src/basic/fd-util.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2 /***
   3   This file is part of systemd.
   4
   5   Copyright 2010 Lennart Poettering
   6 ***/
   7
   8 #include <errno.h>
   9 #include <fcntl.h>
  10 #include <sys/resource.h>
  11 #include <sys/socket.h>
  12 #include <sys/stat.h>
  13 #include <unistd.h>
  14
  15 #include "dirent-util.h"
  16 #include "fd-util.h"
  17 #include "fileio.h"
  18 #include "fs-util.h"
  19 #include "macro.h"
  20 #include "memfd-util.h"
  21 #include "missing.h"
  22 #include "parse-util.h"
  23 #include "path-util.h"
  24 #include "process-util.h"
  25 #include "socket-util.h"
  26 #include "stdio-util.h"
  27 #include "util.h"
  28
  29 int close_nointr(int fd) {
  30         assert(fd >= 0);
  31
  32         if (close(fd) >= 0)
  33                 return 0;
  34
  35         /*
  36          * Just ignore EINTR; a retry loop is the wrong thing to do on
  37          * Linux.
  38          *
  39          * http://lkml.indiana.edu/hypermail/linux/kernel/0509.1/0877.html
  40          * https://bugzilla.gnome.org/show_bug.cgi?id=682819
  41          * http://utcc.utoronto.ca/~cks/space/blog/unix/CloseEINTR
  42          * https://sites.google.com/site/michaelsafyan/software-engineering/checkforeintrwheninvokingclosethinkagain
  43          */
  44         if (errno == EINTR)
  45                 return 0;
  46
  47         return -errno;
  48 }
  49
  50 int safe_close(int fd) {
  51
  52         /*
  53          * Like close_nointr() but cannot fail. Guarantees errno is
  54          * unchanged. Is a NOP with negative fds passed, and returns
  55          * -1, so that it can be used in this syntax:
  56          *
  57          * fd = safe_close(fd);
  58          */
  59
  60         if (fd >= 0) {
  61                 PROTECT_ERRNO;
  62
  63                 /* The kernel might return pretty much any error code
  64                  * via close(), but the fd will be closed anyway. The
  65                  * only condition we want to check for here is whether
  66                  * the fd was invalid at all... */
  67
  68                 assert_se(close_nointr(fd) != -EBADF);
  69         }
  70
  71         return -1;
  72 }
  73
  74 void safe_close_pair(int p[]) {
  75         assert(p);
  76
  77         if (p[0] == p[1]) {
  78                 /* Special case pairs which use the same fd in both
  79                  * directions... */
  80                 p[0] = p[1] = safe_close(p[0]);
  81                 return;
  82         }
  83
  84         p[0] = safe_close(p[0]);
  85         p[1] = safe_close(p[1]);
  86 }
  87
  88 void close_many(const int fds[], size_t n_fd) {
  89         size_t i;
  90
  91         assert(fds || n_fd <= 0);
  92
  93         for (i = 0; i < n_fd; i++)
  94                 safe_close(fds[i]);
  95 }
  96
  97 int fclose_nointr(FILE *f) {
  98         assert(f);
  99
 100         /* Same as close_nointr(), but for fclose() */
 101
 102         if (fclose(f) == 0)
 103                 return 0;
 104
 105         if (errno == EINTR)
 106                 return 0;
 107
 108         return -errno;
 109 }
 110
 111 FILE* safe_fclose(FILE *f) {
 112
 113         /* Same as safe_close(), but for fclose() */
 114
 115         if (f) {
 116                 PROTECT_ERRNO;
 117
 118                 assert_se(fclose_nointr(f) != EBADF);
 119         }
 120
 121         return NULL;
 122 }
 123
 124 DIR* safe_closedir(DIR *d) {
 125
 126         if (d) {
 127                 PROTECT_ERRNO;
 128
 129                 assert_se(closedir(d) >= 0 || errno != EBADF);
 130         }
 131
 132         return NULL;
 133 }
 134
 135 int fd_nonblock(int fd, bool nonblock) {
 136         int flags, nflags;
 137
 138         assert(fd >= 0);
 139
 140         flags = fcntl(fd, F_GETFL, 0);
 141         if (flags < 0)
 142                 return -errno;
 143
 144         if (nonblock)
 145                 nflags = flags | O_NONBLOCK;
 146         else
 147                 nflags = flags & ~O_NONBLOCK;
 148
 149         if (nflags == flags)
 150                 return 0;
 151
 152         if (fcntl(fd, F_SETFL, nflags) < 0)
 153                 return -errno;
 154
 155         return 0;
 156 }
 157
 158 int fd_cloexec(int fd, bool cloexec) {
 159         int flags, nflags;
 160
 161         assert(fd >= 0);
 162
 163         flags = fcntl(fd, F_GETFD, 0);
 164         if (flags < 0)
 165                 return -errno;
 166
 167         if (cloexec)
