src/basic/fs-util.c

   1 /***
   2   This file is part of systemd.
   3
   4   Copyright 2010 Lennart Poettering
   5
   6   systemd is free software; you can redistribute it and/or modify it
   7   under the terms of the GNU Lesser General Public License as published by
   8   the Free Software Foundation; either version 2.1 of the License, or
   9   (at your option) any later version.
  10
  11   systemd is distributed in the hope that it will be useful, but
  12   WITHOUT ANY WARRANTY; without even the implied warranty of
  13   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  14   Lesser General Public License for more details.
  15
  16   You should have received a copy of the GNU Lesser General Public License
  17   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  18 ***/
  19
  20 #include <errno.h>
  21 #include <stddef.h>
  22 #include <stdio.h>
  23 #include <stdlib.h>
  24 #include <string.h>
  25 #include <sys/stat.h>
  26 #include <linux/magic.h>
  27 #include <time.h>
  28 #include <unistd.h>
  29
  30 #include "alloc-util.h"
  31 #include "dirent-util.h"
  32 #include "fd-util.h"
  33 #include "fileio.h"
  34 #include "fs-util.h"
  35 #include "log.h"
  36 #include "macro.h"
  37 #include "missing.h"
  38 #include "mkdir.h"
  39 #include "parse-util.h"
  40 #include "path-util.h"
  41 #include "stat-util.h"
  42 #include "stdio-util.h"
  43 #include "string-util.h"
  44 #include "strv.h"
  45 #include "time-util.h"
  46 #include "user-util.h"
  47 #include "util.h"
  48
  49 int unlink_noerrno(const char *path) {
  50         PROTECT_ERRNO;
  51         int r;
  52
  53         r = unlink(path);
  54         if (r < 0)
  55                 return -errno;
  56
  57         return 0;
  58 }
  59
  60 int rmdir_parents(const char *path, const char *stop) {
  61         size_t l;
  62         int r = 0;
  63
  64         assert(path);
  65         assert(stop);
  66
  67         l = strlen(path);
  68
  69         /* Skip trailing slashes */
  70         while (l > 0 && path[l-1] == '/')
  71                 l--;
  72
  73         while (l > 0) {
  74                 char *t;
  75
  76                 /* Skip last component */
  77                 while (l > 0 && path[l-1] != '/')
  78                         l--;
  79
  80                 /* Skip trailing slashes */
  81                 while (l > 0 && path[l-1] == '/')
  82                         l--;
  83
  84                 if (l <= 0)
  85                         break;
  86
  87                 t = strndup(path, l);
  88                 if (!t)
  89                         return -ENOMEM;
  90
  91                 if (path_startswith(stop, t)) {
  92                         free(t);
  93                         return 0;
  94                 }
  95
  96                 r = rmdir(t);
  97                 free(t);
  98
  99                 if (r < 0)
 100                         if (errno != ENOENT)
 101                                 return -errno;
 102         }
 103
 104         return 0;
 105 }
 106
 107
 108 int rename_noreplace(int olddirfd, const char *oldpath, int newdirfd, const char *newpath) {
 109         struct stat buf;
 110         int ret;
 111
 112         ret = renameat2(olddirfd, oldpath, newdirfd, newpath, RENAME_NOREPLACE);
 113         if (ret >= 0)
 114                 return 0;
 115
 116         /* renameat2() exists since Linux 3.15, btrfs added support for it later.
