[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, the O_PATH flag, and
 * the asprintf() function.
*/
#define _GNU_SOURCE

#include <dirent.h>     /* readdir(), etc. */
#include <errno.h>      /* EINVAL, ELOOP, ENOTDIR, etc. */
#include <fcntl.h>      /* openat() */
#include <libgen.h>     /* basename(), dirname() */
#include <stdbool.h>    /* the "bool" type */
#include <stdio.h>      /* perror(), asprintf() */
#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 ASPRINTF_ERROR -1
#define STAT_ERROR -1
#define XATTR_ERROR -1


/* Prototypes */
int safe_open_ex(int at_fd, char* pathname, int flags);
int safe_open(const char* pathname, int flags);
int acl_update_entry(acl_t aclp, acl_entry_t entry);
int acl_entry_count(acl_t acl);
int acl_is_minimal(acl_t acl);
int acl_execute_masked(acl_t acl);
int any_can_execute(int fd, const struct stat* sp);
int acl_copy_xattr(int src_fd,
                   acl_type_t src_type,
                   int dst_fd,
                   acl_type_t dst_type);
int has_default_acl_fd(int fd);
int apply_default_acl_fds(int parent_fd, int fd, bool recursive);
int apply_default_acl(const char* path, bool recursive);



/**
 * @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 = NULL;
  int asprintf_result = 0;
  if (strchr(pathname, '/') == pathname) {
    /* pathname is already absolute; just copy it. */
    asprintf_result = asprintf(&abspath, "%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;
    }
    asprintf_result = asprintf(&abspath, "%s/%s", abs_cwd, pathname);
    free(cwd);
  }
  if (asprintf_result == ASPRINTF_ERROR) {
    perror("safe_open (asprintf)");
    return OPEN_ERROR;
  }

  /* Beyond here, asprintf() worked, and we need to free abspath. */
  int result = OPEN_ERROR;

  bool abspath_is_root = (strcmp(abspath, "/") == 0);
  int rootflags = flags | O_DIRECTORY;
  if (!abspath_is_root) {
    /* Use O_PATH for some added safety if "/" is not our target */
    rootflags |= O_PATH;
  }
  int rootfd = open("/", rootflags);
  if (rootfd == OPEN_ERROR) {
    perror("safe_open (open)");
    result = OPEN_ERROR;
    goto cleanup;
  }

  if (abspath_is_root) {
    result = rootfd;
    goto cleanup;
  }

  result = safe_open_ex(rootfd, abspath+1, flags);
  if (close(rootfd) == CLOSE_ERROR) {
    perror("safe_open (close)");
    result = OPEN_ERROR;
    goto cleanup;
  }

 cleanup:
  free(abspath);
  return result;
}




/**
 * @brief Update an entry in an @b minimal ACL.
 *
 * @param aclp
 *   A pointer to the acl_t structure whose entry we want to update.
 *
 * @param entry
 *   The new entry.
 *
 * @return
 *   - @c ACL_SUCCESS - If we update an existing entry.
 *   - @c ACL_FAILURE - If we don't find an entry to update.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int acl_update_entry(acl_t aclp, acl_entry_t entry) {
  if (aclp == NULL || entry == NULL) {
    errno = EINVAL;
    perror("acl_update_entry (args)");
    return ACL_ERROR;
  }

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

  acl_permset_t entry_permset;
  if (acl_get_permset(entry, &entry_permset) == ACL_ERROR) {
    perror("acl_update_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_update_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_update_entry (acl_get_entry)");
    return ACL_ERROR;
  }

