[apply-default-acl.git] / src / apply-default-acl.c

/**
 * @file apply-default-acl.c
 *
 * @brief The entire implementation.
 *
 */

/* On Linux, ftw.h needs this special voodoo to work. */
#define _XOPEN_SOURCE 500
#define _GNU_SOURCE

#include <errno.h>
#include <fcntl.h>  /* AT_FOO constants */
#include <ftw.h>    /* nftw() et al. */
#include <getopt.h>
#include <libgen.h> /* basename(), dirname() */
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>

/* ACLs */
#include <acl/libacl.h> /* acl_get_perm, not portable */
#include <sys/types.h>
#include <sys/acl.h>

/* Most of the libacl functions return 1 for success, 0 for failure,
   and -1 on error */
#define ACL_ERROR -1
#define ACL_FAILURE 0
#define ACL_SUCCESS 1




/**
 * @brief Determine if the given file descriptor might refer to an
 *   (unsafe) hard link.
 *
 * @param fd
 *   The file descriptor whose link count we want to investigate.
 *
 * @return true if we are certain that @c fd does not describe a hard
 *   link, and false otherwise. In case of error, false is returned,
 *   because we are not sure that @c fd is not a hard link.
 */
bool is_hardlink_safe(int fd) {
  if (fd <= 0) {
    return false;
  }
  struct stat s;
  if (fstat(fd, &s) == 0) {
    return (s.st_nlink == 1 || S_ISDIR(s.st_mode));
  }
  else {
    return false;
  }
}


/**
 * @brief Determine whether or not the given file descriptor is for
 *   a regular file.
 *
 * @param fd
 *   The file descriptor to test for regular-fileness.
 *
 * @return true if @c fd describes a regular file, and false otherwise.
 */
bool is_regular_file(int fd) {
  if (fd <= 0) {
    return false;
  }

  struct stat s;
  if (fstat(fd, &s) == 0) {
    return S_ISREG(s.st_mode);
  }
  else {
    return false;
  }
}



/**
 * @brief Determine whether or not the given path is accessible.
 *
 * @param path
 *   The path to test.
 *
 * @return true if @c path is accessible to the current effective
 *   user/group, false otherwise.
 */
bool path_accessible(const char* path) {
  if (path == NULL) {
    return false;
  }

  /*  Test for access using the effective user and group rather than
      the real one. */
  int flags = AT_EACCESS;

  /* Don't follow symlinks when checking for a path's existence,
     since we won't follow them to set its ACLs either. */
  flags |= AT_SYMLINK_NOFOLLOW;

  /* If the path is relative, interpret it relative to the current
     working directory (just like the access() system call). */
  if (faccessat(AT_FDCWD, path, F_OK, flags) == 0) {
    return true;
  }
  else {
    return false;
  }
}



/**
 * @brief Determine whether or not the given path is a directory.
 *
 * @param path
 *   The path to test.
 *
 * @return true if @c path is a directory, false otherwise.
 */
bool is_path_directory(const char* path) {
  if (path == NULL) {
    return false;
  }

  struct stat s;
  if (lstat(path, &s) == 0) {
    return S_ISDIR(s.st_mode);
  }
  else {
    return false;
  }
}


/**
 * @brief Determine whether or not the given file descriptor is for
 *   a directory.
 *
 * @param fd
 *   The file descriptor whose directoryness is in question.
 *
 * @return true if @c fd describes a directory, and false otherwise.
 */
bool is_directory(int fd) {
  if (fd <= 0) {
    return false;
  }

  struct stat s;
  if (fstat(fd, &s) == 0) {
    return S_ISDIR(s.st_mode);
  }
  else {
    return false;
  }
}



