_cookie.Get();
}

void Redefiner::RecordFailure(jvmtiError result, const std::string& error_msg) {
  *error_msg_ = StringPrintf("Unable to perform redefinition of '%s': %s",
                             class_sig_,
                             error_msg.c_str());
  result_ = result;
}

bool Redefiner::FinishRemainingAllocations(
    /*out*/art::MutableHandle<art::mirror::ClassLoader>* source_class_loader,
    /*out*/art::MutableHandle<art::mirror::Object>* java_dex_file_obj,
    /*out*/art::MutableHandle<art::mirror::LongArray>* new_dex_file_cookie,
    /*out*/art::MutableHandle<art::mirror::DexCache>* new_dex_cache) {
  art::StackHandleScope<4> hs(self_);
  // This shouldn't allocate
  art::Handle<art::mirror::ClassLoader> loader(hs.NewHandle(GetClassLoader()));
  if (loader.Get() == nullptr) {
    // TODO Better error msg.
    RecordFailure(ERR(INTERNAL), "Unable to find class loader!");
    return false;
  }
  art::Handle<art::mirror::Object> dex_file_obj(hs.NewHandle(FindSourceDexFileObject(loader)));
  if (dex_file_obj.Get() == nullptr) {
    // TODO Better error msg.
    RecordFailure(ERR(INTERNAL), "Unable to find class loader!");
    return false;
  }
  art::Handle<art::mirror::LongArray> new_cookie(hs.NewHandle(AllocateDexFileCookie(dex_file_obj)));
  if (new_cookie.Get() == nullptr) {
    self_->AssertPendingOOMException();
    self_->ClearException();
    RecordFailure(ERR(OUT_OF_MEMORY), "Unable to allocate dex file array for class loader");
    return false;
  }
  art::Handle<art::mirror::DexCache> dex_cache(hs.NewHandle(CreateNewDexCache(loader)));
  if (dex_cache.Get() == nullptr) {
    self_->AssertPendingOOMException();
    self_->ClearException();
    RecordFailure(ERR(OUT_OF_MEMORY), "Unable to allocate DexCache");
    return false;
  }
  source_class_loader->Assign(loader.Get());
  java_dex_file_obj->Assign(dex_file_obj.Get());
  new_dex_file_cookie->Assign(new_cookie.Get());
  new_dex_cache->Assign(dex_cache.Get());
  return true;
}

jvmtiError Redefiner::Run() {
  art::StackHandleScope<5> hs(self_);
  // TODO We might want to have a global lock (or one based on the class being redefined at least)
  // in order to make cleanup easier. Not a huge deal though.
  //
  // First we just allocate the ClassExt and its fields that we need. These can be updated
  // atomically without any issues (since we allocate the map arrays as empty) so we don't bother
  // doing a try loop. The other allocations we need to ensure that nothing has changed in the time
  // between allocating them and pausing all threads before we can update them so we need to do a
  // try loop.
  if (!EnsureRedefinitionIsValid() || !EnsureClassAllocationsFinished()) {
    return result_;
  }
  art::MutableHandle<art::mirror::ClassLoader> source_class_loader(
      hs.NewHandle<art::mirror::ClassLoader>(nullptr));
  art::MutableHandle<art::mirror::Object> java_dex_file(
      hs.NewHandle<art::mirror::Object>(nullptr));
  art::MutableHandle<art::mirror::LongArray> new_dex_file_cookie(
      hs.NewHandle<art::mirror::LongArray>(nullptr));
  art::MutableHandle<art::mirror::DexCache> new_dex_cache(
      hs.NewHandle<art::mirror::DexCache>(nullptr));
  if (!FinishRemainingAllocations(&source_class_loader,
                                  &java_dex_file,
                                  &new_dex_file_cookie,
                                  &new_dex_cache)) {
    // TODO Null out the ClassExt fields we allocated (if possible, might be racing with another
    // redefineclass call which made it even bigger. Leak shouldn't be huge (2x array of size
    // declared_methods_.length) but would be good to get rid of.
    // new_dex_file_cookie & new_dex_cache should be cleaned up by the GC.
    return result_;
  }
  // Get the mirror class now that we aren't allocating anymore.
  art::Handle<art::mirror::Class> art_class(hs.NewHandle(GetMirrorClass()));
  // Enable assertion that this thread isn't interrupted during this installation.
  // After this we will need to do real cleanup in case of failure. Prior to this we could simply
  // return and would let everything get cleaned up or harmlessly leaked.
  // Do transition to final suspension
  // TODO We might want to give this its own suspended state!
  // TODO This isn't right. We need to change state without any chance of suspend ideally!
  self_->TransitionFromRunnableToSuspended(art::ThreadState::kNative);
  runtime_->GetThreadList()->SuspendAll(
      "Final installation of redefined Class!", /*long_suspend*/true);
  // TODO Might want to move this into a different type.
  // Now we reach the part where we must do active cleanup if something fails.
  // TODO We should really Retry if this fails instead of simply aborting.
  // Set the new DexFileCookie returns the original so we can fix it back up if redefinition fails
  art::ObjPtr<art::mirror::LongArray> original_dex_file_cookie(nullptr);
  if (!UpdateJavaDexFile(java_dex_file.Get(),
                         new_dex_file_cookie.Get(),
                         &original_dex_file_cookie)) {
    // Release suspendAll
    runtime_->GetThreadList()->ResumeAll();
    // Get back shared mutator lock as expected for return.
    self_->TransitionFromSuspendedToRunnable();
    return result_;
  }
  if (!UpdateClass(art_class.Get(), new_dex_cache.Get())) {
    // TODO Should have some form of scope to do this.
    RestoreJavaDexFile(java_dex_file.Get(), original_dex_file_cookie);
    // Release suspendAll
    runtime_->GetThreadList()->ResumeAll();
    // Get back shared mutator lock as expected for return.
    self_->TransitionFromSuspendedToRunnable();
    return result_;
  }
  // Update the ClassObjects Keep the old DexCache (and other stuff) around so we can restore
  // functions/fields.
  // Verify the new Class.
  //   Failure then undo updates to class
  // Do stack walks and allocate obsolete methods
  // Shrink the obsolete method maps if possible?
  // TODO find appropriate class loader. Allocate new dex files array. Pause all java treads.
  // Replace dex files array. Do stack scan + allocate obsoletes. Remove array if possible.
  // TODO We might want to ensure that all threads are stopped for this!
  // AddDexToClassPath();
  // TODO
  // Release suspendAll
  // TODO Put this into a scoped thing.
  runtime_->GetThreadList()->ResumeAll();
  // Get back shared mutator lock as expected for return.
  self_->TransitionFromSuspendedToRunnable();
  // TODO Do this at a more reasonable place.
  dex_file_.release();
  return OK;
}