 168                 nflags = flags | FD_CLOEXEC;
 169         else
 170                 nflags = flags & ~FD_CLOEXEC;
 171
 172         if (nflags == flags)
 173                 return 0;
 174
 175         if (fcntl(fd, F_SETFD, nflags) < 0)
 176                 return -errno;
 177
 178         return 0;
 179 }
 180
 181 _pure_ static bool fd_in_set(int fd, const int fdset[], size_t n_fdset) {
 182         size_t i;
 183
 184         assert(n_fdset == 0 || fdset);
 185
 186         for (i = 0; i < n_fdset; i++)
 187                 if (fdset[i] == fd)
 188                         return true;
 189
 190         return false;
 191 }
 192
 193 int close_all_fds(const int except[], size_t n_except) {
 194         _cleanup_closedir_ DIR *d = NULL;
 195         struct dirent *de;
 196         int r = 0;
 197
 198         assert(n_except == 0 || except);
 199
 200         d = opendir("/proc/self/fd");
 201         if (!d) {
 202                 int fd;
 203                 struct rlimit rl;
 204
 205                 /* When /proc isn't available (for example in chroots)
 206                  * the fallback is brute forcing through the fd
 207                  * table */
 208
 209                 assert_se(getrlimit(RLIMIT_NOFILE, &rl) >= 0);
 210                 for (fd = 3; fd < (int) rl.rlim_max; fd ++) {
 211                         int q;
 212
 213                         if (fd_in_set(fd, except, n_except))
 214                                 continue;
 215
 216                         q = close_nointr(fd);
 217                         if (q < 0 && q != -EBADF && r >= 0)
 218                                 r = q;
 219                 }
 220
 221                 return r;
 222         }
 223
 224         FOREACH_DIRENT(de, d, return -errno) {
 225                 int fd = -1, q;
 226
 227                 if (safe_atoi(de->d_name, &fd) < 0)
 228                         /* Let's better ignore this, just in case */
 229                         continue;
 230
 231                 if (fd < 3)
 232                         continue;
 233
 234                 if (fd == dirfd(d))
 235                         continue;
 236
 237                 if (fd_in_set(fd, except, n_except))
 238                         continue;
 239
 240                 q = close_nointr(fd);
 241                 if (q < 0 && q != -EBADF && r >= 0) /* Valgrind has its own FD and doesn't want to have it closed */
 242                         r = q;
 243         }
 244
 245         return r;
 246 }
 247
 248 int same_fd(int a, int b) {
 249         struct stat sta, stb;
 250         pid_t pid;
 251         int r, fa, fb;
 252
 253         assert(a >= 0);
 254         assert(b >= 0);
 255
 256         /* Compares two file descriptors. Note that semantics are
 257          * quite different depending on whether we have kcmp() or we
 258          * don't. If we have kcmp() this will only return true for
 259          * dup()ed file descriptors, but not otherwise. If we don't
 260          * have kcmp() this will also return true for two fds of the same
 261          * file, created by separate open() calls. Since we use this
 262          * call mostly for filtering out duplicates in the fd store
 263          * this difference hopefully doesn't matter too much. */
 264
 265         if (a == b)
 266                 return true;
 267
 268         /* Try to use kcmp() if we have it. */
 269         pid = getpid_cached();
 270         r = kcmp(pid, pid, KCMP_FILE, a, b);
 271         if (r == 0)
 272                 return true;
 273         if (r > 0)
 274                 return false;
 275         if (errno != ENOSYS)
 276                 return -errno;
 277
 278         /* We don't have kcmp(), use fstat() instead. */
 279         if (fstat(a, &sta) < 0)
 280                 return -errno;
 281
 282         if (fstat(b, &stb) < 0)
 283                 return -errno;
 284
 285         if ((sta.st_mode & S_IFMT) != (stb.st_mode & S_IFMT))
 286                 return false;
 287
 288         /* We consider all device fds different, since two device fds
 289          * might refer to quite different device contexts even though
 290          * they share the same inode and backing dev_t. */
 291
 292         if (S_ISCHR(sta.st_mode) || S_ISBLK(sta.st_mode))
 293                 return false;
 294
 295         if (sta.st_dev != stb.st_dev || sta.st_ino != stb.st_ino)
 296                 return false;
 297
 298         /* The fds refer to the same inode on disk, let's also check
 299          * if they have the same fd flags. This is useful to
