gd/os/linux_generic/files.cc

/*
 * Copyright 2020 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "os/files.h"

#include <fcntl.h>
#include <libgen.h>
#include <sys/stat.h>
#include <unistd.h>

#include <cerrno>
#include <cstring>
#include <fstream>
#include <streambuf>
#include <string>

#include "os/log.h"

namespace {

void HandleError(const std::string& temp_path, int* dir_fd, FILE** fp) {
  // This indicates there is a write issue.  Unlink as partial data is not
  // acceptable.
  unlink(temp_path.c_str());
  if (*fp) {
    fclose(*fp);
    *fp = nullptr;
  }
  if (*dir_fd != -1) {
    close(*dir_fd);
    *dir_fd = -1;
  }
}

}  // namespace

namespace bluetooth {
namespace os {

bool FileExists(const std::string& path) {
  std::ifstream input(path, std::ios::binary | std::ios::ate);
  return input.good();
}

bool RenameFile(const std::string& from, const std::string& to) {
  if (std::rename(from.c_str(), to.c_str()) != 0) {
    LOG_ERROR("unable to rename file from '%s' to '%s', error: %s", from.c_str(), to.c_str(), strerror(errno));
    return false;
  }
  return true;
}

std::optional<std::string> ReadSmallFile(const std::string& path) {
  std::ifstream input(path, std::ios::binary | std::ios::ate);
  if (!input) {
    LOG_WARN("Failed to open file '%s', error: %s", path.c_str(), strerror(errno));
    return std::nullopt;
  }
  auto file_size = input.tellg();
  if (file_size < 0) {
    LOG_WARN("Failed to get file size for '%s', error: %s", path.c_str(), strerror(errno));
    return std::nullopt;
  }
  std::string result(file_size, '\0');
  if (!input.seekg(0)) {
    LOG_WARN("Failed to go back to the beginning of file '%s', error: %s", path.c_str(), strerror(errno));
    return std::nullopt;
  }
  if (!input.read(result.data(), result.size())) {
    LOG_WARN("Failed to read file '%s', error: %s", path.c_str(), strerror(errno));
    return std::nullopt;
  }
  input.close();
  return result;
}

bool WriteToFile(const std::string& path, const std::string& data) {
  ASSERT(!path.empty());
  // Steps to ensure content of data gets to disk:
  //
  // 1) Open and write to temp file (e.g. bt_config.conf.new).
  // 2) Flush the stream buffer to the temp file.
  // 3) Sync the temp file to disk with fsync().
  // 4) Rename temp file to actual config file (e.g. bt_config.conf).
  //    This ensures atomic update.
  // 5) Sync directory that has the conf file with fsync().
  //    This ensures directory entries are up-to-date.
  //
  // We are using traditional C type file methods because C++ std::filesystem and std::ofstream do not support:
  // - Operation on directories
  // - fsync() to ensure content is written to disk

  // Build temp config file based on config file (e.g. bt_config.conf.new).
  const std::string temp_path = path + ".new";

  // Extract directory from file path (e.g. /data/misc/bluedroid).
  // libc++fs is not supported in APEX yet and hence cannot use std::filesystem::path::parent_path
  std::string directory_path;
  {
    // Make a temporary variable as inputs to dirname() will be modified and return value points to input char array
    // temp_path_for_dir must not be destroyed until results from dirname is appended to directory_path
    std::string temp_path_for_dir(path);
    directory_path.append(dirname(temp_path_for_dir.data()));
  }
  if (directory_path.empty()) {
    LOG_ERROR("error extracting directory from '%s', error: %s", path.c_str(), strerror(errno));
    return false;
  }

  int dir_fd = open(directory_path.c_str(), O_RDONLY | O_DIRECTORY);
  if (dir_fd < 0) {
    LOG_ERROR("unable to open dir '%s', error: %s", directory_path.c_str(), strerror(errno));
    return false;
  }

  FILE* fp = std::fopen(temp_path.c_str(), "wt");
  if (!fp) {
    LOG_ERROR("unable to write to file '%s', error: %s", temp_path.c_str(), strerror(errno));
    HandleError(temp_path, &dir_fd, &fp);
    return false;
  }

  if (std::fprintf(fp, "%s", data.c_str()) < 0) {
    LOG_ERROR("unable to write to file '%s', error: %s", temp_path.c_str(), strerror(errno));
    HandleError(temp_path, &dir_fd, &fp);
    return false;
  }

  // Flush the stream buffer to the temp file.
  if (std::fflush(fp) != 0) {
    LOG_ERROR("unable to write flush buffer to file '%s', error: %s", temp_path.c_str(), strerror(errno));
    HandleError(temp_path, &dir_fd, &fp);
    return false;
  }

  // Sync written temp file out to disk. fsync() is blocking until data makes it
  // to disk.
  if (fsync(fileno(fp)) != 0) {
    LOG_WARN("unable to fsync file '%s', error: %s", temp_path.c_str(), strerror(errno));
    // Allow fsync to fail and continue
  }

  if (std::fclose(fp) != 0) {
    LOG_ERROR("unable to close file '%s', error: %s", temp_path.c_str(), strerror(errno));
    HandleError(temp_path, &dir_fd, &fp);
    return false;
  }
  fp = nullptr;

  // Change the file's permissions to Read/Write by User and Group
  if (chmod(temp_path.c_str(), S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP) != 0) {
    LOG_ERROR("unable to change file permissions '%s', error: %s", temp_path.c_str(), strerror(errno));
    HandleError(temp_path, &dir_fd, &fp);
    return false;
  }

  // Rename written temp file to the actual config file.
  if (std::rename(temp_path.c_str(), path.c_str()) != 0) {
    LOG_ERROR("unable to commit file from '%s' to '%s', error: %s", temp_path.c_str(), path.c_str(), strerror(errno));
    HandleError(temp_path, &dir_fd, &fp);
    return false;
  }

  // This should ensure the directory is updated as well.
  if (fsync(dir_fd) != 0) {
    LOG_WARN("unable to fsync dir '%s', error: %s", directory_path.c_str(), strerror(errno));
  }

  if (close(dir_fd) != 0) {
    LOG_ERROR("unable to close dir '%s', error: %s", directory_path.c_str(), strerror(errno));
    HandleError(temp_path, &dir_fd, &fp);
    return false;
  }
  return true;
}

bool RemoveFile(const std::string& path) {
  if (remove(path.c_str()) != 0) {
    LOG_ERROR("unable to remove file '%s', error: %s", path.c_str(), strerror(errno));
    return false;
  }
  return true;
}

}  // namespace os
}  // namespace bluetooth