library_prefetcher.cc

// Copyright 2015 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/library_loader/library_prefetcher.h"

#include <stddef.h>
#include <sys/mman.h>
#include <sys/resource.h>
#include <sys/wait.h>
#include <unistd.h>

#include <atomic>
#include <cstdlib>
#include <memory>
#include <utility>
#include <vector>

#include "base/android/library_loader/anchor_functions.h"
#include "base/android/orderfile/orderfile_buildflags.h"
#include "base/bits.h"
#include "base/files/file.h"
#include "base/format_macros.h"
#include "base/logging.h"
#include "base/posix/eintr_wrapper.h"
#include "base/process/process_metrics.h"
#include "base/ranges/algorithm.h"
#include "base/strings/string_util.h"
#include "base/strings/stringprintf.h"
#include "build/build_config.h"

#if BUILDFLAG(ORDERFILE_INSTRUMENTATION)
#include "base/android/orderfile/orderfile_instrumentation.h"
#endif

#if BUILDFLAG(SUPPORTS_CODE_ORDERING)

namespace base {
namespace android {

namespace {

// Valid for all Android architectures.
constexpr size_t kPageSize = 4096;

// Populates the per-page residency between |start| and |end| in |residency|. If
// successful, |residency| has the size of |end| - |start| in pages.
// Returns true for success.
bool Mincore(size_t start, size_t end, std::vector<unsigned char>* residency) {
  if (start % kPageSize || end % kPageSize)
    return false;
  size_t size = end - start;
  size_t size_in_pages = size / kPageSize;
  if (residency->size() != size_in_pages)
    residency->resize(size_in_pages);
  int err = HANDLE_EINTR(
      mincore(reinterpret_cast<void*>(start), size, &(*residency)[0]));
  PLOG_IF(ERROR, err) << "mincore() failed";
  return !err;
}

// Returns the start and end of .text, aligned to the lower and upper page
// boundaries, respectively.
std::pair<size_t, size_t> GetTextRange() {
  // |kStartOfText| may not be at the beginning of a page, since .plt can be
  // before it, yet in the same mapping for instance.
  size_t start_page = kStartOfText - kStartOfText % kPageSize;
  // Set the end to the page on which the beginning of the last symbol is. The
  // actual symbol may spill into the next page by a few bytes, but this is
  // outside of the executable code range anyway.
  size_t end_page = bits::AlignUp(kEndOfText, kPageSize);
  return {start_page, end_page};
}

// Returns the start and end pages of the unordered section of .text, aligned to
// lower and upper page boundaries, respectively.
std::pair<size_t, size_t> GetOrderedTextRange() {
  size_t start_page = kStartOfOrderedText - kStartOfOrderedText % kPageSize;
  // kEndOfUnorderedText is not considered ordered, but the byte immediately
  // before is considered ordered and so can not be contained in the start page.
  size_t end_page = bits::AlignUp(kEndOfOrderedText, kPageSize);
  return {start_page, end_page};
}

// Calls madvise(advice) on the specified range. Does nothing if the range is
// empty.
void MadviseOnRange(const std::pair<size_t, size_t>& range, int advice) {
  if (range.first >= range.second) {
    return;
  }
  size_t size = range.second - range.first;
  int err = madvise(reinterpret_cast<void*>(range.first), size, advice);
  if (err) {
    PLOG(ERROR) << "madvise() failed";
  }
}

// Timestamp in ns since Unix Epoch, and residency, as returned by mincore().
struct TimestampAndResidency {
  uint64_t timestamp_nanos;
  std::vector<unsigned char> residency;

  TimestampAndResidency(uint64_t timestamp_nanos,
                        std::vector<unsigned char>&& residency)
      : timestamp_nanos(timestamp_nanos), residency(residency) {}
};