 300          * distinguish the read and write side of a pipe created with
 301          * pipe(). */
 302         fa = fcntl(a, F_GETFL);
 303         if (fa < 0)
 304                 return -errno;
 305
 306         fb = fcntl(b, F_GETFL);
 307         if (fb < 0)
 308                 return -errno;
 309
 310         return fa == fb;
 311 }
 312
 313 void cmsg_close_all(struct msghdr *mh) {
 314         struct cmsghdr *cmsg;
 315
 316         assert(mh);
 317
 318         CMSG_FOREACH(cmsg, mh)
 319                 if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS)
 320                         close_many((int*) CMSG_DATA(cmsg), (cmsg->cmsg_len - CMSG_LEN(0)) / sizeof(int));
 321 }
 322
 323 bool fdname_is_valid(const char *s) {
 324         const char *p;
 325
 326         /* Validates a name for $LISTEN_FDNAMES. We basically allow
 327          * everything ASCII that's not a control character. Also, as
 328          * special exception the ":" character is not allowed, as we
 329          * use that as field separator in $LISTEN_FDNAMES.
 330          *
 331          * Note that the empty string is explicitly allowed
 332          * here. However, we limit the length of the names to 255
 333          * characters. */
 334
 335         if (!s)
 336                 return false;
 337
 338         for (p = s; *p; p++) {
 339                 if (*p < ' ')
 340                         return false;
 341                 if (*p >= 127)
 342                         return false;
 343                 if (*p == ':')
 344                         return false;
 345         }
 346
 347         return p - s < 256;
 348 }
 349
 350 int fd_get_path(int fd, char **ret) {
 351         _cleanup_close_ int dir = -1;
 352         char fdname[DECIMAL_STR_MAX(int)];
 353         int r;
 354
 355         dir = open("/proc/self/fd/", O_CLOEXEC | O_DIRECTORY | O_PATH);
 356         if (dir < 0)
 357                 /* /proc is not available or not set up properly, we're most likely
 358                  * in some chroot environment. */
 359                 return errno == ENOENT ? -EOPNOTSUPP : -errno;
 360
 361         xsprintf(fdname, "%i", fd);
 362
 363         r = readlinkat_malloc(dir, fdname, ret);
 364         if (r == -ENOENT)
 365                 /* If the file doesn't exist the fd is invalid */
 366                 return -EBADF;
 367
 368         return r;
 369 }
 370
 371 int move_fd(int from, int to, int cloexec) {
 372         int r;
 373
 374         /* Move fd 'from' to 'to', make sure FD_CLOEXEC remains equal if requested, and release the old fd. If
 375          * 'cloexec' is passed as -1, the original FD_CLOEXEC is inherited for the new fd. If it is 0, it is turned
 376          * off, if it is > 0 it is turned on. */
 377
 378         if (from < 0)
 379                 return -EBADF;
 380         if (to < 0)
 381                 return -EBADF;
 382
 383         if (from == to) {
 384
 385                 if (cloexec >= 0) {
 386                         r = fd_cloexec(to, cloexec);
 387                         if (r < 0)
 388                                 return r;
 389                 }
 390
 391                 return to;
 392         }
 393
 394         if (cloexec < 0) {
 395                 int fl;
 396
 397                 fl = fcntl(from, F_GETFD, 0);
 398                 if (fl < 0)
 399                         return -errno;
 400
 401                 cloexec = !!(fl & FD_CLOEXEC);
 402         }
 403
 404         r = dup3(from, to, cloexec ? O_CLOEXEC : 0);
 405         if (r < 0)
 406                 return -errno;
 407
 408         assert(r == to);
 409
 410         safe_close(from);
 411
 412         return to;
 413 }
 414
 415 int acquire_data_fd(const void *data, size_t size, unsigned flags) {
 416
 417         _cleanup_close_pair_ int pipefds[2] = { -1, -1 };
 418         char pattern[] = "/dev/shm/data-fd-XXXXXX";
 419         _cleanup_close_ int fd = -1;
 420         int isz = 0, r;
 421         ssize_t n;
 422         off_t f;
 423
 424         assert(data || size == 0);
 425
 426         /* Acquire a read-only file descriptor that when read from returns the specified data. This is much more
 427          * complex than I wish it was. But here's why:
 428          *
 429          * a) First we try to use memfds. They are the best option, as we can seal them nicely to make them
 430          *    read-only. Unfortunately they require kernel 3.17, and – at the time of writing – we still support 3.14.
