Prefetch the native library from native code by parsing /proc/pid/maps.

base/android/library_loader/library_prefetcher.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// 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 <sys/resource.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <utility>
+#include <vector>
+
+#include "base/posix/eintr_wrapper.h"
+#include "base/strings/string_number_conversions.h"
+#include "base/strings/string_util.h"
+
+namespace base {
+namespace android {
+
+NativeLibraryPrefetcher::FileLineIterator::FileLineIterator(
+    const base::FilePath& path) {
+  file_ = base::OpenFile(path, "r");
+}
+
+NativeLibraryPrefetcher::FileLineIterator::~FileLineIterator() {
+  if (file_) {
+    base::CloseFile(file_);
+  }
+}
+
+bool NativeLibraryPrefetcher::FileLineIterator::NextLine(std::string* line) {
+  if (!file_) {
+    return false;
+  }
+  char* line_str = fgets(line_buffer_, kMaxLineLength - 1, file_);
+  if (line_str) {
+    *line = std::string(line_str);
+    return true;
+  }
+  return false;
+}
+
+NativeLibraryPrefetcher::ProcMapsIterator::ProcMapsIterator(
+    FileLineIterator* line_iterator)
+    : line_iterator_(line_iterator) {
+}
+
+bool NativeLibraryPrefetcher::ProcMapsIterator::Next(
+    NativeLibraryPrefetcher::ProcMapsIterator::Mapping* mapping) {
+  std::string line;
+  bool ok = line_iterator_->NextLine(&line);
+  if (!ok) {
+    return false;
+  }
+  // Example:
+  // b6e62000-b6e64000 r-xp 00000000 b3:15 1204 /system/lib/libstdc++.so
+  // start-end flags offset device inode [filename]
+  std::vector<std::string> pieces;
+  base::SplitStringAlongWhitespace(line, &pieces);
+  size_t pieces_count = pieces.size();
+  // Filename is optional
+  if ((pieces_count != 5) && (pieces_count != 6)) {

[pasko, 2015/03/25 12:57:58]

a filename might contain a space, right?

[Benoit L, 2015/03/26 15:45:03]

Done.

+    return false;
+  }
+
+  std::vector<std::string> start_end;
+  base::SplitString(pieces[0], '-', &start_end);
+  if (start_end.size() != 2) {
+    return false;
+  }
+  ok = base::HexStringToUInt64(start_end[0], &mapping->start);
+  ok &= base::HexStringToUInt64(start_end[1], &mapping->end);
+  mapping->flags = pieces[1];
+  ok &= base::HexStringToUInt64(pieces[2], &mapping->offset);
+  mapping->device = pieces[3];
+  ok &= base::StringToInt64(pieces[4], &mapping->inode);
+  if (!ok) {
+    return false;
+  }
+  if (pieces_count == 5) {
+    mapping->filename = std::string();
+  } else {
+    mapping->filename = pieces[5];
+  }
+  return true;
+}
+
+namespace {
+
+// Android defines the background priority to this value since at least 2009
+// (see Process.java).
+const int kBackgroundPriority = 10;
+// Valid for all the Android architectures.
+const size_t kPageSize = 4096;
+// We may load directly from the APK.
+const int kSuffixesToMatchCount = 2;
+const std::string kSuffixesToMatch[] = {"libchrome.so", "base.apk"};
+
+bool IsReadableAndPrivate(const std::string& flags) {
+  return flags.size() == 4 && flags[0] == 'r' && flags[3] == 'p';
+}
+
+bool FilenameMatchesSuffix(const std::string& filename) {
+  for (int i = 0; i < kSuffixesToMatchCount; i++) {
+    if (EndsWith(filename, kSuffixesToMatch[i], true)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+// For each range, reads a byte per page to force it into the page cache.
+void Prefetch(const std::vector<std::pair<uint64_t, uint64_t>>& ranges) {
+  for (const auto& range : ranges) {
+    const uint64_t page_mask = kPageSize - 1;
+    CHECK(!(range.first & page_mask) && !(range.second & page_mask));
+    unsigned char* start_address = (unsigned char*) range.first;
+    unsigned char* end_address = (unsigned char*) range.second;
+    unsigned char dummy;
+    for (unsigned char* addr = start_address; addr < end_address;
+         addr += kPageSize) {
+      // Volatile is required to prevent the compiler from eliminating this
+      // loop.
+      dummy ^= *((volatile unsigned char*)addr);
+    }
+  }
+}
+
+}  // namespace
+
+std::vector<std::pair<uint64_t, uint64_t>>
+NativeLibraryPrefetcher::ProcMapsIterator::FindRanges(
+    FileLineIterator* file_line_iterator) {
+  std::vector<std::pair<uint64_t, uint64_t>> ranges;
+  ProcMapsIterator maps_iterator(file_line_iterator);
+  NativeLibraryPrefetcher::ProcMapsIterator::Mapping mapping;
+
+  while (maps_iterator.Next(&mapping)) {
+    if (FilenameMatchesSuffix(mapping.filename) &&
+        IsReadableAndPrivate(mapping.flags)) {
+      ranges.push_back(std::make_pair(mapping.start, mapping.end));
+    }
+  }
+  return ranges;
+}
+
+// Forks and waits for a process prefetching the native library. This is done in
+// a forked process for the following reasons:
+// - Isolating the main process from mistakes in the parsing. If the parsing
+//   returns an incorrect address, only the forked process will crash.
+// - Not inflating the memory used by the main process uselessly, which could
+//   increase its likelihood to be killed.
+// The forked process has background priority and, since it is not declared to
+// the android runtime, can be killed at any time, which is not an issue here.
+bool NativeLibraryPrefetcher::ForkAndPrefetchNativeLibrary() {
+  // Looking for ranges is done before the fork, to avoid syscalls and/or
+  // memory allocations in the forked process, that can be problematic.
+  FileLineIterator file_line_iterator(base::FilePath("/proc/self/maps"));
+  std::vector<std::pair<uint64_t, uint64_t>> ranges =
+      ProcMapsIterator::FindRanges(&file_line_iterator);
+  pid_t pid = fork();
+  if (pid == 0) {
+    setpriority(PRIO_PROCESS, 0, kBackgroundPriority);
+    Prefetch(ranges);
+    exit(0);
+  } else {
+    if (pid < 0) {
+      return false;
+    }
+    int status;
+    const pid_t result = HANDLE_EINTR(waitpid(pid, &status, 0));
+    if (result == pid) {
+      if (WIFEXITED(status)) {
+        return WEXITSTATUS(status) == 0;
+      }
+      if (WIFSIGNALED(status)) {

[pasko, 2015/03/25 12:57:58]

why checking this? we would have returned false anyway

[Benoit L, 2015/03/26 15:45:03]

To log whether the process was killed or crashed.
Done.

+        return false;
+      }
+    }
+    return false;
+  }
+}
+
+}  // namespace android
+}  // namespace base