void Redefiner::RestoreJavaDexFile(art::ObjPtr<art::mirror::Object> java_dex_file,
                                   art::ObjPtr<art::mirror::LongArray> orig_cookie) {
  art::ArtField* internal_cookie_field = java_dex_file->GetClass()->FindDeclaredInstanceField(
      "mInternalCookie", "Ljava/lang/Object;");
  art::ArtField* cookie_field = java_dex_file->GetClass()->FindDeclaredInstanceField(
      "mCookie", "Ljava/lang/Object;");
  art::ObjPtr<art::mirror::LongArray> new_cookie(
      cookie_field->GetObject(java_dex_file)->AsLongArray());
  internal_cookie_field->SetObject<false>(java_dex_file, orig_cookie);
  if (!new_cookie.IsNull()) {
    cookie_field->SetObject<false>(java_dex_file, orig_cookie);
  }
}

// Performs updates to class that will allow us to verify it.
bool Redefiner::UpdateClass(art::ObjPtr<art::mirror::Class> mclass,
                            art::ObjPtr<art::mirror::DexCache> new_dex_cache) {
  art::ClassLinker* linker = runtime_->GetClassLinker();
  art::PointerSize image_pointer_size = linker->GetImagePointerSize();
  const art::DexFile::ClassDef* class_def = art::OatFile::OatDexFile::FindClassDef(
      *dex_file_, class_sig_, art::ComputeModifiedUtf8Hash(class_sig_));
  if (class_def == nullptr) {
    RecordFailure(ERR(INVALID_CLASS_FORMAT), "Unable to find ClassDef!");
    return false;
  }
  const art::DexFile::TypeId& declaring_class_id = dex_file_->GetTypeId(class_def->class_idx_);
  const art::DexFile& old_dex_file = mclass->GetDexFile();
  for (art::ArtMethod& method : mclass->GetMethods(image_pointer_size)) {
    const art::DexFile::StringId* new_name_id = dex_file_->FindStringId(method.GetName());
    art::dex::TypeIndex method_return_idx =
        dex_file_->GetIndexForTypeId(*dex_file_->FindTypeId(method.GetReturnTypeDescriptor()));
    const auto* old_type_list = method.GetParameterTypeList();
    std::vector<art::dex::TypeIndex> new_type_list;
    for (uint32_t i = 0; old_type_list != nullptr && i < old_type_list->Size(); i++) {
      new_type_list.push_back(
          dex_file_->GetIndexForTypeId(
              *dex_file_->FindTypeId(
                  old_dex_file.GetTypeDescriptor(
                      old_dex_file.GetTypeId(
                          old_type_list->GetTypeItem(i).type_idx_)))));
    }
    const art::DexFile::ProtoId* proto_id = dex_file_->FindProtoId(method_return_idx,
                                                                   new_type_list);
    CHECK(proto_id != nullptr || old_type_list == nullptr);
    // TODO Return false, cleanup.
    const art::DexFile::MethodId* method_id = dex_file_->FindMethodId(declaring_class_id,
                                                                      *new_name_id,
                                                                      *proto_id);
    CHECK(method_id != nullptr);
    // TODO Return false, cleanup.
    uint32_t dex_method_idx = dex_file_->GetIndexForMethodId(*method_id);
    method.SetDexMethodIndex(dex_method_idx);
    linker->SetEntryPointsToInterpreter(&method);
    method.SetCodeItemOffset(dex_file_->FindCodeItemOffset(*class_def, dex_method_idx));
    method.SetDexCacheResolvedMethods(new_dex_cache->GetResolvedMethods(), image_pointer_size);
    method.SetDexCacheResolvedTypes(new_dex_cache->GetResolvedTypes(), image_pointer_size);
  }
  // Update the class fields.
  // Need to update class last since the ArtMethod gets its DexFile from the class (which is needed
  // to call GetReturnTypeDescriptor and GetParameterTypeList above).
  mclass->SetDexCache(new_dex_cache.Ptr());
  mclass->SetDexCacheStrings(new_dex_cache->GetStrings());
  mclass->SetDexClassDefIndex(dex_file_->GetIndexForClassDef(*class_def));
  mclass->SetDexTypeIndex(dex_file_->GetIndexForTypeId(*dex_file_->FindTypeId(class_sig_)));
  return true;
}