 431          *
 432          * b) Then, we try classic pipes. They are the second best options, as we can close the writing side, retaining
 433          *    a nicely read-only fd in the reading side. However, they are by default quite small, and unprivileged
 434          *    clients can only bump their size to a system-wide limit, which might be quite low.
 435          *
 436          * c) Then, we try an O_TMPFILE file in /dev/shm (that dir is the only suitable one known to exist from
 437          *    earliest boot on). To make it read-only we open the fd a second time with O_RDONLY via
 438          *    /proc/self/<fd>. Unfortunately O_TMPFILE is not available on older kernels on tmpfs.
 439          *
 440          * d) Finally, we try creating a regular file in /dev/shm, which we then delete.
 441          *
 442          * It sucks a bit that depending on the situation we return very different objects here, but that's Linux I
 443          * figure. */
 444
 445         if (size == 0 && ((flags & ACQUIRE_NO_DEV_NULL) == 0)) {
 446                 /* As a special case, return /dev/null if we have been called for an empty data block */
 447                 r = open("/dev/null", O_RDONLY|O_CLOEXEC|O_NOCTTY);
 448                 if (r < 0)
 449                         return -errno;
 450
 451                 return r;
 452         }
 453
 454         if ((flags & ACQUIRE_NO_MEMFD) == 0) {
 455                 fd = memfd_new("data-fd");
 456                 if (fd < 0)
 457                         goto try_pipe;
 458
 459                 n = write(fd, data, size);
 460                 if (n < 0)
 461                         return -errno;
 462                 if ((size_t) n != size)
 463                         return -EIO;
 464
 465                 f = lseek(fd, 0, SEEK_SET);
 466                 if (f != 0)
 467                         return -errno;
 468
 469                 r = memfd_set_sealed(fd);
 470                 if (r < 0)
 471                         return r;
 472
 473                 return TAKE_FD(fd);
 474         }
 475
 476 try_pipe:
 477         if ((flags & ACQUIRE_NO_PIPE) == 0) {
 478                 if (pipe2(pipefds, O_CLOEXEC|O_NONBLOCK) < 0)
 479                         return -errno;
 480
 481                 isz = fcntl(pipefds[1], F_GETPIPE_SZ, 0);
 482                 if (isz < 0)
 483                         return -errno;
 484
 485                 if ((size_t) isz < size) {
 486                         isz = (int) size;
 487                         if (isz < 0 || (size_t) isz != size)
 488                                 return -E2BIG;
 489
 490                         /* Try to bump the pipe size */
 491                         (void) fcntl(pipefds[1], F_SETPIPE_SZ, isz);
 492
 493                         /* See if that worked */
 494                         isz = fcntl(pipefds[1], F_GETPIPE_SZ, 0);
 495                         if (isz < 0)
 496                                 return -errno;
 497
 498                         if ((size_t) isz < size)
 499                                 goto try_dev_shm;
 500                 }
 501
 502                 n = write(pipefds[1], data, size);
 503                 if (n < 0)
 504                         return -errno;
 505                 if ((size_t) n != size)
 506                         return -EIO;
 507
 508                 (void) fd_nonblock(pipefds[0], false);
 509
 510                 return TAKE_FD(pipefds[0]);
 511         }
 512
 513 try_dev_shm:
 514         if ((flags & ACQUIRE_NO_TMPFILE) == 0) {
 515                 fd = open("/dev/shm", O_RDWR|O_TMPFILE|O_CLOEXEC, 0500);
 516                 if (fd < 0)
 517                         goto try_dev_shm_without_o_tmpfile;
 518
 519                 n = write(fd, data, size);
 520                 if (n < 0)
 521                         return -errno;
 522                 if ((size_t) n != size)
 523                         return -EIO;
 524
 525                 /* Let's reopen the thing, in order to get an O_RDONLY fd for the original O_RDWR one */
 526                 return fd_reopen(fd, O_RDONLY|O_CLOEXEC);