/**
 * @brief Update (or create) an entry in an @b minimal ACL.
 *
 * This function will not work if @c aclp contains extended
 * entries. This is fine for our purposes, since we call @c wipe_acls
 * on each path before applying the default to it.
 *
 * The assumption that there are no extended entries makes things much
 * simpler. For example, we only have to update the @c ACL_USER_OBJ,
 * @c ACL_GROUP_OBJ, and @c ACL_OTHER entries -- all others can simply
 * be created anew. This means we don't have to fool around comparing
 * named-user/group entries.
 *
 * @param aclp
 *   A pointer to the acl_t structure whose entry we want to modify.
 *
 * @param entry
 *   The new entry. If @c entry contains a user/group/other entry, we
 *   update the existing one. Otherwise we create a new entry.
 *
 * @return If there is an unexpected library error, @c ACL_ERROR is
 *   returned. Otherwise, @c ACL_SUCCESS.
 *
 */
int acl_set_entry(acl_t* aclp,
                  acl_entry_t entry) {

  acl_tag_t entry_tag;
  if (acl_get_tag_type(entry, &entry_tag) == ACL_ERROR) {
    perror("acl_set_entry (acl_get_tag_type)");
    return ACL_ERROR;
  }

  acl_permset_t entry_permset;
  if (acl_get_permset(entry, &entry_permset) == ACL_ERROR) {
    perror("acl_set_entry (acl_get_permset)");
    return ACL_ERROR;
  }

  acl_entry_t existing_entry;
  /* Loop through the given ACL looking for matching entries. */
  int result = acl_get_entry(*aclp, ACL_FIRST_ENTRY, &existing_entry);

  while (result == ACL_SUCCESS) {
    acl_tag_t existing_tag = ACL_UNDEFINED_TAG;

    if (acl_get_tag_type(existing_entry, &existing_tag) == ACL_ERROR) {
       perror("set_acl_tag_permset (acl_get_tag_type)");
       return ACL_ERROR;
    }

    if (existing_tag == entry_tag) {
      if (entry_tag == ACL_USER_OBJ ||
          entry_tag == ACL_GROUP_OBJ ||
          entry_tag == ACL_OTHER) {
        /* Only update for these three since all other tags will have
           been wiped. These three are guaranteed to exist, so if we
           match one of them, we're allowed to return ACL_SUCCESS
           below and bypass the rest of the function. */
        acl_permset_t existing_permset;
        if (acl_get_permset(existing_entry, &existing_permset) == ACL_ERROR) {
          perror("acl_set_entry (acl_get_permset)");
          return ACL_ERROR;
        }

        if (acl_set_permset(existing_entry, entry_permset) == ACL_ERROR) {
          perror("acl_set_entry (acl_set_permset)");
          return ACL_ERROR;
        }

        return ACL_SUCCESS;
      }

    }

    result = acl_get_entry(*aclp, ACL_NEXT_ENTRY, &existing_entry);
  }

  /* This catches both the initial acl_get_entry and the ones at the
     end of the loop. */
  if (result == ACL_ERROR) {
    perror("acl_set_entry (acl_get_entry)");
    return ACL_ERROR;
  }

  /* If we've made it this far, we need to add a new entry to the
     ACL. */
  acl_entry_t new_entry;

  /* The acl_create_entry() function can allocate new memory and/or
   * change the location of the ACL structure entirely. When that
   * happens, the value pointed to by aclp is updated, which means
   * that a new acl_t gets "passed out" to our caller, eventually to
   * be fed to acl_free(). In other words, we should still be freeing
   * the right thing, even if the value pointed to by aclp changes.
   */
  if (acl_create_entry(aclp, &new_entry) == ACL_ERROR) {
    perror("acl_set_entry (acl_create_entry)");
    return ACL_ERROR;
  }

  if (acl_set_tag_type(new_entry, entry_tag) == ACL_ERROR) {
    perror("acl_set_entry (acl_set_tag_type)");
    return ACL_ERROR;
  }

  if (acl_set_permset(new_entry, entry_permset) == ACL_ERROR) {
    perror("acl_set_entry (acl_set_permset)");
    return ACL_ERROR;
  }

  if (entry_tag == ACL_USER || entry_tag == ACL_GROUP) {
    /* We need to set the qualifier too. */
    void* entry_qual = acl_get_qualifier(entry);
    if (entry_qual == (void*)NULL) {
      perror("acl_set_entry (acl_get_qualifier)");
      return ACL_ERROR;
    }

    if (acl_set_qualifier(new_entry, entry_qual) == ACL_ERROR) {
      perror("acl_set_entry (acl_set_qualifier)");
      return ACL_ERROR;
    }
  }

  return ACL_SUCCESS;
}



/**
 * @brief Determine the number of entries in the given ACL.
 *
 * @param acl
 *   The ACL to inspect.
 *
 * @return Either the non-negative number of entries in @c acl, or
 *   @c ACL_ERROR on error.
 */
int acl_entry_count(acl_t acl) {

