fs/pipe.c

   1 /*
   2  *  linux/fs/pipe.c
   3  *
   4  *  Copyright (C) 1991, 1992, 1999  Linus Torvalds
   5  */
   6
   7 #include <linux/mm.h>
   8 #include <linux/file.h>
   9 #include <linux/poll.h>
  10 #include <linux/slab.h>
  11 #include <linux/module.h>
  12 #include <linux/init.h>
  13 #include <linux/fs.h>
  14 #include <linux/mount.h>
  15 #include <linux/pipe_fs_i.h>
  16 #include <linux/uio.h>
  17 #include <asm/uaccess.h>
  18 #include <asm/ioctls.h>
  19
  20 /*
  21  * We use a start+len construction, which provides full use of the
  22  * allocated memory.
  23  * -- Florian Coosmann (FGC)
  24  *
  25  * Reads with count = 0 should always return 0.
  26  * -- Julian Bradfield 1999-06-07.
  27  *
  28  * FIFOs and Pipes now generate SIGIO for both readers and writers.
  29  * -- Jeremy Elson <jelson@circlemud.org> 2001-08-16
  30  *
  31  * pipe_read & write cleanup
  32  * -- Manfred Spraul <manfred@colorfullife.com> 2002-05-09
  33  */
  34
  35 /* Drop the inode semaphore and wait for a pipe event, atomically */
  36 void pipe_wait(struct inode * inode)
  37 {
  38         DEFINE_WAIT(wait);
  39
  40         prepare_to_wait(PIPE_WAIT(*inode), &wait, TASK_INTERRUPTIBLE);
  41         up(PIPE_SEM(*inode));
  42         schedule();
  43         finish_wait(PIPE_WAIT(*inode), &wait);
  44         down(PIPE_SEM(*inode));
  45 }
  46
  47 static inline int
  48 pipe_iov_copy_from_user(void *to, struct iovec *iov, unsigned long len)
  49 {
  50         unsigned long copy;
  51
  52         while (len > 0) {
  53                 while (!iov->iov_len)
  54                         iov++;
  55                 copy = min_t(unsigned long, len, iov->iov_len);
  56
  57                 if (copy_from_user(to, iov->iov_base, copy))
  58                         return -EFAULT;
  59                 to += copy;
  60                 len -= copy;
  61                 iov->iov_base += copy;
  62                 iov->iov_len -= copy;
  63         }
  64         return 0;
  65 }
  66
  67 static inline int
  68 pipe_iov_copy_to_user(struct iovec *iov, const void *from, unsigned long len)
  69 {
  70         unsigned long copy;
  71
  72         while (len > 0) {
  73                 while (!iov->iov_len)
  74                         iov++;
  75                 copy = min_t(unsigned long, len, iov->iov_len);
  76
  77                 if (copy_to_user(iov->iov_base, from, copy))
  78                         return -EFAULT;
  79                 from += copy;
  80                 len -= copy;
  81                 iov->iov_base += copy;
  82                 iov->iov_len -= copy;
  83         }
  84         return 0;
  85 }
  86
  87 static ssize_t
  88 pipe_readv(struct file *filp, const struct iovec *_iov,
  89            unsigned long nr_segs, loff_t *ppos)
  90 {
  91         struct inode *inode = filp->f_dentry->d_inode;
  92         int do_wakeup;
  93         ssize_t ret;
  94         struct iovec *iov = (struct iovec *)_iov;
  95         size_t total_len;
  96
  97         /* pread is not allowed on pipes. */
  98         if (unlikely(ppos != &filp->f_pos))
  99                 return -ESPIPE;
 100
 101         total_len = iov_length(iov, nr_segs);
 102         /* Null read succeeds. */
 103         if (unlikely(total_len == 0))
 104                 return 0;
 105
 106         do_wakeup = 0;
 107         ret = 0;
 108         down(PIPE_SEM(*inode));
 109         for (;;) {
 110                 int size = PIPE_LEN(*inode);
 111                 if (size) {
 112                         char *pipebuf = PIPE_BASE(*inode) + PIPE_START(*inode);
 113                         ssize_t chars = PIPE_MAX_RCHUNK(*inode);
 114
 115                         if (chars > total_len)
 116                                 chars = total_len;
 117                         if (chars > size)
 118                                 chars = size;
 119
 120                         if (pipe_iov_copy_to_user(iov, pipebuf, chars)) {
 121                                 if (!ret) ret = -EFAULT;
 122                                 break;
 123                         }
 124                         ret += chars;
 125
 126                         PIPE_START(*inode) += chars;
 127                         PIPE_START(*inode) &= (PIPE_SIZE - 1);
 128                         PIPE_LEN(*inode) -= chars;
 129                         total_len -= chars;
 130                         do_wakeup = 1;
 131                         if (!total_len)
 132                                 break;  /* common path: read succeeded */
 133                 }
 134                 if (PIPE_LEN(*inode)) /* test for cyclic buffers */
 135                         continue;
 136                 if (!PIPE_WRITERS(*inode))
 137                         break;
 138                 if (!PIPE_WAITING_WRITERS(*inode)) {
 139                         /* syscall merging: Usually we must not sleep
 140                          * if O_NONBLOCK is set, or if we got some data.