// Returns true for success.
bool CollectResidency(size_t start,
                      size_t end,
                      std::vector<TimestampAndResidency>* data) {
  // Not using TimeTicks() to not call too many base:: symbol that would pollute
  // the reached symbols dumps.
  struct timespec ts;
  if (HANDLE_EINTR(clock_gettime(CLOCK_MONOTONIC, &ts))) {
    PLOG(ERROR) << "Cannot get the time.";
    return false;
  }
  uint64_t now = static_cast<uint64_t>(ts.tv_sec) * 1000 * 1000 * 1000 +
                 static_cast<uint64_t>(ts.tv_nsec);
  std::vector<unsigned char> residency;
  if (!Mincore(start, end, &residency))
    return false;

  data->emplace_back(now, std::move(residency));
  return true;
}

void DumpResidency(size_t start,
                   size_t end,
                   std::unique_ptr<std::vector<TimestampAndResidency>> data) {
  LOG(WARNING) << "Dumping native library residency";
  auto path = FilePath(
      StringPrintf("/data/local/tmp/chrome/residency-%d.txt", getpid()));
  auto file = File(path, File::FLAG_CREATE_ALWAYS | File::FLAG_WRITE);
  if (!file.IsValid()) {
    PLOG(ERROR) << "Cannot open file to dump the residency data "
                << path.value();
    return;
  }

  // First line: start-end of text range.
  CHECK(AreAnchorsSane());
  CHECK_LE(start, kStartOfText);
  CHECK_LE(kEndOfText, end);
  auto start_end = StringPrintf("%" PRIuS " %" PRIuS "\n", kStartOfText - start,
                                kEndOfText - start);
  file.WriteAtCurrentPos(start_end.c_str(), static_cast<int>(start_end.size()));

  for (const auto& data_point : *data) {
    auto timestamp = StringPrintf("%" PRIu64 " ", data_point.timestamp_nanos);
    file.WriteAtCurrentPos(timestamp.c_str(),
                           static_cast<int>(timestamp.size()));

    std::vector<char> dump;
    dump.reserve(data_point.residency.size() + 1);
    for (auto c : data_point.residency)
      dump.push_back(c ? '1' : '0');
    dump[dump.size() - 1] = '\n';
    file.WriteAtCurrentPos(&dump[0], checked_cast<int>(dump.size()));
  }
}

#if !BUILDFLAG(ORDERFILE_INSTRUMENTATION)
// Reads a byte per page between |start| and |end| to force it into the page
// cache.
// Heap allocations, syscalls and library functions are not allowed in this
// function.
// Returns true for success.
#if defined(ADDRESS_SANITIZER)
// Disable AddressSanitizer instrumentation for this function. It is touching
// memory that hasn't been allocated by the app, though the addresses are
// valid. Furthermore, this takes place in a child process. See crbug.com/653372
// for the context.
__attribute__((no_sanitize_address))
#endif
void Prefetch(size_t start, size_t end) {
  unsigned char* start_ptr = reinterpret_cast<unsigned char*>(start);
  unsigned char* end_ptr = reinterpret_cast<unsigned char*>(end);
  [[maybe_unused]] unsigned char dummy = 0;
  for (unsigned char* ptr = start_ptr; ptr < end_ptr; ptr += kPageSize) {
    // Volatile is required to prevent the compiler from eliminating this
    // loop.
    dummy ^= *static_cast<volatile unsigned char*>(ptr);
  }
}

// These values were used in the past for recording
// "LibraryLoader.PrefetchDetailedStatus".
enum class PrefetchStatus {
  kSuccess = 0,
  kWrongOrdering = 1,
  kForkFailed = 2,
  kChildProcessCrashed = 3,
  kChildProcessKilled = 4,
  kMaxValue = kChildProcessKilled
};

PrefetchStatus ForkAndPrefetch(bool ordered_only) {
  if (!IsOrderingSane()) {
    LOG(WARNING) << "Incorrect code ordering";
    return PrefetchStatus::kWrongOrdering;
  }

  // Looking for ranges is done before the fork, to avoid syscalls and/or memory
  // allocations in the forked process. The child process inherits the lock
  // state of its parent thread. It cannot rely on being able to acquire any
  // lock (unless special care is taken in a pre-fork handler), including being
  // able to call malloc().
  //
  // Always prefetch the ordered section first, as it's reached early during
  // startup, and not necessarily located at the beginning of .text.
  std::vector<std::pair<size_t, size_t>> ranges = {GetOrderedTextRange()};
  if (!ordered_only)
    ranges.push_back(GetTextRange());

