Eliminate the wipe_acls() function that is apparently not needed.

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

/**
 * @file libadacl.c
 *
 * @brief The adacl (apply default acl) shared library.
 *
 */

/* Enables get_current_dir_name() in unistd.h and the O_PATH flag. */
#define _GNU_SOURCE

#include <errno.h>      /* EINVAL, ELOOP, ENOTDIR, etc. */
#include <fcntl.h>      /* openat() */
#include <libgen.h>     /* basename(), dirname() */
#include <limits.h>     /* PATH_MAX */
#include <stdbool.h>    /* the "bool" type */
#include <stdio.h>      /* perror(), snprintf() */
#include <stdlib.h>     /* free() */
#include <string.h>     /* strdup() */
#include <sys/stat.h>   /* fstat() */
#include <sys/xattr.h>  /* fgetxattr(), fsetxattr() */
#include <unistd.h>     /* get_current_dir_name() */

/* ACLs */
#include <acl/libacl.h> /* acl_get_perm, not portable */
#include <sys/acl.h>    /* all other acl_foo functions */

/* XATTR_NAME_POSIX_ACL_ACCESS and XATTR_NAME_POSIX_ACL_DEFAULT */
#include <linux/xattr.h>

#include "libadacl.h"


/* Even though most other library functions reliably return -1 for
 * error, it feels a little wrong to re-use the ACL_ERROR constant.
 */
#define CLOSE_ERROR -1
#define OPEN_ERROR -1
#define SNPRINTF_ERROR -1
#define STAT_ERROR -1
#define XATTR_ERROR -1


/**
 * @brief The recursive portion of the @c safe_open function, used to
 *   open a file descriptor in a symlink-safe way when combined with
 *   the @c O_NOFOLLOW flag.
 *
 * @param at_fd
 *   A file descriptor relative to which @c pathname will be opened.
 *
 * @param pathname
 *   The path to the file/directory/whatever whose descriptor you want.
 *
 * @param flags
 *   File status flags to be passed to @c openat.
 *
 * @return a file descriptor for @c pathname if everything goes well,
 *   and @c OPEN_ERROR if not.
 */
int safe_open_ex(int at_fd, char* pathname, int flags) {
  if (pathname == NULL) {
    errno = EINVAL;
    perror("safe_open_ex (args)");
    return OPEN_ERROR;
  }

  char* firstslash = strchr(pathname, '/');
  if (firstslash == NULL) {
    /* No more slashes, this is the base case. */
    return openat(at_fd, pathname, flags);
  }
  if (firstslash[1] == '\0') {
    /* The first slash is the last character; ensure that we open
       a directory. */
    firstslash[0] = '\0';
    return openat(at_fd, pathname, flags | O_DIRECTORY);
  }

  /* The first slash exists and isn't the last character in the path,
     so we can split the path wherever that first slash lies and
     recurse. */
   *firstslash = '\0';
   int fd = openat(at_fd, pathname, flags | O_DIRECTORY | O_PATH);
   if (fd == OPEN_ERROR) {
     if (errno != ENOTDIR) {
       /* Don't output anything if we ignore a symlink */
       perror("safe_open_ex (safe_open_ex)");
     }
     return OPEN_ERROR;
   }

   /* The +1 is safe because there needs to be at least one character
      after the first slash (we checked this above). */
   int result = safe_open_ex(fd, firstslash+1, flags);
   if (close(fd) == CLOSE_ERROR) {
      perror("safe_open_ex (close)");
      return OPEN_ERROR;
    }
   return result;
}