 141                          * But if a writer sleeps in kernel space, then
 142                          * we can wait for that data without violating POSIX.
 143                          */
 144                         if (ret)
 145                                 break;
 146                         if (filp->f_flags & O_NONBLOCK) {
 147                                 ret = -EAGAIN;
 148                                 break;
 149                         }
 150                 }
 151                 if (signal_pending(current)) {
 152                         if (!ret) ret = -ERESTARTSYS;
 153                         break;
 154                 }
 155                 if (do_wakeup) {
 156                         wake_up_interruptible_sync(PIPE_WAIT(*inode));
 157                         kill_fasync(PIPE_FASYNC_WRITERS(*inode), SIGIO, POLL_OUT);
 158                 }
 159                 pipe_wait(inode);
 160         }
 161         up(PIPE_SEM(*inode));
 162         /* Signal writers asynchronously that there is more room.  */
 163         if (do_wakeup) {
 164                 wake_up_interruptible(PIPE_WAIT(*inode));
 165                 kill_fasync(PIPE_FASYNC_WRITERS(*inode), SIGIO, POLL_OUT);
 166         }
 167         if (ret > 0)
 168                 file_accessed(filp);
 169         return ret;
 170 }
 171
 172 static ssize_t
 173 pipe_read(struct file *filp, char __user *buf, size_t count, loff_t *ppos)
 174 {
 175         struct iovec iov = { .iov_base = buf, .iov_len = count };
 176         return pipe_readv(filp, &iov, 1, ppos);
 177 }
 178
 179 static ssize_t
 180 pipe_writev(struct file *filp, const struct iovec *_iov,
 181             unsigned long nr_segs, loff_t *ppos)
 182 {
 183         struct inode *inode = filp->f_dentry->d_inode;
 184         ssize_t ret;
 185         size_t min;
 186         int do_wakeup;
 187         struct iovec *iov = (struct iovec *)_iov;
 188         size_t total_len;
 189
 190         /* pwrite is not allowed on pipes. */
 191         if (unlikely(ppos != &filp->f_pos))
 192                 return -ESPIPE;
 193
 194         total_len = iov_length(iov, nr_segs);
 195         /* Null write succeeds. */
 196         if (unlikely(total_len == 0))
 197                 return 0;
 198
 199         do_wakeup = 0;
 200         ret = 0;
 201         min = total_len;
 202         if (min > PIPE_BUF)
 203                 min = 1;
 204         down(PIPE_SEM(*inode));
 205         for (;;) {
 206                 int free;
 207                 if (!PIPE_READERS(*inode)) {
 208                         send_sig(SIGPIPE, current, 0);
 209                         if (!ret) ret = -EPIPE;
 210                         break;
 211                 }
 212                 free = PIPE_FREE(*inode);
 213                 if (free >= min) {
 214                         /* transfer data */
 215                         ssize_t chars = PIPE_MAX_WCHUNK(*inode);
 216                         char *pipebuf = PIPE_BASE(*inode) + PIPE_END(*inode);
 217                         /* Always wakeup, even if the copy fails. Otherwise
 218                          * we lock up (O_NONBLOCK-)readers that sleep due to
 219                          * syscall merging.