  pid_t pid = fork();
  if (pid == 0) {
    // Android defines the background priority to this value since at least 2009
    // (see Process.java).
    constexpr int kBackgroundPriority = 10;
    setpriority(PRIO_PROCESS, 0, kBackgroundPriority);
    // _exit() doesn't call the atexit() handlers.
    for (const auto& range : ranges) {
      Prefetch(range.first, range.second);
    }
    _exit(EXIT_SUCCESS);
  } else {
    if (pid < 0) {
      return PrefetchStatus::kForkFailed;
    }
    int status;
    const pid_t result = HANDLE_EINTR(waitpid(pid, &status, 0));
    if (result == pid) {
      if (WIFEXITED(status))
        return PrefetchStatus::kSuccess;
      if (WIFSIGNALED(status)) {
        int signal = WTERMSIG(status);
        switch (signal) {
          case SIGSEGV:
          case SIGBUS:
            return PrefetchStatus::kChildProcessCrashed;
          case SIGKILL:
          case SIGTERM:
          default:
            return PrefetchStatus::kChildProcessKilled;
        }
      }
    }
    // Should not happen. Per man waitpid(2), errors are:
    // - EINTR: handled.
    // - ECHILD if the process doesn't have an unwaited-for child with this PID.
    // - EINVAL.
    return PrefetchStatus::kChildProcessKilled;
  }
}
#endif  // !BUILDFLAG(ORDERFILE_INSTRUMENTATION)

}  // namespace

// static
void NativeLibraryPrefetcher::ForkAndPrefetchNativeLibrary(bool ordered_only) {
#if BUILDFLAG(ORDERFILE_INSTRUMENTATION)
  // Avoid forking with orderfile instrumentation because the child process
  // would create a dump as well.
  return;
#else
  PrefetchStatus status = ForkAndPrefetch(ordered_only);
  if (status != PrefetchStatus::kSuccess) {
    LOG(WARNING) << "Cannot prefetch the library. status = "
                 << static_cast<int>(status);
  }
#endif  // BUILDFLAG(ORDERFILE_INSTRUMENTATION)
}

// static
int NativeLibraryPrefetcher::PercentageOfResidentCode(size_t start,
                                                      size_t end) {
  size_t total_pages = 0;
  size_t resident_pages = 0;

  std::vector<unsigned char> residency;
  bool ok = Mincore(start, end, &residency);
  if (!ok)
    return -1;
  total_pages += residency.size();
  resident_pages += static_cast<size_t>(
      ranges::count_if(residency, [](unsigned char x) { return x & 1; }));
  if (total_pages == 0)
    return -1;
  return static_cast<int>((100 * resident_pages) / total_pages);
}

// static
int NativeLibraryPrefetcher::PercentageOfResidentNativeLibraryCode() {
  if (!AreAnchorsSane()) {
    LOG(WARNING) << "Incorrect code ordering";
    return -1;
  }
  const auto& range = GetTextRange();
  return PercentageOfResidentCode(range.first, range.second);
}

// static
void NativeLibraryPrefetcher::PeriodicallyCollectResidency() {
  CHECK_EQ(static_cast<long>(kPageSize), sysconf(_SC_PAGESIZE));

  LOG(WARNING) << "Spawning thread to periodically collect residency";
  const auto& range = GetTextRange();
  auto data = std::make_unique<std::vector<TimestampAndResidency>>();
  // Collect residency for about minute (the actual time spent collecting
  // residency can vary, so this is only approximate).
  for (int i = 0; i < 120; ++i) {
    if (!CollectResidency(range.first, range.second, data.get()))
      return;
    usleep(5e5);
  }
  DumpResidency(range.first, range.second, std::move(data));
}

// static
void NativeLibraryPrefetcher::MadviseForOrderfile() {
  if (!IsOrderingSane()) {
    LOG(WARNING) << "Code not ordered, madvise optimization skipped";
    return;
  }
  // First MADV_RANDOM on all of text, then turn the ordered text range back to
  // normal. The ordered range may be placed anywhere within .text.
  MadviseOnRange(GetTextRange(), MADV_RANDOM);
  MadviseOnRange(GetOrderedTextRange(), MADV_NORMAL);
}

// static
void NativeLibraryPrefetcher::MadviseForResidencyCollection() {
  if (!AreAnchorsSane()) {
    LOG(WARNING) << "Code not ordered, cannot madvise";
    return;
  }
  LOG(WARNING) << "Performing madvise for residency collection";
  MadviseOnRange(GetTextRange(), MADV_RANDOM);
}

}  // namespace android
}  // namespace base
#endif  // BUILDFLAG(SUPPORTS_CODE_ORDERING)