 527         }
 528
 529 try_dev_shm_without_o_tmpfile:
 530         if ((flags & ACQUIRE_NO_REGULAR) == 0) {
 531                 fd = mkostemp_safe(pattern);
 532                 if (fd < 0)
 533                         return fd;
 534
 535                 n = write(fd, data, size);
 536                 if (n < 0) {
 537                         r = -errno;
 538                         goto unlink_and_return;
 539                 }
 540                 if ((size_t) n != size) {
 541                         r = -EIO;
 542                         goto unlink_and_return;
 543                 }
 544
 545                 /* Let's reopen the thing, in order to get an O_RDONLY fd for the original O_RDWR one */
 546                 r = open(pattern, O_RDONLY|O_CLOEXEC);
 547                 if (r < 0)
 548                         r = -errno;
 549
 550         unlink_and_return:
 551                 (void) unlink(pattern);
 552                 return r;
 553         }
 554
 555         return -EOPNOTSUPP;
 556 }
 557
 558 int fd_move_above_stdio(int fd) {
 559         int flags, copy;
 560         PROTECT_ERRNO;
 561
 562         /* Moves the specified file descriptor if possible out of the range [0…2], i.e. the range of
 563          * stdin/stdout/stderr. If it can't be moved outside of this range the original file descriptor is
 564          * returned. This call is supposed to be used for long-lasting file descriptors we allocate in our code that
 565          * might get loaded into foreign code, and where we want ensure our fds are unlikely used accidentally as
 566          * stdin/stdout/stderr of unrelated code.
 567          *
 568          * Note that this doesn't fix any real bugs, it just makes it less likely that our code will be affected by
 569          * buggy code from others that mindlessly invokes 'fprintf(stderr, …' or similar in places where stderr has
 570          * been closed before.
 571          *
 572          * This function is written in a "best-effort" and "least-impact" style. This means whenever we encounter an
 573          * error we simply return the original file descriptor, and we do not touch errno. */
 574
 575         if (fd < 0 || fd > 2)
 576                 return fd;
 577
 578         flags = fcntl(fd, F_GETFD, 0);
 579         if (flags < 0)
 580                 return fd;
 581
 582         if (flags & FD_CLOEXEC)
 583                 copy = fcntl(fd, F_DUPFD_CLOEXEC, 3);
 584         else
 585                 copy = fcntl(fd, F_DUPFD, 3);
 586         if (copy < 0)
 587                 return fd;
 588
 589         assert(copy > 2);
 590
 591         (void) close(fd);
 592         return copy;
 593 }
 594
 595 int rearrange_stdio(int original_input_fd, int original_output_fd, int original_error_fd) {
 596
 597         int fd[3] = { /* Put together an array of fds we work on */
 598                 original_input_fd,
 599                 original_output_fd,
 600                 original_error_fd
 601         };
 602
 603         int r, i,
 604                 null_fd = -1,                /* if we open /dev/null, we store the fd to it here */
 605                 copy_fd[3] = { -1, -1, -1 }; /* This contains all fds we duplicate here temporarily, and hence need to close at the end */
 606         bool null_readable, null_writable;
 607
 608         /* Sets up stdin, stdout, stderr with the three file descriptors passed in. If any of the descriptors is
 609          * specified as -1 it will be connected with /dev/null instead. If any of the file descriptors is passed as
 610          * itself (e.g. stdin as STDIN_FILENO) it is left unmodified, but the O_CLOEXEC bit is turned off should it be
 611          * on.
 612          *
 613          * Note that if any of the passed file descriptors are > 2 they will be closed — both on success and on
 614          * failure! Thus, callers should assume that when this function returns the input fds are invalidated.
 615          *
 616          * Note that when this function fails stdin/stdout/stderr might remain half set up!