 220                          */
 221                         do_wakeup = 1;
 222                         if (chars > total_len)
 223                                 chars = total_len;
 224                         if (chars > free)
 225                                 chars = free;
 226
 227                         if (pipe_iov_copy_from_user(pipebuf, iov, chars)) {
 228                                 if (!ret) ret = -EFAULT;
 229                                 break;
 230                         }
 231                         ret += chars;
 232
 233                         PIPE_LEN(*inode) += chars;
 234                         total_len -= chars;
 235                         if (!total_len)
 236                                 break;
 237                 }
 238                 if (PIPE_FREE(*inode) && ret) {
 239                         /* handle cyclic data buffers */
 240                         min = 1;
 241                         continue;
 242                 }
 243                 if (filp->f_flags & O_NONBLOCK) {
 244                         if (!ret) ret = -EAGAIN;
 245                         break;
 246                 }
 247                 if (signal_pending(current)) {
 248                         if (!ret) ret = -ERESTARTSYS;
 249                         break;
 250                 }
 251                 if (do_wakeup) {
 252                         wake_up_interruptible_sync(PIPE_WAIT(*inode));
 253                         kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN);
 254                         do_wakeup = 0;
 255                 }
 256                 PIPE_WAITING_WRITERS(*inode)++;
 257                 pipe_wait(inode);
 258                 PIPE_WAITING_WRITERS(*inode)--;
 259         }
 260         up(PIPE_SEM(*inode));
 261         if (do_wakeup) {
 262                 wake_up_interruptible(PIPE_WAIT(*inode));
 263                 kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN);
 264         }
 265         if (ret > 0)
 266                 inode_update_time(inode, 1);    /* mtime and ctime */
 267         return ret;
 268 }
 269
 270 static ssize_t
 271 pipe_write(struct file *filp, const char __user *buf,
 272            size_t count, loff_t *ppos)
 273 {
 274         struct iovec iov = { .iov_base = (void __user *)buf, .iov_len = count };
 275         return pipe_writev(filp, &iov, 1, ppos);
 276 }
 277
 278 static ssize_t
 279 bad_pipe_r(struct file *filp, char __user *buf, size_t count, loff_t *ppos)
 280 {
 281         return -EBADF;
 282 }
 283
 284 static ssize_t
 285 bad_pipe_w(struct file *filp, const char __user *buf, size_t count, loff_t *ppos)
 286 {
 287         return -EBADF;
 288 }
 289
 290 static int
 291 pipe_ioctl(struct inode *pino, struct file *filp,
 292            unsigned int cmd, unsigned long arg)
 293 {
 294         switch (cmd) {
 295                 case FIONREAD:
 296                         return put_user(PIPE_LEN(*pino), (int __user *)arg);
 297                 default:
 298                         return -EINVAL;
 299         }
 300 }
 301
 302 /* No kernel lock held - fine */
 303 static unsigned int
 304 pipe_poll(struct file *filp, poll_table *wait)
 305 {
 306         unsigned int mask;
 307         struct inode *inode = filp->f_dentry->d_inode;
 308
 309         poll_wait(filp, PIPE_WAIT(*inode), wait);
 310
 311         /* Reading only -- no need for acquiring the semaphore.  */
 312         mask = POLLIN | POLLRDNORM;
 313         if (PIPE_EMPTY(*inode))
 314                 mask = POLLOUT | POLLWRNORM;
 315         if (!PIPE_WRITERS(*inode) && filp->f_version != PIPE_WCOUNTER(*inode))
 316                 mask |= POLLHUP;
 317         if (!PIPE_READERS(*inode))
 318                 mask |= POLLERR;
 319
 320         return mask;
 321 }
 322
 323 /* FIXME: most Unices do not set POLLERR for fifos */
 324 #define fifo_poll pipe_poll
 325
 326 static int
 327 pipe_release(struct inode *inode, int decr, int decw)
 328 {
 329         down(PIPE_SEM(*inode));