  acl_entry_t entry;
  int entry_count = 0;
  int result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry);

  while (result == ACL_SUCCESS) {
    entry_count++;
    result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry);
  }

  if (result == ACL_ERROR) {
    perror("acl_entry_count (acl_get_entry)");
    return ACL_ERROR;
  }

  return entry_count;
}



/**
 * @brief Determine whether or not the given ACL is minimal.
 *
 * An ACL is minimal if it has fewer than four entries.
 *
 * @param acl
 *   The ACL whose minimality is in question.
 *
 * @return
 *   - @c ACL_SUCCESS - @c acl is minimal
 *   - @c ACL_FAILURE - @c acl is not minimal
 *   - @c ACL_ERROR - Unexpected library error
 */
int acl_is_minimal(acl_t acl) {

  int ec = acl_entry_count(acl);

  if (ec == ACL_ERROR) {
    perror("acl_is_minimal (acl_entry_count)");
    return ACL_ERROR;
  }

  if (ec < 4) {
    return ACL_SUCCESS;
  }
  else {
    return ACL_FAILURE;
  }
}



/**
 * @brief Determine whether the given ACL's mask denies execute.
 *
 * @param acl
 *   The ACL whose mask we want to check.
 *
 * @return
 *   - @c ACL_SUCCESS - The @c acl has a mask which denies execute.
 *   - @c ACL_FAILURE - The @c acl has a mask which does not deny execute.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int acl_execute_masked(acl_t acl) {

  acl_entry_t entry;
  int ge_result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry);

  while (ge_result == ACL_SUCCESS) {
    acl_tag_t tag = ACL_UNDEFINED_TAG;

    if (acl_get_tag_type(entry, &tag) == ACL_ERROR) {
       perror("acl_execute_masked (acl_get_tag_type)");
       return ACL_ERROR;
    }

    if (tag == ACL_MASK) {
      /* This is the mask entry, get its permissions, and see if
         execute is specified. */
      acl_permset_t permset;

      if (acl_get_permset(entry, &permset) == ACL_ERROR) {
        perror("acl_execute_masked (acl_get_permset)");
        return ACL_ERROR;
      }

      int gp_result = acl_get_perm(permset, ACL_EXECUTE);
      if (gp_result == ACL_ERROR) {
        perror("acl_execute_masked (acl_get_perm)");
        return ACL_ERROR;
      }

      if (gp_result == ACL_FAILURE) {
        /* No execute bit set in the mask; execute not allowed. */
        return ACL_SUCCESS;
      }
    }

    ge_result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry);
  }

  return ACL_FAILURE;
}



/**
 * @brief Determine whether @c fd is executable (by anyone) or a
 *   directory.
 *
 * This is used as part of the heuristic to determine whether or not
 * we should mask the execute bit when inheriting an ACL. If @c fd
 * describes a directory, the answer is a clear-cut yes. This behavior
 * is modeled after the capital 'X' perms of setfacl.
 *
 * If @c fd describes a file, we check the @a effective permissions,
 * contrary to what setfacl does.
 *
 * @param fd
 *   The file descriptor to check.
 *
 * @return
 *   - @c ACL_SUCCESS - @c fd describes a directory, or someone has effective
       execute permissions.
 *   - @c ACL_FAILURE - @c fd describes a regular file and nobody can execute
       it.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int any_can_execute_or_dir(int fd) {

  if (is_directory(fd)) {
    /* That was easy... */
    return ACL_SUCCESS;
  }

  acl_t acl = acl_get_fd(fd);

  if (acl == (acl_t)NULL) {
    perror("any_can_execute_or_dir (acl_get_file)");
    return ACL_ERROR;
  }