  return ACL_FAILURE;
}



/**
 * @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;
  }

  ssize_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. */
  ssize_t src_size = fgetxattr(src_fd, src_name, src_acl_p, 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 The recursive portion of @c apply_default_acl.
 *
 * The @c apply_default_acl function takes a path, but then opens file
 * descriptors for the path and its parent. Afterwards, everything is
 * done using file descriptors, including the recursive application on
 * the path's children. This function encapsulates the portion of @c
 * apply_default_acl that uses only file descriptors; for the
 * recursion, this function ultimately calls itself.
 *
 * This overwrites any existing ACLs on @c fd and, if @c recursive is
 * @c true, its children. When @c recursive is @c true, the "worst"
 * result encountered is returned as the overall result.
 *
 * @param parent_fd
 *   A file descriptor for the parent directory of @c fd.
 *
 * @param fd
 *   The file descriptor that should inherit its parent's default ACL.
 *
 * @param recursive
 *   Should we recurse into subdirectories?
 *
 * @return
 *   - @c ACL_SUCCESS - The parent default ACLs were inherited successfully.
 *   - @c ACL_FAILURE - If symlinks or hard links are encountered.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int apply_default_acl_fds(int parent_fd, int fd, bool recursive) {
  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;

  /* 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.
  */
  struct stat s;
  if (fstat(fd, &s) == STAT_ERROR) {
    perror("apply_default_acl_fds (fstat)");
    /* We can't recurse without the stat struct for fd */
    goto cleanup;
  }


  /* Check to make sure the parent descriptor actually has a default
     ACL. If it doesn't, then we can "succeed" immediately, saving a
     little work, particularly in any_can_execute(). Note that we
     can't skip the fstat() above, because we need it in case we
     recurse. */
  if (has_default_acl_fd(parent_fd) == ACL_FAILURE) {
    result = ACL_SUCCESS;
    /* Just because this target can't inherit anything doesn't mean
       that one of it's children can't. For example, if there's a
       default on "c" in "a/b/c/d", then we don't want to skip all
       children of "a"! */
    goto recurse;
  }


  if (!S_ISDIR(s.st_mode)) {
    /* If it's not a directory, make sure it's a regular,
       non-hard-linked file. */
    if (!S_ISREG(s.st_mode) || s.st_nlink != 1) {
      result = ACL_FAILURE;
      goto cleanup; /* It's not a directory, so we can skip the recursion. */
    }
  }


  /* Next We try to guess whether or not to strip the execute bits.
   * This behavior is modeled after the capital 'X' perms of setfacl.
  */
  int ace_result = any_can_execute(fd, &s);

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

  /* Never mask the execute bit on directories. */
  bool allow_exec = (bool)ace_result || S_ISDIR(s.st_mode);


  /* If it's a directory, inherit the parent's default.  */
  if (S_ISDIR(s.st_mode)) {
    if (acl_copy_xattr(parent_fd,
                       ACL_TYPE_DEFAULT,
                       fd,
                       ACL_TYPE_DEFAULT) == ACL_ERROR) {
      perror("apply_default_acl_fds (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_fds (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 to 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 not. */
  if (allow_exec) {
    /* Skip the mask code for this target, but don't skip its children! */
    goto recurse;
  }

  /* 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_fds (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_fds (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_fds (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_fds (acl_get_permset)");
      result = ACL_ERROR;
      goto cleanup;
    }

    /* To mimic what the kernel does, I think we could drop
       ACL_GROUP_OBJ from the list below? */
    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_fds (acl_delete_perm)");
        result = ACL_ERROR;
        goto cleanup;
      }

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

    if (acl_update_entry(new_acl, entry) == ACL_ERROR) {
      perror("apply_default_acl_fds (acl_update_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_fds (acl_get_entry)");
    result = ACL_ERROR;
    goto cleanup;
  }

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

 recurse:
  if (recursive && S_ISDIR(s.st_mode)) {
    /* Recurse into subdirectories. Don't call closedir() on d! It
       closes the open file descriptor as well, and subsequent calls
       to close() then throw errors. */
    DIR* d = fdopendir(fd);
    if (d == NULL) {
      perror("apply_default_acl_fds (fdopendir)");
      result = ACL_ERROR;
      goto cleanup;
    }

    struct dirent* de;
    int new_fd = 0;
    while ((de = readdir(d)) != NULL) {
      if (de->d_type != DT_DIR && de->d_type != DT_REG) {
        /* Hit a symlink or whatever. */
        result = ACL_FAILURE;
        continue;
      }
      if (strcmp(de->d_name, ".") == 0) { continue; }
      if (strcmp(de->d_name, "..") == 0) { continue; }

