src/hotspot/share/cds/cppVtables.cpp

/*
 * Copyright (c) 2020, 2025, 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.
 *
 * 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 "cds/archiveUtils.hpp"
#include "cds/archiveBuilder.hpp"
#include "cds/cdsConfig.hpp"
#include "cds/cppVtables.hpp"
#include "cds/metaspaceShared.hpp"
#include "logging/log.hpp"
#include "oops/instanceClassLoaderKlass.hpp"
#include "oops/instanceMirrorKlass.hpp"
#include "oops/instanceRefKlass.hpp"
#include "oops/instanceStackChunkKlass.hpp"
#include "oops/methodCounters.hpp"
#include "oops/methodData.hpp"
#include "oops/trainingData.hpp"
#include "oops/objArrayKlass.hpp"
#include "oops/typeArrayKlass.hpp"
#include "runtime/arguments.hpp"
#include "utilities/globalDefinitions.hpp"

// Objects of the Metadata types (such as Klass and ConstantPool) have C++ vtables.
// (In GCC this is the field <Type>::_vptr, i.e., first word in the object.)
//
// Addresses of the vtables and the methods may be different across JVM runs,
// if libjvm.so is dynamically loaded at a different base address.
//
// To ensure that the Metadata objects in the CDS archive always have the correct vtable:
//
// + at dump time:  we redirect the _vptr to point to our own vtables inside
//                  the CDS image
// + at run time:   we clone the actual contents of the vtables from libjvm.so
//                  into our own tables.

// Currently, the archive contains ONLY the following types of objects that have C++ vtables.
#define CPP_VTABLE_TYPES_DO(f) \
  f(ConstantPool) \
  f(InstanceKlass) \
  f(InstanceClassLoaderKlass) \
  f(InstanceMirrorKlass) \
  f(InstanceRefKlass) \
  f(InstanceStackChunkKlass) \
  f(Method) \
  f(MethodData) \
  f(MethodCounters) \
  f(ObjArrayKlass) \
  f(TypeArrayKlass) \
  f(KlassTrainingData) \
  f(MethodTrainingData) \
  f(CompileTrainingData)

class CppVtableInfo {
  intptr_t _vtable_size;
  intptr_t _cloned_vtable[1]; // Pseudo flexible array member.
  static size_t cloned_vtable_offset() { return offset_of(CppVtableInfo, _cloned_vtable); }
public:
  int vtable_size()           { return int(uintx(_vtable_size)); }
  void set_vtable_size(int n) { _vtable_size = intptr_t(n); }
  // Using _cloned_vtable[i] for i > 0 causes undefined behavior. We use address calculation instead.
  intptr_t* cloned_vtable()   { return (intptr_t*)((char*)this + cloned_vtable_offset()); }
  void zero()                 { memset(cloned_vtable(), 0, sizeof(intptr_t) * vtable_size()); }
  // Returns the address of the next CppVtableInfo that can be placed immediately after this CppVtableInfo
  static size_t byte_size(int vtable_size) {
    return cloned_vtable_offset() + (sizeof(intptr_t) * vtable_size);
  }
};

static inline intptr_t* vtable_of(const Metadata* m) {
  return *((intptr_t**)m);
}

template <class T> class CppVtableCloner {
  static int get_vtable_length(const char* name);

public:
  // Allocate a clone of the vtable of T from the shared metaspace;
  // Initialize the contents of this clone.
  static CppVtableInfo* allocate_and_initialize(const char* name);

  // Copy the contents of the vtable of T into info->_cloned_vtable;
  static void initialize(const char* name, CppVtableInfo* info);

  static void init_orig_cpp_vtptr(int kind);
};

template <class T>
CppVtableInfo* CppVtableCloner<T>::allocate_and_initialize(const char* name) {
  int n = get_vtable_length(name);
  CppVtableInfo* info =
      (CppVtableInfo*)ArchiveBuilder::current()->rw_region()->allocate(CppVtableInfo::byte_size(n));
  info->set_vtable_size(n);
  initialize(name, info);
  return info;
}

template <class T>
void CppVtableCloner<T>::initialize(const char* name, CppVtableInfo* info) {
  T tmp; // Allocate temporary dummy metadata object to get to the original vtable.
  int n = info->vtable_size();
  intptr_t* srcvtable = vtable_of(&tmp);
  intptr_t* dstvtable = info->cloned_vtable();

  // We already checked (and, if necessary, adjusted n) when the vtables were allocated, so we are
  // safe to do memcpy.
  log_debug(aot, vtables)("Copying %3d vtable entries for %s", n, name);
  memcpy(dstvtable, srcvtable, sizeof(intptr_t) * n);
}