  /* Our return value. */
  int result = ACL_FAILURE;

  if (acl_is_minimal(acl)) {
    struct stat s;
    if (fstat(fd, &s) == -1) {
      perror("any_can_execute_or_dir (fstat)");
      result = ACL_ERROR;
      goto cleanup;
    }
    if (s.st_mode & (S_IXUSR | S_IXOTH | S_IXGRP)) {
      result = ACL_SUCCESS;
      goto cleanup;
    }
    else {
      result = ACL_FAILURE;
      goto cleanup;
    }
  }

  acl_entry_t entry;
  int ge_result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry);

  while (ge_result == ACL_SUCCESS) {
    /* The first thing we do is check to see if this is a mask
       entry. If it is, we skip it entirely. */
    acl_tag_t tag = ACL_UNDEFINED_TAG;

    if (acl_get_tag_type(entry, &tag) == ACL_ERROR) {
      perror("any_can_execute_or_dir (acl_get_tag_type)");
      result = ACL_ERROR;
      goto cleanup;
    }

    if (tag == ACL_MASK) {
      ge_result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry);
      continue;
    }

    /* Ok, so it's not a mask entry. Check the execute perms. */
    acl_permset_t permset;

    if (acl_get_permset(entry, &permset) == ACL_ERROR) {
      perror("any_can_execute_or_dir (acl_get_permset)");
      result = ACL_ERROR;
      goto cleanup;
    }

    int gp_result = acl_get_perm(permset, ACL_EXECUTE);
    if (gp_result == ACL_ERROR) {
      perror("any_can_execute_or_dir (acl_get_perm)");
      result = ACL_ERROR;
      goto cleanup;
    }

    if (gp_result == ACL_SUCCESS) {
      /* Only return ACL_SUCCESS if this execute bit is not masked. */
      if (acl_execute_masked(acl) != ACL_SUCCESS) {
        result = ACL_SUCCESS;
        goto cleanup;
      }
    }

    ge_result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry);
  }

  if (ge_result == ACL_ERROR) {
    perror("any_can_execute_or_dir (acl_get_entry)");
    result = ACL_ERROR;
    goto cleanup;
  }

 cleanup:
  acl_free(acl);
  return result;
}



/**
 * @brief Set @c acl as the default ACL on @c path if it's a directory.
 *
 * This overwrites any existing default ACL on @c path. If no default
 * ACL exists, then one is created. If @c path is not a directory, we
 * return ACL_FAILURE but no error is raised.
 *
 * @param path
 *   The target directory whose ACL we wish to replace or create.
 *
  * @param acl
 *   The ACL to set as default on @c path.
 *
 * @return
 *   - @c ACL_SUCCESS - The default ACL was assigned successfully.
 *   - @c ACL_FAILURE - If @c path is not a directory.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int assign_default_acl(const char* path, acl_t acl) {

  if (path == NULL) {
    errno = ENOENT;
    return ACL_ERROR;
  }

  if (!is_path_directory(path)) {
    return ACL_FAILURE;
  }

    /* Our return value; success unless something bad happens. */
  int result = ACL_SUCCESS;
  acl_t path_acl = acl_dup(acl);

  if (path_acl == (acl_t)NULL) {
    perror("assign_default_acl (acl_dup)");
    return ACL_ERROR; /* Nothing to clean up in this case. */
  }

  if (acl_set_file(path, ACL_TYPE_DEFAULT, path_acl) == ACL_ERROR) {
    perror("assign_default_acl (acl_set_file)");
    result = ACL_ERROR;
  }

  acl_free(path_acl);
  return result;
}



/**
 * @brief Remove all @c ACL_TYPE_ACCESS entries from the given file
 *   descriptor, leaving the UNIX permission bits.
 *
 * @param fd
 *   The file descriptor whose ACLs we want to wipe.
 *
 * @return
 *   - @c ACL_SUCCESS - The ACLs were wiped successfully, or none
 *     existed in the first place.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int wipe_acls(int fd) {
  /* Initialize an empty ACL, and then overwrite the one on "fd" with it. */
  acl_t empty_acl = acl_init(0);

  if (empty_acl == (acl_t)NULL) {
    perror("wipe_acls (acl_init)");
    return ACL_ERROR;
  }

  if (acl_set_fd(fd, empty_acl) == ACL_ERROR) {
    perror("wipe_acls (acl_set_fd)");
    acl_free(empty_acl);
    return ACL_ERROR;
  }

  acl_free(empty_acl);
  return ACL_SUCCESS;
}



/**
 * @brief Apply parent default ACL to a path.
 *
 * This overwrites any existing ACLs on @c path.
 *
 * @param path
 *   The path whose ACL we would like to reset to its default.
 *
 * @param no_exec_mask
 *   The value (either true or false) of the --no-exec-mask flag.
 *
 * @return
 *   - @c ACL_SUCCESS - The parent default ACL was inherited successfully.
 *   - @c ACL_FAILURE - The target path is not a regular file/directory,
 *     or the parent of @c path is not a directory.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int apply_default_acl(const char* path, bool no_exec_mask) {

  if (path == NULL) {
    errno = ENOENT;
    return ACL_ERROR;
  }

  /* Define these next three variables here because we may have to
   * jump to the cleanup routine which expects them to exist.
   */