 117          * If it is not implemented, fallback to another method. */
 118         if (!IN_SET(errno, EINVAL, ENOSYS))
 119                 return -errno;
 120
 121         /* The link()/unlink() fallback does not work on directories. But
 122          * renameat() without RENAME_NOREPLACE gives the same semantics on
 123          * directories, except when newpath is an *empty* directory. This is
 124          * good enough. */
 125         ret = fstatat(olddirfd, oldpath, &buf, AT_SYMLINK_NOFOLLOW);
 126         if (ret >= 0 && S_ISDIR(buf.st_mode)) {
 127                 ret = renameat(olddirfd, oldpath, newdirfd, newpath);
 128                 return ret >= 0 ? 0 : -errno;
 129         }
 130
 131         /* If it is not a directory, use the link()/unlink() fallback. */
 132         ret = linkat(olddirfd, oldpath, newdirfd, newpath, 0);
 133         if (ret < 0)
 134                 return -errno;
 135
 136         ret = unlinkat(olddirfd, oldpath, 0);
 137         if (ret < 0) {
 138                 /* backup errno before the following unlinkat() alters it */
 139                 ret = errno;
 140                 (void) unlinkat(newdirfd, newpath, 0);
 141                 errno = ret;
 142                 return -errno;
 143         }
 144
 145         return 0;
 146 }
 147
 148 int readlinkat_malloc(int fd, const char *p, char **ret) {
 149         size_t l = 100;
 150         int r;
 151
 152         assert(p);
 153         assert(ret);
 154
 155         for (;;) {
 156                 char *c;
 157                 ssize_t n;
 158
 159                 c = new(char, l);
 160                 if (!c)
 161                         return -ENOMEM;
 162
 163                 n = readlinkat(fd, p, c, l-1);
 164                 if (n < 0) {
 165                         r = -errno;
 166                         free(c);
 167                         return r;
 168                 }
 169
 170                 if ((size_t) n < l-1) {
 171                         c[n] = 0;
 172                         *ret = c;
 173                         return 0;
 174                 }
 175
 176                 free(c);
 177                 l *= 2;
 178         }
 179 }
 180
 181 int readlink_malloc(const char *p, char **ret) {
 182         return readlinkat_malloc(AT_FDCWD, p, ret);
 183 }
 184
 185 int readlink_value(const char *p, char **ret) {
 186         _cleanup_free_ char *link = NULL;
 187         char *value;
 188         int r;
 189
 190         r = readlink_malloc(p, &link);
 191         if (r < 0)
 192                 return r;
 193
 194         value = basename(link);
 195         if (!value)
 196                 return -ENOENT;
 197
 198         value = strdup(value);
 199         if (!value)
 200                 return -ENOMEM;
 201
 202         *ret = value;
 203
 204         return 0;
 205 }
 206
 207 int readlink_and_make_absolute(const char *p, char **r) {
 208         _cleanup_free_ char *target = NULL;
 209         char *k;
 210         int j;
 211
 212         assert(p);
 213         assert(r);
 214
 215         j = readlink_malloc(p, &target);
 216         if (j < 0)
 217                 return j;
 218
 219         k = file_in_same_dir(p, target);
 220         if (!k)
 221                 return -ENOMEM;
 222
 223         *r = k;
 224         return 0;
 225 }
 226
 227 int readlink_and_canonicalize(const char *p, const char *root, char **ret) {
 228         char *t, *s;
 229         int r;
 230
 231         assert(p);
 232         assert(ret);
 233
 234         r = readlink_and_make_absolute(p, &t);
 235         if (r < 0)
 236                 return r;
 237
 238         r = chase_symlinks(t, root, 0, &s);
 239         if (r < 0)
 240                 /* If we can't follow up, then let's return the original string, slightly cleaned up. */
 241                 *ret = path_kill_slashes(t);
 242         else {
 243                 *ret = s;
 244                 free(t);
 245         }
 246
 247         return 0;
 248 }
 249
 250 int readlink_and_make_absolute_root(const char *root, const char *path, char **ret) {
 251         _cleanup_free_ char *target = NULL, *t = NULL;
 252         const char *full;
 253         int r;
 254
 255         full = prefix_roota(root, path);
 256         r = readlink_malloc(full, &target);
 257         if (r < 0)
 258                 return r;
 259
 260         t = file_in_same_dir(path, target);
 261         if (!t)
 262                 return -ENOMEM;
 263
 264         *ret = t;
 265         t = NULL;
 266
 267         return 0;
 268 }
 269
 270 int chmod_and_chown(const char *path, mode_t mode, uid_t uid, gid_t gid) {
 271         assert(path);
 272
 273         /* Under the assumption that we are running privileged we
 274          * first change the access mode and only then hand out
 275          * ownership to avoid a window where access is too open. */
 276
 277         if (mode != MODE_INVALID)
 278                 if (chmod(path, mode) < 0)
 279                         return -errno;
 280
 281         if (uid != UID_INVALID || gid != GID_INVALID)
 282                 if (chown(path, uid, gid) < 0)