/**
 * @brief A version of @c open that is completely symlink-safe when
 *   used with the @c O_NOFOLLOW flag.
 *
 * The @c openat function exists to ensure that you can anchor one
 * path to a particular directory while opening it; however, if you
 * open "b/c/d" relative to "/a", then even the @c openat function will
 * still follow symlinks in the "b" component. This can be exploited
 * by an attacker to make you open the wrong path.
 *
 * To avoid that problem, this function uses a recursive
 * implementation that opens every path from the root, one level at a
 * time. So "a" is opened relative to "/", and then "b" is opened
 * relative to "/a", and then "c" is opened relative to "/a/b",
 * etc. When the @c O_NOFOLLOW flag is used, this approach ensures
 * that no symlinks in any component are followed.
 *
 * @param pathname
 *   The path to the file/directory/whatever whose descriptor you want.
 *
 * @param flags
 *   File status flags to be passed to @c openat.
 *
 * @return a file descriptor for @c pathname if everything goes well,
 *   and @c OPEN_ERROR if not.
 */
int safe_open(const char* pathname, int flags) {
  if (pathname == NULL) {
    errno = EINVAL;
    perror("safe_open (args)");
    return OPEN_ERROR;
  }

  char abspath[PATH_MAX];
  int snprintf_result = 0;
  if (strchr(pathname, '/') == pathname) {
    /* pathname is already absolute; just copy it. */
    snprintf_result = snprintf(abspath, PATH_MAX, "%s", pathname);
  }
  else {
    /* Concatenate the current working directory and pathname into an
     * absolute path. We use realpath() ONLY on the cwd part, and not
     * on the pathname part, because realpath() resolves symlinks. And
     * the whole point of all this crap is to avoid following symlinks
     * in the pathname.
     *
     * Using realpath() on the cwd lets us operate on relative paths
     * while we're sitting in a directory that happens to have a
     * symlink in it; for example: cd /var/run && apply-default-acl foo.
     */
    char* cwd = get_current_dir_name();
    if (cwd == NULL) {
      perror("safe_open (get_current_dir_name)");
      return OPEN_ERROR;
    }

    char abs_cwd[PATH_MAX];
    if (realpath(cwd, abs_cwd) == NULL) {
      perror("safe_open (realpath)");
      free(cwd);
      return OPEN_ERROR;
    }
    snprintf_result = snprintf(abspath, PATH_MAX, "%s/%s", abs_cwd, pathname);
    free(cwd);
  }
  if (snprintf_result == SNPRINTF_ERROR || snprintf_result > PATH_MAX) {
    perror("safe_open (snprintf)");
    return OPEN_ERROR;
  }

  int fd = 0;
  if (strcmp(abspath, "/") == 0) {
    fd = open("/", flags | O_DIRECTORY);
  }
  else {
    /* Use O_PATH for some added safety if "/" is not our target */
    fd = open("/", flags | O_DIRECTORY | O_PATH);
  }
  if (fd == OPEN_ERROR) {
    perror("safe_open (open)");
    return OPEN_ERROR;
  }

  if (strcmp(abspath, "/") == 0) {
    return fd;
  }

  int result = safe_open_ex(fd, abspath+1, flags);
  if (close(fd) == CLOSE_ERROR) {
    perror("safe_open (close)");
    return OPEN_ERROR;
  }
  return result;
}




/**
 * @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) {
  if (aclp == NULL || entry == NULL) {
    errno = EINVAL;
    perror("acl_set_entry (args)");
    return ACL_ERROR;
  }

  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 we update something, we're done and return ACL_SUCCESS */
      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) {
  if (acl == NULL) {
    errno = EINVAL;
    perror("acl_is_minimal (args)");
    return ACL_ERROR;
  }

  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) {
  if (acl == NULL) {
    errno = EINVAL;
    perror("acl_execute_masked (args)");
    return ACL_ERROR;
  }

  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.
 *
 *
 * 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 file, we check the @a effective permissions, contrary
 * to what setfacl does.
 *
 * @param fd
 *   The file descriptor to check.
 *
 * @param sp
 *   A pointer to a stat structure for @c fd.
 *
 * @return
 *   - @c ACL_SUCCESS - Someone has effective execute permissions on @c fd.
 *   - @c ACL_FAILURE - Nobody can execute @c fd.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int any_can_execute(int fd, const struct stat* sp) {
  if (sp == NULL) {
    errno = EINVAL;
    perror("any_can_execute (args)");
    return ACL_ERROR;
  }

  acl_t acl = acl_get_fd(fd);

  if (acl == (acl_t)NULL) {
    perror("any_can_execute (acl_get_fd)");
    return ACL_ERROR;
  }