  /* Our return value. */
  int result = ACL_SUCCESS;

  /* The default ACL on path's parent directory */
  acl_t defacl = (acl_t)NULL;

  /* The file descriptor corresponding to "path" */
  int fd = 0;

  /* Split "path" into base/dirname parts to be used with openat().
   * We duplicate the strings involved because dirname/basename mangle
   * their arguments.
   */
  char* path_copy = strdup(path);
  if (path_copy == NULL) {
    perror("apply_default_acl (strdup)");
    return ACL_ERROR;
  }
  char* parent = dirname(path_copy);

  fd = open(path, O_NOFOLLOW);
  if (fd == -1) {
    if (errno == ELOOP) {
      result = ACL_FAILURE; /* hit a symlink */
      goto cleanup;
    }
    else {
      perror("apply_default_acl (open fd)");
      result = ACL_ERROR;
      goto cleanup;
    }
  }


  /* Refuse to operate on hard links, which can be abused by an
   * attacker to trick us into changing the ACL on a file we didn't
   * intend to; namely the "target" of the hard link. There is TOCTOU
   * race condition here, but the window is as small as possible
   * between when we open the file descriptor (look above) and when we
   * fstat it.
  */
  if (!is_hardlink_safe(fd)) {
    result = ACL_FAILURE;
    goto cleanup;
  }

  if (!is_regular_file(fd) && !is_directory(fd)) {
    result = ACL_FAILURE;
    goto cleanup;
  }

  /* Default to not masking the exec bit; i.e. applying the default
     ACL literally. If --no-exec-mask was not specified, then we try
     to "guess" whether or not to mask the exec bit. */
  bool allow_exec = true;

  if (!no_exec_mask) {
    int ace_result = any_can_execute_or_dir(fd);

    if (ace_result == ACL_ERROR) {
      perror("apply_default_acl (any_can_execute_or_dir)");
      result = ACL_ERROR;
      goto cleanup;
    }

    allow_exec = (bool)ace_result;
  }

  defacl = acl_get_file(parent, ACL_TYPE_DEFAULT);

  if (defacl == (acl_t)NULL) {
    perror("apply_default_acl (acl_get_file)");
    result = ACL_ERROR;
    goto cleanup;
  }

  if (wipe_acls(fd) == ACL_ERROR) {
    perror("apply_default_acl (wipe_acls)");
    result = ACL_ERROR;
    goto cleanup;
  }

  /* Do this after wipe_acls(), otherwise we'll overwrite the wiped
     ACL with this one. */
  acl_t acl = acl_get_fd(fd);
  if (acl == (acl_t)NULL) {
    perror("apply_default_acl (acl_get_fd)");
    result = ACL_ERROR;
    goto cleanup;
  }

  /* If it's a directory, inherit the parent's default. */
  if (assign_default_acl(path, defacl) == ACL_ERROR) {
    perror("apply_default_acl (assign_default_acl)");
    result = ACL_ERROR;
    goto cleanup;
  }

  acl_entry_t entry;
  int ge_result = acl_get_entry(defacl, ACL_FIRST_ENTRY, &entry);

  while (ge_result == ACL_SUCCESS) {
    acl_tag_t tag = ACL_UNDEFINED_TAG;

    if (acl_get_tag_type(entry, &tag) == ACL_ERROR) {
       perror("apply_default_acl (acl_get_tag_type)");
       result = ACL_ERROR;
       goto cleanup;
    }


    /* We've got an entry/tag from the default ACL. Get its permset. */
    acl_permset_t permset;
    if (acl_get_permset(entry, &permset) == ACL_ERROR) {
      perror("apply_default_acl (acl_get_permset)");
      result = ACL_ERROR;
      goto cleanup;
    }