 283                         return -errno;
 284
 285         return 0;
 286 }
 287
 288 int fchmod_umask(int fd, mode_t m) {
 289         mode_t u;
 290         int r;
 291
 292         u = umask(0777);
 293         r = fchmod(fd, m & (~u)) < 0 ? -errno : 0;
 294         umask(u);
 295
 296         return r;
 297 }
 298
 299 int fd_warn_permissions(const char *path, int fd) {
 300         struct stat st;
 301
 302         if (fstat(fd, &st) < 0)
 303                 return -errno;
 304
 305         if (st.st_mode & 0111)
 306                 log_warning("Configuration file %s is marked executable. Please remove executable permission bits. Proceeding anyway.", path);
 307
 308         if (st.st_mode & 0002)
 309                 log_warning("Configuration file %s is marked world-writable. Please remove world writability permission bits. Proceeding anyway.", path);
 310
 311         if (getpid_cached() == 1 && (st.st_mode & 0044) != 0044)
 312                 log_warning("Configuration file %s is marked world-inaccessible. This has no effect as configuration data is accessible via APIs without restrictions. Proceeding anyway.", path);
 313
 314         return 0;
 315 }
 316
 317 int touch_file(const char *path, bool parents, usec_t stamp, uid_t uid, gid_t gid, mode_t mode) {
 318         _cleanup_close_ int fd;
 319         int r;
 320
 321         assert(path);
 322
 323         if (parents)
 324                 mkdir_parents(path, 0755);
 325
 326         fd = open(path, O_WRONLY|O_CREAT|O_CLOEXEC|O_NOCTTY,
 327                         (mode == 0 || mode == MODE_INVALID) ? 0644 : mode);
 328         if (fd < 0)
 329                 return -errno;
 330
 331         if (mode != MODE_INVALID) {
 332                 r = fchmod(fd, mode);
 333                 if (r < 0)
 334                         return -errno;
 335         }
 336
 337         if (uid != UID_INVALID || gid != GID_INVALID) {
 338                 r = fchown(fd, uid, gid);
 339                 if (r < 0)
 340                         return -errno;
 341         }
 342
 343         if (stamp != USEC_INFINITY) {
 344                 struct timespec ts[2];
 345
 346                 timespec_store(&ts[0], stamp);
 347                 ts[1] = ts[0];
 348                 r = futimens(fd, ts);
 349         } else
 350                 r = futimens(fd, NULL);
 351         if (r < 0)
 352                 return -errno;
 353
 354         return 0;
 355 }
 356
 357 int touch(const char *path) {
 358         return touch_file(path, false, USEC_INFINITY, UID_INVALID, GID_INVALID, MODE_INVALID);
 359 }
 360
 361 int symlink_idempotent(const char *from, const char *to) {
 362         _cleanup_free_ char *p = NULL;
 363         int r;
 364
 365         assert(from);
 366         assert(to);
 367
 368         if (symlink(from, to) < 0) {
 369                 if (errno != EEXIST)
 370                         return -errno;
 371
 372                 r = readlink_malloc(to, &p);
 373                 if (r < 0)
 374                         return r;
 375
 376                 if (!streq(p, from))
 377                         return -EINVAL;
 378         }
 379
 380         return 0;
 381 }
 382
 383 int symlink_atomic(const char *from, const char *to) {
 384         _cleanup_free_ char *t = NULL;
 385         int r;
 386
 387         assert(from);
 388         assert(to);
 389
 390         r = tempfn_random(to, NULL, &t);
 391         if (r < 0)
 392                 return r;
 393
 394         if (symlink(from, t) < 0)
 395                 return -errno;
 396
 397         if (rename(t, to) < 0) {
 398                 unlink_noerrno(t);
 399                 return -errno;
 400         }
 401
 402         return 0;
 403 }
 404
 405 int mknod_atomic(const char *path, mode_t mode, dev_t dev) {
 406         _cleanup_free_ char *t = NULL;
 407         int r;
 408
 409         assert(path);
 410
 411         r = tempfn_random(path, NULL, &t);
 412         if (r < 0)
 413                 return r;
 414
 415         if (mknod(t, mode, dev) < 0)
 416                 return -errno;
 417
 418         if (rename(t, path) < 0) {
 419                 unlink_noerrno(t);
 420                 return -errno;
 421         }
 422
 423         return 0;
 424 }
 425
 426 int mkfifo_atomic(const char *path, mode_t mode) {
 427         _cleanup_free_ char *t = NULL;
 428         int r;
 429
 430         assert(path);
 431
 432         r = tempfn_random(path, NULL, &t);
 433         if (r < 0)
 434                 return r;
 435
 436         if (mkfifo(t, mode) < 0)
 437                 return -errno;
 438
 439         if (rename(t, path) < 0) {
 440                 unlink_noerrno(t);
 441                 return -errno;
 442         }
 443
 444         return 0;
 445 }
 446
 447 int get_files_in_directory(const char *path, char ***list) {
 448         _cleanup_closedir_ DIR *d = NULL;
 449         struct dirent *de;
 450         size_t bufsize = 0, n = 0;
 451         _cleanup_strv_free_ char **l = NULL;
 452
 453         assert(path);
 454
 455         /* Returns all files in a directory in *list, and the number
 456          * of files as return value. If list is NULL returns only the