 330         PIPE_READERS(*inode) -= decr;
 331         PIPE_WRITERS(*inode) -= decw;
 332         if (!PIPE_READERS(*inode) && !PIPE_WRITERS(*inode)) {
 333                 struct pipe_inode_info *info = inode->i_pipe;
 334                 inode->i_pipe = NULL;
 335                 free_page((unsigned long) info->base);
 336                 kfree(info);
 337         } else {
 338                 wake_up_interruptible(PIPE_WAIT(*inode));
 339                 kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN);
 340                 kill_fasync(PIPE_FASYNC_WRITERS(*inode), SIGIO, POLL_OUT);
 341         }
 342         up(PIPE_SEM(*inode));
 343
 344         return 0;
 345 }
 346
 347 static int
 348 pipe_read_fasync(int fd, struct file *filp, int on)
 349 {
 350         struct inode *inode = filp->f_dentry->d_inode;
 351         int retval;
 352
 353         down(PIPE_SEM(*inode));
 354         retval = fasync_helper(fd, filp, on, PIPE_FASYNC_READERS(*inode));
 355         up(PIPE_SEM(*inode));
 356
 357         if (retval < 0)
 358                 return retval;
 359
 360         return 0;
 361 }
 362
 363
 364 static int
 365 pipe_write_fasync(int fd, struct file *filp, int on)
 366 {
 367         struct inode *inode = filp->f_dentry->d_inode;
 368         int retval;
 369
 370         down(PIPE_SEM(*inode));
 371         retval = fasync_helper(fd, filp, on, PIPE_FASYNC_WRITERS(*inode));
 372         up(PIPE_SEM(*inode));
 373
 374         if (retval < 0)
 375                 return retval;
 376
 377         return 0;
 378 }
 379
 380
 381 static int
 382 pipe_rdwr_fasync(int fd, struct file *filp, int on)
 383 {
 384         struct inode *inode = filp->f_dentry->d_inode;
 385         int retval;
 386
 387         down(PIPE_SEM(*inode));
 388
 389         retval = fasync_helper(fd, filp, on, PIPE_FASYNC_READERS(*inode));
 390
 391         if (retval >= 0)
 392                 retval = fasync_helper(fd, filp, on, PIPE_FASYNC_WRITERS(*inode));
 393
 394         up(PIPE_SEM(*inode));
 395
 396         if (retval < 0)
 397                 return retval;
 398
 399         return 0;
 400 }
 401
 402
 403 static int
 404 pipe_read_release(struct inode *inode, struct file *filp)
 405 {
 406         pipe_read_fasync(-1, filp, 0);
 407         return pipe_release(inode, 1, 0);
 408 }
 409
 410 static int
 411 pipe_write_release(struct inode *inode, struct file *filp)
 412 {
 413         pipe_write_fasync(-1, filp, 0);
 414         return pipe_release(inode, 0, 1);
 415 }
 416
 417 static int
 418 pipe_rdwr_release(struct inode *inode, struct file *filp)
 419 {
 420         int decr, decw;
 421
 422         pipe_rdwr_fasync(-1, filp, 0);
 423         decr = (filp->f_mode & FMODE_READ) != 0;
 424         decw = (filp->f_mode & FMODE_WRITE) != 0;
 425         return pipe_release(inode, decr, decw);
 426 }
 427
 428 static int
 429 pipe_read_open(struct inode *inode, struct file *filp)
 430 {
 431         /* We could have perhaps used atomic_t, but this and friends
 432            below are the only places.  So it doesn't seem worthwhile.  */
 433         down(PIPE_SEM(*inode));
 434         PIPE_READERS(*inode)++;
 435         up(PIPE_SEM(*inode));
 436
 437         return 0;
 438 }
 439
 440 static int
 441 pipe_write_open(struct inode *inode, struct file *filp)
 442 {
 443         down(PIPE_SEM(*inode));
 444         PIPE_WRITERS(*inode)++;
 445         up(PIPE_SEM(*inode));
 446
 447         return 0;
 448 }
 449
 450 static int
 451 pipe_rdwr_open(struct inode *inode, struct file *filp)
 452 {
 453         down(PIPE_SEM(*inode));
 454         if (filp->f_mode & FMODE_READ)
 455                 PIPE_READERS(*inode)++;
 456         if (filp->f_mode & FMODE_WRITE)
 457                 PIPE_WRITERS(*inode)++;
 458         up(PIPE_SEM(*inode));
 459
 460         return 0;
 461 }
 462
 463 /*
 464  * The file_operations structs are not static because they
 465  * are also used in linux/fs/fifo.c to do operations on FIFOs.