      /* Be careful not to "return" out of this loop and leave the
         new_fd open! */
      new_fd = openat(fd, de->d_name, O_NOFOLLOW);
      if (new_fd == OPEN_ERROR) {
        if (errno == ELOOP || errno == ENOTDIR) {
          /* We hit a symlink, either in the last path component (ELOOP)
             or higher up (ENOTDIR). */
          if (result == ACL_SUCCESS) {
            /* Don't overwrite an error result with success/failure. */
            result = ACL_FAILURE;
          }
          continue;
        }
        else {
          perror("apply_default_acl_fds (openat)");
          result = ACL_ERROR;
          continue;
        }
      }
      switch (apply_default_acl_fds(fd, new_fd, recursive)) {
        /* Don't overwrite an error result with success/failure. */
        case ACL_FAILURE:
          if (result == ACL_SUCCESS) {
            result = ACL_FAILURE;
          }
          break;
        case ACL_ERROR:
          result = ACL_ERROR;
        default:
          if (close(new_fd) == CLOSE_ERROR) {
            perror("apply_default_acl_fds (close)");
            result = ACL_ERROR;
          }
      }
    }
  }

 cleanup:
  acl_free(new_acl);
  acl_free(new_acl_unmasked);
  return result;
}


/**
 * @brief Apply parent default ACL to a path and optionally its children.
 *
 * This overwrites any existing ACLs on the target, and, if @c
 * recursive is @c true, its children. When @c recursive is @c true,
 * the "worst" result encountered is returned as the overall result.
 *
 * @param path
 *   The path whose ACL we would like to reset to its default.
 *
 * @param recursive
 *   Should we recurse into subdirectories?
 *
 * @return
 *   - @c ACL_SUCCESS - The parent default ACLs were inherited successfully.
 *   - @c ACL_FAILURE - If symlinks or hard links are encountered.
 *   - @c ACL_ERROR - Unexpected library error.
 */
int apply_default_acl(const char* path, bool recursive) {

  if (path == NULL) {
    errno = EINVAL;
    perror("apply_default_acl (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 file descriptor corresponding to "path" */
  int fd = 0;

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

  /* dirname() and basename() mangle their arguments, so we need
     to make copies of "path" before using them. */
  char* dirname_path_copy = NULL;
  char* basename_path_copy = NULL;

  /* Get the parent directory of "path" with dirname(), which happens
   * to murder its argument and necessitates a path_copy. */
  dirname_path_copy = strdup(path);
  if (dirname_path_copy == NULL) {
    perror("apply_default_acl (strdup)");
    return ACL_ERROR;
  }
  char* parent = dirname(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 (open parent fd)");
      result = ACL_ERROR;
      goto cleanup;
    }
  }

  /* We already obtained the parent fd safely, so if we use the
     basename of path here instead of the full thing, then we can get
     away with using openat() and spare ourselves the slowness of
     another safe_open(). */
  basename_path_copy = strdup(path);
  if (basename_path_copy == NULL) {
    perror("apply_default_acl (strdup)");
    return ACL_ERROR;
  }
  fd = openat(parent_fd, basename(basename_path_copy), 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 (open fd)");
      result = ACL_ERROR;
      goto cleanup;
    }
  }

  result = apply_default_acl_fds(parent_fd, fd, recursive);

 cleanup:
  free(dirname_path_copy);
  free(basename_path_copy);

  if (parent_fd > 0 && close(parent_fd) == CLOSE_ERROR) {
    perror("apply_default_acl (close parent_fd)");
    result = ACL_ERROR;
  }
  if (fd > 0 && close(fd) == CLOSE_ERROR) {
    perror("apply_default_acl (close fd)");
    result = ACL_ERROR;
  }
  return result;
}