 457          * number. */
 458
 459         d = opendir(path);
 460         if (!d)
 461                 return -errno;
 462
 463         FOREACH_DIRENT_ALL(de, d, return -errno) {
 464                 dirent_ensure_type(d, de);
 465
 466                 if (!dirent_is_file(de))
 467                         continue;
 468
 469                 if (list) {
 470                         /* one extra slot is needed for the terminating NULL */
 471                         if (!GREEDY_REALLOC(l, bufsize, n + 2))
 472                                 return -ENOMEM;
 473
 474                         l[n] = strdup(de->d_name);
 475                         if (!l[n])
 476                                 return -ENOMEM;
 477
 478                         l[++n] = NULL;
 479                 } else
 480                         n++;
 481         }
 482
 483         if (list) {
 484                 *list = l;
 485                 l = NULL; /* avoid freeing */
 486         }
 487
 488         return n;
 489 }
 490
 491 static int getenv_tmp_dir(const char **ret_path) {
 492         const char *n;
 493         int r, ret = 0;
 494
 495         assert(ret_path);
 496
 497         /* We use the same order of environment variables python uses in tempfile.gettempdir():
 498          * https://docs.python.org/3/library/tempfile.html#tempfile.gettempdir */
 499         FOREACH_STRING(n, "TMPDIR", "TEMP", "TMP") {
 500                 const char *e;
 501
 502                 e = secure_getenv(n);
 503                 if (!e)
 504                         continue;
 505                 if (!path_is_absolute(e)) {
 506                         r = -ENOTDIR;
 507                         goto next;
 508                 }
 509                 if (!path_is_safe(e)) {
 510                         r = -EPERM;
 511                         goto next;
 512                 }
 513
 514                 r = is_dir(e, true);
 515                 if (r < 0)
 516                         goto next;
 517                 if (r == 0) {
 518                         r = -ENOTDIR;
 519                         goto next;
 520                 }
 521
 522                 *ret_path = e;
 523                 return 1;
 524
 525         next:
 526                 /* Remember first error, to make this more debuggable */
 527                 if (ret >= 0)
 528                         ret = r;
 529         }
 530
 531         if (ret < 0)
 532                 return ret;
 533
 534         *ret_path = NULL;
 535         return ret;
 536 }
 537
 538 static int tmp_dir_internal(const char *def, const char **ret) {
 539         const char *e;
 540         int r, k;
 541
 542         assert(def);
 543         assert(ret);
 544
 545         r = getenv_tmp_dir(&e);
 546         if (r > 0) {
 547                 *ret = e;
 548                 return 0;
 549         }
 550
 551         k = is_dir(def, true);
 552         if (k == 0)
 553                 k = -ENOTDIR;
 554         if (k < 0)
 555                 return r < 0 ? r : k;
 556
 557         *ret = def;
 558         return 0;
 559 }
 560
 561 int var_tmp_dir(const char **ret) {
 562
 563         /* Returns the location for "larger" temporary files, that is backed by physical storage if available, and thus
 564          * even might survive a boot: /var/tmp. If $TMPDIR (or related environment variables) are set, its value is
 565          * returned preferably however. Note that both this function and tmp_dir() below are affected by $TMPDIR,
 566          * making it a variable that overrides all temporary file storage locations. */
 567
 568         return tmp_dir_internal("/var/tmp", ret);
 569 }
 570
 571 int tmp_dir(const char **ret) {
 572
 573         /* Similar to var_tmp_dir() above, but returns the location for "smaller" temporary files, which is usually
 574          * backed by an in-memory file system: /tmp. */
 575
 576         return tmp_dir_internal("/tmp", ret);
 577 }
 578
 579 int inotify_add_watch_fd(int fd, int what, uint32_t mask) {
 580         char path[strlen("/proc/self/fd/") + DECIMAL_STR_MAX(int) + 1];
 581         int r;
 582
 583         /* This is like inotify_add_watch(), except that the file to watch is not referenced by a path, but by an fd */
 584         xsprintf(path, "/proc/self/fd/%i", what);
 585
 586         r = inotify_add_watch(fd, path, mask);
 587         if (r < 0)
 588                 return -errno;
 589
 590         return r;
 591 }
 592
 593 int chase_symlinks(const char *path, const char *original_root, unsigned flags, char **ret) {
 594         _cleanup_free_ char *buffer = NULL, *done = NULL, *root = NULL;
 595         _cleanup_close_ int fd = -1;
 596         unsigned max_follow = 32; /* how many symlinks to follow before giving up and returning ELOOP */
 597         bool exists = true;
 598         char *todo;
 599         int r;
 600
 601         assert(path);
 602
 603         /* This is a lot like canonicalize_file_name(), but takes an additional "root" parameter, that allows following
 604          * symlinks relative to a root directory, instead of the root of the host.