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

  if (acl_is_minimal(acl)) {
    if (sp->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 (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 (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 (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 (acl_get_entry)");
    result = ACL_ERROR;
    goto cleanup;
  }

 cleanup:
  acl_free(acl);
  return result;
}



/**
 * @brief Copy ACLs between file descriptors as xattrs, verbatim.
 *
 * There is a small deficiency in libacl, namely that there is no way
 * to get or set default ACLs through file descriptors. The @c
 * acl_get_file and @c acl_set_file functions can do it, but they use
 * paths, and are vulnerable to symlink attacks.
 *
 * Fortunately, when inheriting an ACL, we don't really need to look
 * at what it contains. That means that we can copy the on-disk xattrs
 * from the source directory to the destination file/directory without
 * passing through libacl, and this can be done with file descriptors
 * through @c fgetxattr and @c fsetxattr. That's what this function
 * does.
 *
 * @param src_fd
 *   The file descriptor from which the ACL will be copied.
 *
 * @param src_type
 *   The type of ACL (either @c ACL_TYPE_ACCESS or @c ACL_TYPE_DEFAULT)
 *   to copy from @c src_fd.
 *
 * @param dst_fd
 *   The file descriptor whose ACL will be overwritten with the one
 *   from @c src_fd.
 *
 * @param dst_type
 *   The type of ACL (either @c ACL_TYPE_ACCESS or @c ACL_TYPE_DEFAULT)
 *   to replace on @c dst_fd.
 *
 * @return
 *   - @c ACL_SUCCESS - The ACL was copied successfully.
 *   - @c ACL_FAILURE - There was no ACL on @c src_fd.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int acl_copy_xattr(int src_fd,
                   acl_type_t src_type,
                   int dst_fd,
                   acl_type_t dst_type) {

  const char* src_name;
  if (src_type == ACL_TYPE_ACCESS) {
    src_name = XATTR_NAME_POSIX_ACL_ACCESS;
  }
  else if (src_type == ACL_TYPE_DEFAULT) {
    src_name = XATTR_NAME_POSIX_ACL_DEFAULT;
  }
  else {
    errno = EINVAL;
    perror("acl_copy_xattr (src type)");
    return ACL_ERROR;
  }

  const char* dst_name;
  if (dst_type == ACL_TYPE_ACCESS) {
    dst_name = XATTR_NAME_POSIX_ACL_ACCESS;
  }
  else if (dst_type == ACL_TYPE_DEFAULT) {
    dst_name = XATTR_NAME_POSIX_ACL_DEFAULT;
  }
  else {
    errno = EINVAL;
    perror("acl_copy_xattr (dst type)");
    return ACL_ERROR;
  }

  size_t src_size_guess = fgetxattr(src_fd, src_name, NULL, 0);
  if (src_size_guess == XATTR_ERROR) {
    if (errno == ENODATA) {
      /* A missing ACL isn't really an error. ENOATTR and ENODATA are
         synonyms, but using ENODATA here lets us avoid another
         "include" directive. */
      return ACL_FAILURE;
    }
    perror("acl_copy_xattr (fgetxattr size guess)");
    return ACL_ERROR;
  }
  char* src_acl_p = alloca(src_size_guess);
  /* The actual size may be smaller than our guess? I don't know. */
  size_t src_size = fgetxattr(src_fd, src_name, src_acl_p, (int)src_size_guess);
  if (src_size == XATTR_ERROR) {
    if (errno == ENODATA) {
      /* A missing ACL isn't an error. */
      return ACL_FAILURE;
    }
    perror("acl_copy_xattr (fgetxattr)");
    return ACL_ERROR;
  }

  if (fsetxattr(dst_fd, dst_name, src_acl_p, src_size, 0) == XATTR_ERROR) {
    perror("acl_copy_xattr (fsetxattr)");
    return ACL_ERROR;
  }

  return ACL_SUCCESS;
}


/**
 * @brief Determine if a file descriptor has a default ACL.
 *
 * @param fd
 *   The file descriptor whose default ACL is in question.
 *
 * @return
 *   - @c ACL_SUCCESS - If @c fd has a default ACL.
 *   - @c ACL_FAILURE - If @c fd does not have a default ACL.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int has_default_acl_fd(int fd) {
  if (fgetxattr(fd, XATTR_NAME_POSIX_ACL_DEFAULT, NULL, 0) == XATTR_ERROR) {
    if (errno == ENODATA) {
      return ACL_FAILURE;
    }
    perror("has_default_acl_fd (fgetxattr)");
    return ACL_ERROR;
  }