 617          *
 618          * O_CLOEXEC is turned off for all three file descriptors (which is how it should be for
 619          * stdin/stdout/stderr). */
 620
 621         null_readable = original_input_fd < 0;
 622         null_writable = original_output_fd < 0 || original_error_fd < 0;
 623
 624         /* First step, open /dev/null once, if we need it */
 625         if (null_readable || null_writable) {
 626
 627                 /* Let's open this with O_CLOEXEC first, and convert it to non-O_CLOEXEC when we move the fd to the final position. */
 628                 null_fd = open("/dev/null", (null_readable && null_writable ? O_RDWR :
 629                                              null_readable ? O_RDONLY : O_WRONLY) | O_CLOEXEC);
 630                 if (null_fd < 0) {
 631                         r = -errno;
 632                         goto finish;
 633                 }
 634
 635                 /* If this fd is in the 0…2 range, let's move it out of it */
 636                 if (null_fd < 3) {
 637                         int copy;
 638
 639                         copy = fcntl(null_fd, F_DUPFD_CLOEXEC, 3); /* Duplicate this with O_CLOEXEC set */
 640                         if (copy < 0) {
 641                                 r = -errno;
 642                                 goto finish;
 643                         }
 644
 645                         safe_close(null_fd);
 646                         null_fd = copy;
 647                 }
 648         }
 649
 650         /* Let's assemble fd[] with the fds to install in place of stdin/stdout/stderr */
 651         for (i = 0; i < 3; i++) {
 652
 653                 if (fd[i] < 0)
 654                         fd[i] = null_fd;        /* A negative parameter means: connect this one to /dev/null */
 655                 else if (fd[i] != i && fd[i] < 3) {
 656                         /* This fd is in the 0…2 territory, but not at its intended place, move it out of there, so that we can work there. */
 657                         copy_fd[i] = fcntl(fd[i], F_DUPFD_CLOEXEC, 3); /* Duplicate this with O_CLOEXEC set */
 658                         if (copy_fd[i] < 0) {
 659                                 r = -errno;
 660                                 goto finish;
 661                         }
 662
 663                         fd[i] = copy_fd[i];
 664                 }
 665         }
 666
 667         /* At this point we now have the fds to use in fd[], and they are all above the stdio range, so that we
 668          * have freedom to move them around. If the fds already were at the right places then the specific fds are
 669          * -1. Let's now move them to the right places. This is the point of no return. */
 670         for (i = 0; i < 3; i++) {
 671
 672                 if (fd[i] == i) {
 673
 674                         /* fd is already in place, but let's make sure O_CLOEXEC is off */
 675                         r = fd_cloexec(i, false);
 676                         if (r < 0)
 677                                 goto finish;
 678
 679                 } else {
 680                         assert(fd[i] > 2);
 681
 682                         if (dup2(fd[i], i) < 0) { /* Turns off O_CLOEXEC on the new fd. */
 683                                 r = -errno;
 684                                 goto finish;
 685                         }
 686                 }
 687         }
 688
 689         r = 0;
 690
 691 finish:
 692         /* Close the original fds, but only if they were outside of the stdio range. Also, properly check for the same
 693          * fd passed in multiple times. */
 694         safe_close_above_stdio(original_input_fd);
 695         if (original_output_fd != original_input_fd)
 696                 safe_close_above_stdio(original_output_fd);
 697         if (original_error_fd != original_input_fd && original_error_fd != original_output_fd)
 698                 safe_close_above_stdio(original_error_fd);
 699
 700         /* Close the copies we moved > 2 */
 701         for (i = 0; i < 3; i++)
 702                 safe_close(copy_fd[i]);
 703
 704         /* Close our null fd, if it's > 2 */
 705         safe_close_above_stdio(null_fd);
 706
 707         return r;
 708 }
 709
 710 int fd_reopen(int fd, int flags) {
 711         char procfs_path[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int)];
 712         int new_fd;
 713
 714         /* Reopens the specified fd with new flags. This is useful for convert an O_PATH fd into a regular one, or to
 715          * turn O_RDWR fds into O_RDONLY fds.
 716          *
 717          * This doesn't work on sockets (since they cannot be open()ed, ever).
 718          *
 719          * This implicitly resets the file read index to 0. */
 720
 721         xsprintf(procfs_path, "/proc/self/fd/%i", fd);
 722         new_fd = open(procfs_path, flags);
 723         if (new_fd < 0)
 724                 return -errno;
 725
 726         return new_fd;
 727 }