 605          *
 606          * Note that "root" primarily matters if we encounter an absolute symlink. It is also used when following
 607          * relative symlinks to ensure they cannot be used to "escape" the root directory. The path parameter passed is
 608          * assumed to be already prefixed by it, except if the CHASE_PREFIX_ROOT flag is set, in which case it is first
 609          * prefixed accordingly.
 610          *
 611          * Algorithmically this operates on two path buffers: "done" are the components of the path we already
 612          * processed and resolved symlinks, "." and ".." of. "todo" are the components of the path we still need to
 613          * process. On each iteration, we move one component from "todo" to "done", processing it's special meaning
 614          * each time. The "todo" path always starts with at least one slash, the "done" path always ends in no
 615          * slash. We always keep an O_PATH fd to the component we are currently processing, thus keeping lookup races
 616          * at a minimum.
 617          *
 618          * Suggested usage: whenever you want to canonicalize a path, use this function. Pass the absolute path you got
 619          * as-is: fully qualified and relative to your host's root. Optionally, specify the root parameter to tell this
 620          * function what to do when encountering a symlink with an absolute path as directory: prefix it by the
 621          * specified path.
 622          *
 623          * Note: there's also chase_symlinks_prefix() (see below), which as first step prefixes the passed path by the
 624          * passed root. */
 625
 626         if (original_root) {
 627                 r = path_make_absolute_cwd(original_root, &root);
 628                 if (r < 0)
 629                         return r;
 630
 631                 if (flags & CHASE_PREFIX_ROOT)
 632                         path = prefix_roota(root, path);
 633         }
 634
 635         r = path_make_absolute_cwd(path, &buffer);
 636         if (r < 0)
 637                 return r;
 638
 639         fd = open("/", O_CLOEXEC|O_NOFOLLOW|O_PATH);
 640         if (fd < 0)
 641                 return -errno;
 642
 643         todo = buffer;
 644         for (;;) {
 645                 _cleanup_free_ char *first = NULL;
 646                 _cleanup_close_ int child = -1;
 647                 struct stat st;
 648                 size_t n, m;
 649
 650                 /* Determine length of first component in the path */
 651                 n = strspn(todo, "/");                  /* The slashes */
 652                 m = n + strcspn(todo + n, "/");         /* The entire length of the component */
 653
 654                 /* Extract the first component. */
 655                 first = strndup(todo, m);
 656                 if (!first)
 657                         return -ENOMEM;
 658
 659                 todo += m;
 660
 661                 /* Just a single slash? Then we reached the end. */
 662                 if (isempty(first) || path_equal(first, "/"))
 663                         break;
 664
 665                 /* Just a dot? Then let's eat this up. */
 666                 if (path_equal(first, "/."))
