v8-sandbox.h

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// Copyright 2024 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.
 
#ifndef INCLUDE_V8_SANDBOX_H_
#define INCLUDE_V8_SANDBOX_H_
 
#include <cstdint>
 
#include "v8-internal.h"  // NOLINT(build/include_directory)
#include "v8config.h"     // NOLINT(build/include_directory)
 
namespace v8 {
 
enum class CppHeapPointerTag : uint16_t {
  kFirstTag = 0,
  kNullTag = 0,
 
  kDefaultTag = 0x7000,
 
  kZappedEntryTag = 0x7ffd,
  kEvacuationEntryTag = 0x7ffe,
  kFreeEntryTag = 0x7fff,
  // The tags are limited to 15 bits, so the last tag is 0x7fff.
  kLastTag = 0x7fff,
};
 
using CppHeapPointerTagRange = internal::TagRange<CppHeapPointerTag>;
 
constexpr CppHeapPointerTagRange kAnyCppHeapPointer(
    CppHeapPointerTag::kFirstTag, CppHeapPointerTag::kLastTag);
 
class SandboxHardwareSupport {
 public:
  V8_EXPORT static void InitializeBeforeThreadCreation();
};
 
namespace internal {
 
#ifdef V8_COMPRESS_POINTERS
V8_INLINE static Address* GetCppHeapPointerTableBase(v8::Isolate* isolate) {
  Address addr = reinterpret_cast<Address>(isolate) +
                 Internals::kIsolateCppHeapPointerTableOffset +
                 Internals::kExternalPointerTableBasePointerOffset;
  return *reinterpret_cast<Address**>(addr);
}
#endif  // V8_COMPRESS_POINTERS
 
template <typename T>
V8_INLINE static T* ReadCppHeapPointerField(v8::Isolate* isolate,
                                            Address heap_object_ptr, int offset,
                                            CppHeapPointerTagRange tag_range) {
  // This is a specialized version of the the CppHeapPointerTable accessors
  // which (1) allows the code to be inlined into the callers for performance
  // and (2) is optimized for code size as there are a huge number of callers
  // from auto-generated bindings code.
 
#ifdef V8_COMPRESS_POINTERS
  const CppHeapPointerHandle handle =
      Internals::ReadRawField<CppHeapPointerHandle>(heap_object_ptr, offset);
  const uint32_t index = handle >> kExternalPointerIndexShift;
  const Address* table = GetCppHeapPointerTableBase(isolate);
  const std::atomic<Address>* ptr =
      reinterpret_cast<const std::atomic<Address>*>(&table[index]);
  Address entry = std::atomic_load_explicit(ptr, std::memory_order_relaxed);
 
  // Note: the cast to uint32_t is important here. Otherwise, the uint16_t's
  // would be promoted to int in the range check below, which would result in
  // undefined behavior (signed integer underflow) if the actual value is less
  // than the lower bound. Then, the compiler would take advantage of the
  // undefined behavior and turn the range check into a simple
  // `actual_tag <= last_tag` comparison, which is incorrect.
  uint32_t actual_tag = static_cast<uint16_t>(entry);
  // The actual_tag is shifted to the left by one and contains the marking
  // bit in the LSB. To ignore that during the type check, simply add one to
  // the (shifted) range.
  constexpr int kTagShift = internal::kCppHeapPointerTagShift;
  uint32_t first_tag = static_cast<uint32_t>(tag_range.first) << kTagShift;
  uint32_t last_tag = (static_cast<uint32_t>(tag_range.last) << kTagShift) + 1;
  if (V8_LIKELY(actual_tag >= first_tag && actual_tag <= last_tag)) {
    entry = entry >> kCppHeapPointerPayloadShift;
  } else {
    // If the type check failed, we simply return nullptr here. That way:
    //  1. The null handle always results in nullptr being returned here, which
    //     is a desired property. Otherwise, we would need an explicit check for
    //     the null handle above, and therefore an additional branch. This
    //     works because the 0th entry of the table always contains nullptr
    //     tagged with the null tag (i.e. an all-zeros entry). As such,
    //     regardless of whether the type check succeeds, the result will
    //     always be nullptr.
    //  2. The returned pointer is guaranteed to crash even on platforms with
    //     top byte ignore (TBI), such as Arm64. The alternative would be to
    //     simply return the original entry with the left-shifted payload.
    //     However, due to TBI, an access to that may not always result in a
    //     crash (specifically, if the second most significant byte happens to
    //     be zero). In addition, there shouldn't be a difference on Arm64
    //     between returning nullptr or the original entry, since it will
    //     simply compile to a `csel x0, x8, xzr, lo` instead of a
    //     `csel x0, x10, x8, lo` instruction.
    //  3. The machine code sequence ends up being pretty short, which is
    //     important here as this code will be inlined into a lot of functions.
    entry = 0;
  }
  return reinterpret_cast<T*>(entry);
#else   // !V8_COMPRESS_POINTERS
  return reinterpret_cast<T*>(
      Internals::ReadRawField<Address>(heap_object_ptr, offset));
#endif  // !V8_COMPRESS_POINTERS
}
 
// TODO(saelo): temporary workaround needed to introduce range-based type
// checks for the external pointer table. See comment above
// ExternalPointerCanBeEmpty(ExternalPointerTagRange) function for details.
V8_INLINE static constexpr bool ExternalPointerCanBeEmpty(
    CppHeapPointerTagRange tag_range) {
  return true;
}
 
}  // namespace internal
}  // namespace v8
 
#endif  // INCLUDE_V8_SANDBOX_H_