bool Redefiner::UpdateJavaDexFile(art::ObjPtr<art::mirror::Object> java_dex_file,
                                  art::ObjPtr<art::mirror::LongArray> new_cookie,
                                  /*out*/art::ObjPtr<art::mirror::LongArray>* original_cookie) {
  art::ArtField* internal_cookie_field = java_dex_file->GetClass()->FindDeclaredInstanceField(
      "mInternalCookie", "Ljava/lang/Object;");
  art::ArtField* cookie_field = java_dex_file->GetClass()->FindDeclaredInstanceField(
      "mCookie", "Ljava/lang/Object;");
  CHECK(internal_cookie_field != nullptr);
  art::ObjPtr<art::mirror::LongArray> orig_internal_cookie(
      internal_cookie_field->GetObject(java_dex_file)->AsLongArray());
  art::ObjPtr<art::mirror::LongArray> orig_cookie(
      cookie_field->GetObject(java_dex_file)->AsLongArray());
  internal_cookie_field->SetObject<false>(java_dex_file, new_cookie);
  *original_cookie = orig_internal_cookie;
  if (!orig_cookie.IsNull()) {
    cookie_field->SetObject<false>(java_dex_file, new_cookie);
  }
  return true;
}

// This function does all (java) allocations we need to do for the Class being redefined.
// TODO Change this name maybe?
bool Redefiner::EnsureClassAllocationsFinished() {
  art::StackHandleScope<2> hs(self_);
  art::Handle<art::mirror::Class> klass(hs.NewHandle(self_->DecodeJObject(klass_)->AsClass()));
  if (klass.Get() == nullptr) {
    RecordFailure(ERR(INVALID_CLASS), "Unable to decode class argument!");
    return false;
  }
  // Allocate the classExt
  art::Handle<art::mirror::ClassExt> ext(hs.NewHandle(klass->EnsureExtDataPresent(self_)));
  if (ext.Get() == nullptr) {
    // No memory. Clear exception (it's not useful) and return error.
    // TODO This doesn't need to be fatal. We could just not support obsolete methods after hitting
    // this case.
    self_->AssertPendingOOMException();
    self_->ClearException();
    RecordFailure(ERR(OUT_OF_MEMORY), "Could not allocate ClassExt");
    return false;
  }
  // Allocate the 2 arrays that make up the obsolete methods map.  Since the contents of the arrays
  // are only modified when all threads (other than the modifying one) are suspended we don't need
  // to worry about missing the unsyncronized writes to the array. We do synchronize when setting it
  // however, since that can happen at any time.
  // TODO Clear these after we walk the stacks in order to free them in the (likely?) event there
  // are no obsolete methods.
  {
    art::ObjectLock<art::mirror::ClassExt> lock(self_, ext);
    if (!ext->ExtendObsoleteArrays(
          self_, klass->GetDeclaredMethodsSlice(art::kRuntimePointerSize).size())) {
      // OOM. Clear exception and return error.
      self_->AssertPendingOOMException();
      self_->ClearException();
      RecordFailure(ERR(OUT_OF_MEMORY), "Unable to allocate/extend obsolete methods map");
      return false;
    }
  }
  return true;
}

}  // namespace openjdkjvmti