base/android/reached_addresses_bitset.cc

// Copyright 2019 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "base/android/reached_addresses_bitset.h"

#include "base/android/library_loader/anchor_functions.h"
#include "base/android/library_loader/anchor_functions_buildflags.h"
#include "base/check_op.h"
#include "base/numerics/safe_conversions.h"

namespace base {
namespace android {

namespace {
constexpr size_t kBitsPerElement = sizeof(uint32_t) * 8;

// Below an array of uint32_t in BSS is introduced and then casted to an array
// of std::atomic<uint32_t>. In C++20 constructing an std::atomic is not
// 'trivial'. See https://github.com/microsoft/STL/issues/661 for reasons of
// this change in the standard.
//
// Assert that both types have the same size. The sizes do not have to match
// according to a note in [atomics.types.generic] in C++17. With this assertion
// in place it is unlikely that the constructor produces the value other than
// (uint32_t)0.
static_assert(sizeof(uint32_t) == sizeof(std::atomic<uint32_t>), "");

// Keep the array in BSS only for non-official builds to avoid potential harm to
// data locality and unspecified behavior from the reinterpret_cast below.
// In order to start new experiments with base::Feature(ReachedCodeProfiler) on
// Canary/Dev this array will need to be reintroduced to official builds. When
// doing so, don't forget to update `kConfigurationSupported` in
// `reached_code_profiler.cc`
#if BUILDFLAG(SUPPORTS_CODE_ORDERING) && !defined(OFFICIAL_BUILD)
// Enough for 1 << 29 bytes of code, 512MB.
constexpr size_t kTextBitfieldSize = 1 << 20;
uint32_t g_text_bitfield[kTextBitfieldSize];
#endif
}  // namespace

// static
ReachedAddressesBitset* ReachedAddressesBitset::GetTextBitset() {
#if BUILDFLAG(SUPPORTS_CODE_ORDERING) && !defined(OFFICIAL_BUILD)
  static ReachedAddressesBitset text_bitset(
      kStartOfText, kEndOfText,
      reinterpret_cast<std::atomic<uint32_t>*>(g_text_bitfield),
      kTextBitfieldSize);
  return &text_bitset;
#else
  return nullptr;
#endif
}

void ReachedAddressesBitset::RecordAddress(uintptr_t address) {
  // |address| is outside of the range.
  if (address < start_address_ || address >= end_address_)
    return;

  size_t offset = static_cast<size_t>(address - start_address_);
  uint32_t offset_index = checked_cast<uint32_t>(offset / kBytesGranularity);

  // Atomically set the corresponding bit in the array.
  std::atomic<uint32_t>* element = reached_ + (offset_index / kBitsPerElement);

  // First, a racy check. This saves a CAS if the bit is already set, and
  // allows the cache line to remain shared across CPUs in this case.
  uint32_t value = element->load(std::memory_order_relaxed);
  uint32_t mask = 1 << (offset_index % kBitsPerElement);
  if (value & mask)
    return;
  element->fetch_or(mask, std::memory_order_relaxed);
}

std::vector<uint32_t> ReachedAddressesBitset::GetReachedOffsets() const {
  std::vector<uint32_t> offsets;
  const size_t elements = NumberOfReachableElements();

  for (size_t i = 0; i < elements; ++i) {
    uint32_t element = reached_[i].load(std::memory_order_relaxed);
    // No reached addresses at this element.
    if (element == 0)
      continue;

    for (size_t j = 0; j < 32; ++j) {
      if (!((element >> j) & 1))
        continue;

      size_t offset_index = i * 32 + j;
      size_t offset = offset_index * kBytesGranularity;
      offsets.push_back(checked_cast<uint32_t>(offset));
    }
  }

  return offsets;
}

ReachedAddressesBitset::ReachedAddressesBitset(
    uintptr_t start_address,
    uintptr_t end_address,
    std::atomic<uint32_t>* storage_ptr,
    size_t storage_size)
    : start_address_(start_address),
      end_address_(end_address),
      reached_(storage_ptr) {
  DCHECK_LE(start_address_, end_address_);
  DCHECK_LE(NumberOfReachableElements(), storage_size * kBitsPerElement);
}

size_t ReachedAddressesBitset::NumberOfReachableElements() const {
  size_t reachable_bits =
      (end_address_ + kBytesGranularity - 1) / kBytesGranularity -
      start_address_ / kBytesGranularity;
  return (reachable_bits + kBitsPerElement - 1) / kBitsPerElement;
}

}  // namespace android
}  // namespace base