  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 sp
 *   A pointer to a stat structure for @c path, or @c NULL if you don't
 *   have one handy.
 *
 * @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 - If symlinks or hard links are encountered.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int apply_default_acl_ex(const char* path,
                         const struct stat* sp,
                         bool no_exec_mask) {

  if (path == NULL) {
    errno = EINVAL;
    perror("apply_default_acl_ex (args)");
    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 new ACL for this path */
  acl_t new_acl = (acl_t)NULL;

  /* A copy of new_acl, to be made before we begin mangling new_acl in
     order to mask the execute bit. */
  acl_t new_acl_unmasked = (acl_t)NULL;

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

  /* The file descriptor for the directory containing "path" */
  int parent_fd = 0;

  /* Get the parent directory of "path" with dirname(), which happens
   * to murder its argument and necessitates a path_copy. */
  char* path_copy = strdup(path);
  if (path_copy == NULL) {
    perror("apply_default_acl_ex (strdup)");
    return ACL_ERROR;
  }
  char* parent = dirname(path_copy);
  parent_fd = safe_open(parent, O_DIRECTORY | O_NOFOLLOW);
  if (parent_fd == OPEN_ERROR) {
    if (errno == ELOOP || errno == ENOTDIR) {
      /* We hit a symlink, either in the last path component (ELOOP)
         or higher up (ENOTDIR). */
      result = ACL_FAILURE;
      goto cleanup;
    }
    else {
      perror("apply_default_acl_ex (open parent fd)");
      result = ACL_ERROR;
      goto cleanup;
    }
  }

  /* Check to make sure the parent descriptor actually has a default
     ACL. If it doesn't, then we can "succeed" immediately. */
  if (has_default_acl_fd(parent_fd) == ACL_FAILURE) {
    result = ACL_SUCCESS;
    goto cleanup;
  }

  fd = safe_open(path, O_NOFOLLOW);
  if (fd == OPEN_ERROR) {
    if (errno == ELOOP || errno == ENOTDIR) {
      /* We hit a symlink, either in the last path component (ELOOP)
         or higher up (ENOTDIR). */
      result = ACL_FAILURE;
      goto cleanup;
    }
    else {
      perror("apply_default_acl_ex (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.
   *
   * Note: we only need to call fstat ourselves if we weren't passed a
   * valid pointer to a stat structure (nftw does that).
  */
  if (sp == NULL) {
    struct stat s;
    if (fstat(fd, &s) == STAT_ERROR) {
      perror("apply_default_acl_ex (fstat)");
      goto cleanup;
    }

    sp = &s;
  }

  if (!S_ISDIR(sp->st_mode)) {
    /* If it's not a directory, make sure it's a regular,
       non-hard-linked file. */
    if (!S_ISREG(sp->st_mode) || sp->st_nlink != 1) {
      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. This behavior
     is modeled after the capital 'X' perms of setfacl. */
  bool allow_exec = true;

  if (!no_exec_mask) {
    /* Never mask the execute bit on directories. */
    int ace_result = any_can_execute(fd,sp) || S_ISDIR(sp->st_mode);

    if (ace_result == ACL_ERROR) {
      perror("apply_default_acl_ex (any_can_execute)");
      result = ACL_ERROR;
      goto cleanup;
    }

    allow_exec = (bool)ace_result;
  }

  /* If it's a directory, inherit the parent's default.  */
  if (S_ISDIR(sp->st_mode)) {
    if (acl_copy_xattr(parent_fd,
                       ACL_TYPE_DEFAULT,
                       fd,
                       ACL_TYPE_DEFAULT) == ACL_ERROR) {
      perror("apply_default_acl_ex (acl_copy_xattr default)");
      result = ACL_ERROR;
      goto cleanup;
    }
  }