// To determine the size of the vtable for each type, we use the following
// trick by declaring 2 subclasses:
//
//   class CppVtableTesterA: public InstanceKlass {virtual int   last_virtual_method() {return 1;}    };
//   class CppVtableTesterB: public InstanceKlass {virtual void* last_virtual_method() {return nullptr}; };
//
// CppVtableTesterA and CppVtableTesterB's vtables have the following properties:
// - Their size (N+1) is exactly one more than the size of InstanceKlass's vtable (N)
// - The first N entries have are exactly the same as in InstanceKlass's vtable.
// - Their last entry is different.
//
// So to determine the value of N, we just walk CppVtableTesterA and CppVtableTesterB's tables
// and find the first entry that's different.
//
// This works on all C++ compilers supported by Oracle, but you may need to tweak it for more
// esoteric compilers.

template <class T> class CppVtableTesterB: public T {
public:
  virtual int last_virtual_method() {return 1;}
};

template <class T> class CppVtableTesterA : public T {
public:
  virtual void* last_virtual_method() {
    // Make this different than CppVtableTesterB::last_virtual_method so the C++
    // compiler/linker won't alias the two functions.
    return nullptr;
  }
};

template <class T>
int CppVtableCloner<T>::get_vtable_length(const char* name) {
  CppVtableTesterA<T> a;
  CppVtableTesterB<T> b;

  intptr_t* avtable = vtable_of(&a);
  intptr_t* bvtable = vtable_of(&b);

  // Start at slot 1, because slot 0 may be RTTI (on Solaris/Sparc)
  int vtable_len = 1;
  for (; ; vtable_len++) {
    if (avtable[vtable_len] != bvtable[vtable_len]) {
      break;
    }
  }
  log_debug(aot, vtables)("Found   %3d vtable entries for %s", vtable_len, name);

  return vtable_len;
}

#define ALLOCATE_AND_INITIALIZE_VTABLE(c) \
  _index[c##_Kind] = CppVtableCloner<c>::allocate_and_initialize(#c); \
  ArchivePtrMarker::mark_pointer(&_index[c##_Kind]);

#define INITIALIZE_VTABLE(c) \
  CppVtableCloner<c>::initialize(#c, _index[c##_Kind]);

#define INIT_ORIG_CPP_VTPTRS(c) \
  CppVtableCloner<c>::init_orig_cpp_vtptr(c##_Kind);

#define DECLARE_CLONED_VTABLE_KIND(c) c ## _Kind,

enum ClonedVtableKind {
  // E.g., ConstantPool_Kind == 0, InstanceKlass_Kind == 1, etc.
  CPP_VTABLE_TYPES_DO(DECLARE_CLONED_VTABLE_KIND)
  _num_cloned_vtable_kinds
};

// _orig_cpp_vtptrs and _archived_cpp_vtptrs are used for type checking in
// CppVtables::get_archived_vtable().
//
// _orig_cpp_vtptrs is a map of all the original vtptrs. E.g., for
//     ConstantPool *cp = new (...) ConstantPool(...) ; // a dynamically allocated constant pool
// the following holds true:
//     _orig_cpp_vtptrs[ConstantPool_Kind] == ((intptr_t**)cp)[0]
//
// _archived_cpp_vtptrs is a map of all the vptprs used by classes in a preimage. E.g., for
//    InstanceKlass* k = a class loaded from the preimage;
//    ConstantPool* cp = k->constants();
// the following holds true:
//     _archived_cpp_vtptrs[ConstantPool_Kind] == ((intptr_t**)cp)[0]
static bool _orig_cpp_vtptrs_inited = false;
static intptr_t* _orig_cpp_vtptrs[_num_cloned_vtable_kinds];
static intptr_t* _archived_cpp_vtptrs[_num_cloned_vtable_kinds];

template <class T>
void CppVtableCloner<T>::init_orig_cpp_vtptr(int kind) {
  assert(kind < _num_cloned_vtable_kinds, "sanity");
  T tmp; // Allocate temporary dummy metadata object to get to the original vtable.
  intptr_t* srcvtable = vtable_of(&tmp);
  _orig_cpp_vtptrs[kind] = srcvtable;
}

// This is the index of all the cloned vtables. E.g., for
//     ConstantPool* cp = ....; // an archived constant pool
//     InstanceKlass* ik = ....;// an archived class
// the following holds true:
//     _index[ConstantPool_Kind]->cloned_vtable()  == ((intptr_t**)cp)[0]
//     _index[InstanceKlass_Kind]->cloned_vtable() == ((intptr_t**)ik)[0]
static CppVtableInfo* _index[_num_cloned_vtable_kinds];