 667                         continue;
 668
 669                 /* Two dots? Then chop off the last bit of what we already found out. */
 670                 if (path_equal(first, "/..")) {
 671                         _cleanup_free_ char *parent = NULL;
 672                         int fd_parent = -1;
 673
 674                         /* If we already are at the top, then going up will not change anything. This is in-line with
 675                          * how the kernel handles this. */
 676                         if (isempty(done) || path_equal(done, "/"))
 677                                 continue;
 678
 679                         parent = dirname_malloc(done);
 680                         if (!parent)
 681                                 return -ENOMEM;
 682
 683                         /* Don't allow this to leave the root dir.  */
 684                         if (root &&
 685                             path_startswith(done, root) &&
 686                             !path_startswith(parent, root))
 687                                 continue;
 688
 689                         free_and_replace(done, parent);
 690
 691                         fd_parent = openat(fd, "..", O_CLOEXEC|O_NOFOLLOW|O_PATH);
 692                         if (fd_parent < 0)
 693                                 return -errno;
 694
 695                         safe_close(fd);
 696                         fd = fd_parent;
 697
 698                         continue;
 699                 }
 700
 701                 /* Otherwise let's see what this is. */
 702                 child = openat(fd, first + n, O_CLOEXEC|O_NOFOLLOW|O_PATH);
 703                 if (child < 0) {
 704
 705                         if (errno == ENOENT &&
 706                             (flags & CHASE_NONEXISTENT) &&
 707                             (isempty(todo) || path_is_safe(todo))) {
 708
 709                                 /* If CHASE_NONEXISTENT is set, and the path does not exist, then that's OK, return
 710                                  * what we got so far. But don't allow this if the remaining path contains "../ or "./"
 711                                  * or something else weird. */
 712
 713                                 if (!strextend(&done, first, todo, NULL))
 714                                         return -ENOMEM;
 715
 716                                 exists = false;
 717                                 break;
 718                         }
 719
 720                         return -errno;
 721                 }
 722
 723                 if (fstat(child, &st) < 0)
 724                         return -errno;
 725                 if ((flags & CHASE_NO_AUTOFS) &&
 726                     fd_check_fstype(child, AUTOFS_SUPER_MAGIC) > 0)
 727                         return -EREMOTE;
 728
 729                 if (S_ISLNK(st.st_mode)) {
 730                         char *joined;
 731
 732                         _cleanup_free_ char *destination = NULL;
 733
 734                         /* This is a symlink, in this case read the destination. But let's make sure we don't follow
 735                          * symlinks without bounds. */
 736                         if (--max_follow <= 0)
 737                                 return -ELOOP;
 738
 739                         r = readlinkat_malloc(fd, first + n, &destination);
 740                         if (r < 0)
 741                                 return r;
 742                         if (isempty(destination))
 743                                 return -EINVAL;
 744
 745                         if (path_is_absolute(destination)) {
 746
 747                                 /* An absolute destination. Start the loop from the beginning, but use the root
 748                                  * directory as base. */
 749
 750                                 safe_close(fd);
 751                                 fd = open(root ?: "/", O_CLOEXEC|O_NOFOLLOW|O_PATH);
 752                                 if (fd < 0)
 753                                         return -errno;
 754
 755                                 free(done);
 756
 757                                 /* Note that we do not revalidate the root, we take it as is. */
 758                                 if (isempty(root))
 759                                         done = NULL;
 760                                 else {
 761                                         done = strdup(root);
 762                                         if (!done)
 763                                                 return -ENOMEM;
 764                                 }
 765
 766                         }
 767
 768                         /* Prefix what's left to do with what we just read, and start the loop again,
 769                          * but remain in the current directory. */
 770
 771                         joined = strjoin("/", destination, todo);
 772                         if (!joined)
 773                                 return -ENOMEM;
 774
 775                         free(buffer);
 776                         todo = buffer = joined;
 777
 778                         continue;
 779                 }
 780
 781                 /* If this is not a symlink, then let's just add the name we read to what we already verified. */
 782                 if (!done) {
 783                         done = first;
 784                         first = NULL;
 785                 } else {
 786                         if (!strextend(&done, first, NULL))
 787                                 return -ENOMEM;
 788                 }
 789
 790                 /* And iterate again, but go one directory further down. */
 791                 safe_close(fd);
 792                 fd = child;
 793                 child = -1;
 794         }
 795
 796         if (!done) {
 797                 /* Special case, turn the empty string into "/", to indicate the root directory. */
 798                 done = strdup("/");
 799                 if (!done)
 800                         return -ENOMEM;
 801         }
 802
 803         if (ret) {
 804                 *ret = done;
 805                 done = NULL;
 806         }
 807
 808         return exists;
 809 }