    /* If this is a default mask, fix it up. */
    if (tag == ACL_MASK ||
        tag == ACL_USER_OBJ ||
        tag == ACL_GROUP_OBJ ||
        tag == ACL_OTHER) {

      if (!allow_exec) {
        /* The mask doesn't affect acl_user_obj, acl_group_obj (in
           minimal ACLs) or acl_other entries, so if execute should be
           masked, we have to do it manually. */
        if (acl_delete_perm(permset, ACL_EXECUTE) == ACL_ERROR) {
          perror("apply_default_acl (acl_delete_perm)");
          result = ACL_ERROR;
          goto cleanup;
        }

        if (acl_set_permset(entry, permset) == ACL_ERROR) {
          perror("apply_default_acl (acl_set_permset)");
          result = ACL_ERROR;
          goto cleanup;
        }
      }
    }

    /* Finally, add the permset to the access ACL. It's actually
     * important that we pass in the address of "acl" here, and not
     * "acl" itself. Why? The call to acl_create_entry() within
     * acl_set_entry() can allocate new memory for the entry.
     * Sometimes that can be done in-place, in which case everything
     * is cool and the new memory gets released when we call
     * acl_free(acl).
     *
     * But occasionally, the whole ACL structure will have to be moved
     * in order to allocate the extra space. When that happens,
     * acl_create_entry() modifies the pointer it was passed (in this
     * case, &acl) to point to the new location. We want to call
     * acl_free() on the new location, and since acl_free() gets
     * called right here, we need acl_create_entry() to update the
     * value of "acl". To do that, it needs the address of "acl".
     */
    if (acl_set_entry(&acl, entry) == ACL_ERROR) {
      perror("apply_default_acl (acl_set_entry)");
      result = ACL_ERROR;
      goto cleanup;
    }

    ge_result = acl_get_entry(defacl, ACL_NEXT_ENTRY, &entry);
  }

  /* Catches the first acl_get_entry as well as the ones at the end of
     the loop. */
  if (ge_result == ACL_ERROR) {
    perror("apply_default_acl (acl_get_entry)");
    result = ACL_ERROR;
    goto cleanup;
  }

  if (acl_set_fd(fd, acl) == ACL_ERROR) {
    perror("apply_default_acl (acl_set_fd)");
    result = ACL_ERROR;
    goto cleanup;
  }

 cleanup:
  free(path_copy);
  if (defacl != (acl_t)NULL) {
    acl_free(defacl);
  }
  if (fd >= 0 && close(fd) == -1) {
    perror("apply_default_acl (close)");
    result = ACL_ERROR;
  }
  return result;
}



/**
 * @brief Display program usage information.
 *
 * @param program_name
 *   The program name to use in the output.
 *
 */
void usage(const char* program_name) {
  printf("Apply any applicable default ACLs to the given files or "
         "directories.\n\n");
  printf("Usage: %s [flags] <target1> [<target2> [ <target3>...]]\n\n",
         program_name);
  printf("Flags:\n");
  printf(" -h, --help         Print this help message\n");
  printf(" -r, --recursive    Act on any given directories recursively\n");
  printf(" -x, --no-exec-mask Apply execute permissions unconditionally\n");

  return;
}


/**
 * @brief Wrapper around @c apply_default_acl() for use with @c nftw().
 *
 * For parameter information, see the @c nftw man page.
 *
 * @return If the ACL was applied to @c target successfully, we return
 *   @c FTW_CONTINUE to signal to @ nftw() that we should proceed onto
 *   the next file or directory. Otherwise, we return @c FTW_STOP to
 *   signal failure.
 *
 */
int apply_default_acl_nftw(const char *target,
                           const struct stat *s,
                           int info,
                           struct FTW *ftw) {

  if (apply_default_acl(target, false)) {
    return FTW_CONTINUE;
  }
  else {
    return FTW_STOP;
  }
}



/**
 * @brief Wrapper around @c apply_default_acl() for use with @c nftw().
 *
 * This is identical to @c apply_default_acl_nftw(), except it passes
 * @c true to @c apply_default_acl() as its no_exec_mask argument.
 *
 */
int apply_default_acl_nftw_x(const char *target,
                             const struct stat *s,
                             int info,
                             struct FTW *ftw) {

  if (apply_default_acl(target, true)) {
    return FTW_CONTINUE;
  }
  else {
    return FTW_STOP;
  }
}