// This marks the location in the archive where _index[0] is stored. This location
// will be stored as FileMapHeader::_cloned_vtables_offset into the archive header.
// Serviceability Agent uses this information to determine the vtables of
// archived Metadata objects.
char* CppVtables::_vtables_serialized_base = nullptr;

void CppVtables::dumptime_init(ArchiveBuilder* builder) {
  assert(CDSConfig::is_dumping_static_archive(), "cpp tables are only dumped into static archive");

  if (CDSConfig::is_dumping_final_static_archive()) {
    // When dumping final archive, _index[kind] at this point is in the preimage.
    // Remember these vtable pointers in _archived_cpp_vtptrs, as _index[kind] will now be rewritten
    // to point to the runtime vtable data.
    for (int i = 0; i < _num_cloned_vtable_kinds; i++) {
      assert(_index[i] != nullptr, "must have been restored by CppVtables::serialize()");
      _archived_cpp_vtptrs[i] = _index[i]->cloned_vtable();
    }
  } else {
    memset(_archived_cpp_vtptrs, 0, sizeof(_archived_cpp_vtptrs));
  }

  CPP_VTABLE_TYPES_DO(ALLOCATE_AND_INITIALIZE_VTABLE);

  size_t cpp_tables_size = builder->rw_region()->top() - builder->rw_region()->base();
  builder->alloc_stats()->record_cpp_vtables((int)cpp_tables_size);
}

void CppVtables::serialize(SerializeClosure* soc) {
  if (!soc->reading()) {
    _vtables_serialized_base = (char*)ArchiveBuilder::current()->buffer_top();
  }
  for (int i = 0; i < _num_cloned_vtable_kinds; i++) {
    soc->do_ptr(&_index[i]);
  }
  if (soc->reading()) {
    CPP_VTABLE_TYPES_DO(INITIALIZE_VTABLE);
  }
}

intptr_t* CppVtables::get_archived_vtable(MetaspaceObj::Type msotype, address obj) {
  if (!_orig_cpp_vtptrs_inited) {
    CPP_VTABLE_TYPES_DO(INIT_ORIG_CPP_VTPTRS);
    _orig_cpp_vtptrs_inited = true;
  }

  assert(CDSConfig::is_dumping_archive(), "sanity");
  int kind = -1;
  switch (msotype) {
  case MetaspaceObj::SymbolType:
  case MetaspaceObj::TypeArrayU1Type:
  case MetaspaceObj::TypeArrayU2Type:
  case MetaspaceObj::TypeArrayU4Type:
  case MetaspaceObj::TypeArrayU8Type:
  case MetaspaceObj::TypeArrayOtherType:
  case MetaspaceObj::ConstMethodType:
  case MetaspaceObj::ConstantPoolCacheType:
  case MetaspaceObj::AnnotationsType:
  case MetaspaceObj::RecordComponentType:
  case MetaspaceObj::AdapterHandlerEntryType:
  case MetaspaceObj::AdapterFingerPrintType:
    // These have no vtables.
    break;
  default:
    for (kind = 0; kind < _num_cloned_vtable_kinds; kind ++) {
      if (vtable_of((Metadata*)obj) == _orig_cpp_vtptrs[kind] ||
          vtable_of((Metadata*)obj) == _archived_cpp_vtptrs[kind]) {
        break;
      }
    }
    if (kind >= _num_cloned_vtable_kinds) {
      fatal("Cannot find C++ vtable for " INTPTR_FORMAT " -- you probably added"
            " a new subtype of Klass or MetaData without updating CPP_VTABLE_TYPES_DO or the cases in this 'switch' statement",
            p2i(obj));
    }
  }

  if (kind >= 0) {
    assert(kind < _num_cloned_vtable_kinds, "must be");
    return _index[kind]->cloned_vtable();
  } else {
    return nullptr;
  }
}

void CppVtables::zero_archived_vtables() {
  assert(CDSConfig::is_dumping_static_archive(), "cpp tables are only dumped into static archive");
  for (int kind = 0; kind < _num_cloned_vtable_kinds; kind ++) {
    _index[kind]->zero();
  }
}

bool CppVtables::is_valid_shared_method(const Method* m) {
  assert(MetaspaceShared::is_in_shared_metaspace(m), "must be");
  return vtable_of(m) == _index[Method_Kind]->cloned_vtable() ||
         vtable_of(m) == _archived_cpp_vtptrs[Method_Kind];
}