 466  */
 467 struct file_operations read_fifo_fops = {
 468         .llseek         = no_llseek,
 469         .read           = pipe_read,
 470         .readv          = pipe_readv,
 471         .write          = bad_pipe_w,
 472         .poll           = fifo_poll,
 473         .ioctl          = pipe_ioctl,
 474         .open           = pipe_read_open,
 475         .release        = pipe_read_release,
 476         .fasync         = pipe_read_fasync,
 477 };
 478
 479 struct file_operations write_fifo_fops = {
 480         .llseek         = no_llseek,
 481         .read           = bad_pipe_r,
 482         .write          = pipe_write,
 483         .writev         = pipe_writev,
 484         .poll           = fifo_poll,
 485         .ioctl          = pipe_ioctl,
 486         .open           = pipe_write_open,
 487         .release        = pipe_write_release,
 488         .fasync         = pipe_write_fasync,
 489 };
 490
 491 struct file_operations rdwr_fifo_fops = {
 492         .llseek         = no_llseek,
 493         .read           = pipe_read,
 494         .readv          = pipe_readv,
 495         .write          = pipe_write,
 496         .writev         = pipe_writev,
 497         .poll           = fifo_poll,
 498         .ioctl          = pipe_ioctl,
 499         .open           = pipe_rdwr_open,
 500         .release        = pipe_rdwr_release,
 501         .fasync         = pipe_rdwr_fasync,
 502 };
 503
 504 struct file_operations read_pipe_fops = {
 505         .llseek         = no_llseek,
 506         .read           = pipe_read,
 507         .readv          = pipe_readv,
 508         .write          = bad_pipe_w,
 509         .poll           = pipe_poll,
 510         .ioctl          = pipe_ioctl,
 511         .open           = pipe_read_open,
 512         .release        = pipe_read_release,
 513         .fasync         = pipe_read_fasync,
 514 };
 515
 516 struct file_operations write_pipe_fops = {
 517         .llseek         = no_llseek,
 518         .read           = bad_pipe_r,
 519         .write          = pipe_write,
 520         .writev         = pipe_writev,
 521         .poll           = pipe_poll,
 522         .ioctl          = pipe_ioctl,
 523         .open           = pipe_write_open,
 524         .release        = pipe_write_release,
 525         .fasync         = pipe_write_fasync,
 526 };
 527
 528 struct file_operations rdwr_pipe_fops = {
 529         .llseek         = no_llseek,
 530         .read           = pipe_read,
 531         .readv          = pipe_readv,
 532         .write          = pipe_write,
 533         .writev         = pipe_writev,
 534         .poll           = pipe_poll,
 535         .ioctl          = pipe_ioctl,
 536         .open           = pipe_rdwr_open,
 537         .release        = pipe_rdwr_release,
 538         .fasync         = pipe_rdwr_fasync,
 539 };
 540
 541 struct inode* pipe_new(struct inode* inode)
 542 {
 543         unsigned long page;
 544
 545         page = __get_free_page(GFP_USER);
 546         if (!page)
 547                 return NULL;
 548
 549         inode->i_pipe = kmalloc(sizeof(struct pipe_inode_info), GFP_KERNEL);
 550         if (!inode->i_pipe)
 551                 goto fail_page;
 552
 553         init_waitqueue_head(PIPE_WAIT(*inode));
 554         PIPE_BASE(*inode) = (char*) page;
 555         PIPE_START(*inode) = PIPE_LEN(*inode) = 0;
 556         PIPE_READERS(*inode) = PIPE_WRITERS(*inode) = 0;
 557         PIPE_WAITING_WRITERS(*inode) = 0;
 558         PIPE_RCOUNTER(*inode) = PIPE_WCOUNTER(*inode) = 1;
 559         *PIPE_FASYNC_READERS(*inode) = *PIPE_FASYNC_WRITERS(*inode) = NULL;
 560
 561         return inode;
 562 fail_page:
 563         free_page(page);
 564         return NULL;
 565 }
 566
 567 static struct vfsmount *pipe_mnt;
 568 static int pipefs_delete_dentry(struct dentry *dentry)
 569 {
 570         return 1;
 571 }
 572 static struct dentry_operations pipefs_dentry_operations = {
 573         .d_delete       = pipefs_delete_dentry,
 574 };
 575
 576 static struct inode * get_pipe_inode(void)
 577 {
 578         struct inode *inode = new_inode(pipe_mnt->mnt_sb);
 579
 580         if (!inode)
 581                 goto fail_inode;
 582
 583         if(!pipe_new(inode))
 584                 goto fail_iput;
 585         PIPE_READERS(*inode) = PIPE_WRITERS(*inode) = 1;
 586         inode->i_fop = &rdwr_pipe_fops;
 587
 588         /*
 589          * Mark the inode dirty from the very beginning,
 590          * that way it will never be moved to the dirty
 591          * list because "mark_inode_dirty()" will think
 592          * that it already _is_ on the dirty list.