/**
 * @brief Recursive version of @c apply_default_acl().
 *
 * If @c target is a directory, we use @c nftw() to call @c
 * apply_default_acl() recursively on all of its children. Otherwise,
 * we just delegate to @c apply_default_acl().
 *
 * We ignore symlinks for consistency with chmod -r.
 *
 * @param target
 *   The root (path) of the recursive application.
 *
 * @param no_exec_mask
 *   The value (either true or false) of the --no-exec-mask flag.
 *
 * @return
 *   If @c target is not a directory, we return the result of
 *   calling @c apply_default_acl() on @c target. Otherwise, we convert
 *   the return value of @c nftw(). If @c nftw() succeeds (returns 0),
 *   then we return @c true. Otherwise, we return @c false.
 *   \n\n
 *   If there is an error, it will be reported via @c perror, but
 *   we still return @c false.
 */
bool apply_default_acl_recursive(const char *target, bool no_exec_mask) {

  if (!is_path_directory(target)) {
    return apply_default_acl(target, no_exec_mask);
  }

  int max_levels = 256;
  int flags = FTW_PHYS; /* Don't follow links. */

  /* There are two separate functions that could be passed to
     nftw(). One passes no_exec_mask = true to apply_default_acl(),
     and the other passes no_exec_mask = false. Since the function we
     pass to nftw() cannot have parameters, we have to create separate
     options and make the decision here. */
  int (*fn)(const char *, const struct stat *, int, struct FTW *) = NULL;
  fn = no_exec_mask ? apply_default_acl_nftw_x : apply_default_acl_nftw;

  int nftw_result = nftw(target, fn, max_levels, flags);

  if (nftw_result == 0) {
    /* Success */
    return true;
  }

  /* nftw will return -1 on error, or if the supplied function
   * (apply_default_acl_nftw) returns a non-zero result, nftw will
   * return that.
   */
  if (nftw_result == -1) {
    perror("apply_default_acl_recursive (nftw)");
  }

  return false;
}



/**
 * @brief Call apply_default_acl (possibly recursively) on each
 * command-line argument.
 *
 * @return Either @c EXIT_FAILURE or @c EXIT_SUCCESS. If everything
 *   goes as expected, we return @c EXIT_SUCCESS. Otherwise, we return
 *   @c EXIT_FAILURE.
 */
int main(int argc, char* argv[]) {

  if (argc < 2) {
    usage(argv[0]);
    return EXIT_FAILURE;
  }

  bool recursive = false;
  bool no_exec_mask = false;

  struct option long_options[] = {
    /* These options set a flag. */
    {"help",      no_argument, NULL, 'h'},
    {"recursive", no_argument, NULL, 'r'},
    {"no-exec-mask", no_argument, NULL, 'x'},
    {NULL,        0,           NULL, 0}
  };

  int opt = 0;

  while ((opt = getopt_long(argc, argv, "hrx", long_options, NULL)) != -1) {
    switch (opt) {
    case 'h':
      usage(argv[0]);
      return EXIT_SUCCESS;
    case 'r':
      recursive = true;
      break;
    case 'x':
      no_exec_mask = true;
      break;
    default:
      usage(argv[0]);
      return EXIT_FAILURE;
    }
  }

  int result = EXIT_SUCCESS;

  int arg_index = 1;
  for (arg_index = optind; arg_index < argc; arg_index++) {
    const char* target = argv[arg_index];
    bool reapp_result = false;

    /* Make sure we can access the given path before we go out of our
     * way to please it. Doing this check outside of
     * apply_default_acl() lets us spit out a better error message for
     * typos, too.
     */
    if (!path_accessible(target)) {
      fprintf(stderr, "%s: %s: No such file or directory\n", argv[0], target);
      result = EXIT_FAILURE;
      continue;
    }

    if (recursive) {
      reapp_result = apply_default_acl_recursive(target, no_exec_mask);
    }
    else {
      /* It's either a normal file, or we're not operating recursively. */
      reapp_result = apply_default_acl(target, no_exec_mask);
    }

    if (!reapp_result) {
      result = EXIT_FAILURE;
    }
  }

  return result;
}