brillo/file_utils.cc

// Copyright 2014 The Chromium OS 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 "brillo/file_utils.h"

#include <fcntl.h>
#include <unistd.h>

#include <base/files/file_path.h>
#include <base/files/file_util.h>
#include <base/files/scoped_file.h>
#include <base/logging.h>
#include <base/posix/eintr_wrapper.h>

namespace brillo {

namespace {

enum {
  kPermissions600 = S_IRUSR | S_IWUSR,
  kPermissions777 = S_IRWXU | S_IRWXG | S_IRWXO
};

// Verify that base file permission enums are compatible with S_Ixxx. If these
// asserts ever fail, we'll need to ensure that users of these functions switch
// away from using base permission enums and add a note to the function comments
// indicating that base enums can not be used.
static_assert(base::FILE_PERMISSION_READ_BY_USER == S_IRUSR,
              "base file permissions don't match unistd.h permissions");
static_assert(base::FILE_PERMISSION_WRITE_BY_USER == S_IWUSR,
              "base file permissions don't match unistd.h permissions");
static_assert(base::FILE_PERMISSION_EXECUTE_BY_USER == S_IXUSR,
              "base file permissions don't match unistd.h permissions");
static_assert(base::FILE_PERMISSION_READ_BY_GROUP == S_IRGRP,
              "base file permissions don't match unistd.h permissions");
static_assert(base::FILE_PERMISSION_WRITE_BY_GROUP == S_IWGRP,
              "base file permissions don't match unistd.h permissions");
static_assert(base::FILE_PERMISSION_EXECUTE_BY_GROUP == S_IXGRP,
              "base file permissions don't match unistd.h permissions");
static_assert(base::FILE_PERMISSION_READ_BY_OTHERS == S_IROTH,
              "base file permissions don't match unistd.h permissions");
static_assert(base::FILE_PERMISSION_WRITE_BY_OTHERS == S_IWOTH,
              "base file permissions don't match unistd.h permissions");
static_assert(base::FILE_PERMISSION_EXECUTE_BY_OTHERS == S_IXOTH,
              "base file permissions don't match unistd.h permissions");

enum RegularFileOrDeleteResult {
  kFailure = 0,      // Failed to delete whatever was at the path.
  kRegularFile = 1,  // Regular file existed and was unchanged.
  kEmpty = 2         // Anything that was at the path has been deleted.
};

// Checks if a regular file owned by |uid| and |gid| exists at |path|, otherwise
// deletes anything that might be at |path|. Returns a RegularFileOrDeleteResult
// enum indicating what is at |path| after the function finishes.
RegularFileOrDeleteResult RegularFileOrDelete(const base::FilePath& path,
                                              uid_t uid,
                                              gid_t gid) {
  // Check for symlinks by setting O_NOFOLLOW and checking for ELOOP. This lets
  // us use the safer fstat() instead of having to use lstat().
  base::ScopedFD scoped_fd(HANDLE_EINTR(openat(
      AT_FDCWD, path.value().c_str(), O_RDONLY | O_CLOEXEC | O_NOFOLLOW)));
  bool path_not_empty = (errno == ELOOP || scoped_fd != -1);

  // If there is a file/directory at |path|, see if it matches our criteria.
  if (scoped_fd != -1) {
    struct stat file_stat;
    if (fstat(scoped_fd.get(), &file_stat) != -1 &&
        S_ISREG(file_stat.st_mode) && file_stat.st_uid == uid &&
        file_stat.st_gid == gid) {
      return kRegularFile;
    }
  }

  // If we get here and anything was at |path|, try to delete it so we can put
  // our file there.
  if (path_not_empty) {
    if (!base::DeleteFile(path, true)) {
      PLOG(WARNING) << "Failed to delete entity at \"" << path.value() << '"';
      return kFailure;
    }
  }

  return kEmpty;
}

// Handles common touch functionality but also provides an optional |fd_out|
// so that any further modifications to the file (e.g. permissions) can safely
// use the fd rather than the path. |fd_out| will only be set if a new file
// is created, otherwise it will be unchanged.
// If |fd_out| is null, this function will close the file, otherwise it's
// expected that |fd_out| will close the file when it goes out of scope.
bool TouchFileInternal(const base::FilePath& path,
                       uid_t uid,
                       gid_t gid,
                       base::ScopedFD* fd_out) {
  RegularFileOrDeleteResult result = RegularFileOrDelete(path, uid, gid);
  switch (result) {
    case kFailure:
      return false;
    case kRegularFile:
      return true;
    case kEmpty:
      break;
  }

  // base::CreateDirectory() returns true if the directory already existed.
  if (!base::CreateDirectory(path.DirName())) {
    PLOG(WARNING) << "Failed to create directory for \"" << path.value() << '"';
    return false;
  }

  // Create the file as owner-only initially.
  base::ScopedFD scoped_fd(
      HANDLE_EINTR(openat(AT_FDCWD,
                          path.value().c_str(),
                          O_RDONLY | O_NOFOLLOW | O_CREAT | O_EXCL | O_CLOEXEC,
                          kPermissions600)));
  if (scoped_fd == -1) {
    PLOG(WARNING) << "Failed to create file \"" << path.value() << '"';
    return false;
  }

  if (fd_out) {
    fd_out->swap(scoped_fd);
  }
  return true;
}

}  // namespace

bool TouchFile(const base::FilePath& path,
               int new_file_permissions,
               uid_t uid,
               gid_t gid) {
  // Make sure |permissions| doesn't have any out-of-range bits.
  if (new_file_permissions & ~kPermissions777) {
    LOG(WARNING) << "Illegal permissions: " << new_file_permissions;
    return false;
  }

  base::ScopedFD scoped_fd;
  if (!TouchFileInternal(path, uid, gid, &scoped_fd)) {
    return false;
  }

  // scoped_fd is valid only if a new file was created.
  if (scoped_fd != -1 &&
      HANDLE_EINTR(fchmod(scoped_fd.get(), new_file_permissions)) == -1) {
    PLOG(WARNING) << "Failed to set permissions for \"" << path.value() << '"';
    base::DeleteFile(path, false);
    return false;
  }

  return true;
}

bool TouchFile(const base::FilePath& path) {
  // Use TouchFile() instead of TouchFileInternal() to explicitly set
  // permissions to 600 in case umask is set strangely.
  return TouchFile(path, kPermissions600, geteuid(), getegid());
}

}  // namespace brillo