  /* If it's anything, _apply_ the parent's default. */
  if (acl_copy_xattr(parent_fd,
                     ACL_TYPE_DEFAULT,
                     fd,
                     ACL_TYPE_ACCESS) == ACL_ERROR) {
    perror("apply_default_acl_ex (acl_copy_xattr access)");
    result = ACL_ERROR;
    goto cleanup;
  }

  /* There's a good reason why we saved the ACL above, even though
   * we're about tto read it back into memory and mess with it on the
   * next line. The acl_copy_xattr() function is already a hack to let
   * us copy default ACLs without resorting to path names; we simply
   * have no way to read the parent's default ACL into memory using
   * parent_fd. We can, however, copy the parent's ACL to a file (with
   * acl_copy_xattr), and then read the ACL from a file using
   * "fd". It's quite the circus, but it works and should be safe from
   * sym/hardlink attacks.
  */

  /* Now we potentially need to mask the execute permissions in the
     ACL on fd; or maybe now. */
  if (allow_exec) {
    goto cleanup;
  }

  /* OK, we need to mask some execute permissions. First obtain the
     current ACL... */
  new_acl = acl_get_fd(fd);
  if (new_acl == (acl_t)NULL) {
    perror("apply_default_acl_ex (acl_get_fd)");
    result = ACL_ERROR;
    goto cleanup;
  }

  /* ...and now make a copy of it, because otherwise when we loop
     below, some shit gets stuck (modifying the structure while
     looping over it no worky). */
  new_acl_unmasked = acl_dup(new_acl);
  if (new_acl_unmasked == (acl_t)NULL) {
    perror("apply_default_acl_ex (acl_dup)");
    result = ACL_ERROR;
    goto cleanup;
  }

  acl_entry_t entry;
  int ge_result = acl_get_entry(new_acl_unmasked, 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_ex (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_ex (acl_get_permset)");
      result = ACL_ERROR;
      goto cleanup;
    }

    if (tag == ACL_MASK ||
        tag == ACL_USER_OBJ ||
        tag == ACL_GROUP_OBJ ||
        tag == ACL_OTHER) {

      /* 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_ex (acl_delete_perm)");
        result = ACL_ERROR;
        goto cleanup;
      }

      if (acl_set_permset(entry, permset) == ACL_ERROR) {
        perror("apply_default_acl_ex (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 "new_acl" here, and not
     * "new_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 "new_acl". To do that, it needs the address of "new_acl".
     */

    if (acl_set_entry(&new_acl, entry) == ACL_ERROR) {
      perror("apply_default_acl_ex (acl_set_entry)");
      result = ACL_ERROR;
      goto cleanup;
    }

    ge_result = acl_get_entry(new_acl_unmasked, 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_ex (acl_get_entry)");
    result = ACL_ERROR;
    goto cleanup;
  }

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

 cleanup:
  free(path_copy);
  if (new_acl != (acl_t)NULL) {
    acl_free(new_acl);
  }
  if (new_acl_unmasked != (acl_t)NULL) {
    acl_free(new_acl_unmasked);
  }
  if (fd > 0 && close(fd) == CLOSE_ERROR) {
    perror("apply_default_acl_ex (close fd)");
    result = ACL_ERROR;
  }
  if (parent_fd > 0 && close(parent_fd) == CLOSE_ERROR) {
    perror("apply_default_acl_ex (close parent_fd)");
    result = ACL_ERROR;
  }
  return result;
}



/**
 * @brief The friendly interface to @c apply_default_acl_ex.
 *
 * The @c apply_default_acl_ex function holds the real implementation
 * of this function, but it takes a weird second argument that most
 * people won't care about (a stat structure). But, we use that
 * argument for the recursive mode of the CLI, so it's there.
 *
 * If you don't have a stat structure for your @c path, use this instead.
 *
 * @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 - If symlinks or hard links are encountered.
 *     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) {
  return apply_default_acl_ex(path, NULL, no_exec_mask);
}