 593          */
 594         inode->i_state = I_DIRTY;
 595         inode->i_mode = S_IFIFO | S_IRUSR | S_IWUSR;
 596         inode->i_uid = current->fsuid;
 597         inode->i_gid = current->fsgid;
 598         inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 599         inode->i_blksize = PAGE_SIZE;
 600         return inode;
 601
 602 fail_iput:
 603         iput(inode);
 604 fail_inode:
 605         return NULL;
 606 }
 607
 608 int do_pipe(int *fd)
 609 {
 610         struct qstr this;
 611         char name[32];
 612         struct dentry *dentry;
 613         struct inode * inode;
 614         struct file *f1, *f2;
 615         int error;
 616         int i,j;
 617
 618         error = -ENFILE;
 619         f1 = get_empty_filp();
 620         if (!f1)
 621                 goto no_files;
 622
 623         f2 = get_empty_filp();
 624         if (!f2)
 625                 goto close_f1;
 626
 627         inode = get_pipe_inode();
 628         if (!inode)
 629                 goto close_f12;
 630
 631         error = get_unused_fd();
 632         if (error < 0)
 633                 goto close_f12_inode;
 634         i = error;
 635
 636         error = get_unused_fd();
 637         if (error < 0)
 638                 goto close_f12_inode_i;
 639         j = error;
 640
 641         error = -ENOMEM;
 642         sprintf(name, "[%lu]", inode->i_ino);
 643         this.name = name;
 644         this.len = strlen(name);
 645         this.hash = inode->i_ino; /* will go */
 646         dentry = d_alloc(pipe_mnt->mnt_sb->s_root, &this);
 647         if (!dentry)
 648                 goto close_f12_inode_i_j;
 649         dentry->d_op = &pipefs_dentry_operations;
 650         d_add(dentry, inode);
 651         f1->f_vfsmnt = f2->f_vfsmnt = mntget(mntget(pipe_mnt));
 652         f1->f_dentry = f2->f_dentry = dget(dentry);
 653         f1->f_mapping = f2->f_mapping = inode->i_mapping;
 654
 655         /* read file */
 656         f1->f_pos = f2->f_pos = 0;
 657         f1->f_flags = O_RDONLY;
 658         f1->f_op = &read_pipe_fops;
 659         f1->f_mode = 1;
 660         f1->f_version = 0;
 661
 662         /* write file */
 663         f2->f_flags = O_WRONLY;
 664         f2->f_op = &write_pipe_fops;
 665         f2->f_mode = 2;
 666         f2->f_version = 0;
 667
 668         fd_install(i, f1);
 669         fd_install(j, f2);
 670         fd[0] = i;
 671         fd[1] = j;
 672         return 0;
 673
 674 close_f12_inode_i_j:
 675         put_unused_fd(j);
 676 close_f12_inode_i:
 677         put_unused_fd(i);
 678 close_f12_inode:
 679         free_page((unsigned long) PIPE_BASE(*inode));
 680         kfree(inode->i_pipe);
 681         inode->i_pipe = NULL;
 682         iput(inode);
 683 close_f12:
 684         put_filp(f2);
 685 close_f1:
 686         put_filp(f1);
 687 no_files:
 688         return error;
 689 }
 690
 691 /*
 692  * pipefs should _never_ be mounted by userland - too much of security hassle,
 693  * no real gain from having the whole whorehouse mounted. So we don't need
 694  * any operations on the root directory. However, we need a non-trivial
 695  * d_name - pipe: will go nicely and kill the special-casing in procfs.
 696  */
 697
 698 static struct super_block *pipefs_get_sb(struct file_system_type *fs_type,
 699         int flags, const char *dev_name, void *data)
 700 {
 701         return get_sb_pseudo(fs_type, "pipe:", NULL, PIPEFS_MAGIC);
 702 }
 703
 704 static struct file_system_type pipe_fs_type = {
 705         .name           = "pipefs",
 706         .get_sb         = pipefs_get_sb,
 707         .kill_sb        = kill_anon_super,
 708 };
 709
 710 static int __init init_pipe_fs(void)
 711 {
 712         int err = register_filesystem(&pipe_fs_type);
 713         if (!err) {
 714                 pipe_mnt = kern_mount(&pipe_fs_type);
 715                 if (IS_ERR(pipe_mnt)) {
 716                         err = PTR_ERR(pipe_mnt);
 717                         unregister_filesystem(&pipe_fs_type);
 718                 }
 719         }
 720         return err;
 721 }
 722
 723 static void __exit exit_pipe_fs(void)
 724 {
 725         unregister_filesystem(&pipe_fs_type);
 726         mntput(pipe_mnt);
 727 }
 728
 729 module_init(init_pipe_fs)
 730 module_exit(exit_pipe_fs)