fs/devpts/inode.c

   1 /* -*- linux-c -*- --------------------------------------------------------- *
   2  *
   3  * linux/fs/devpts/inode.c
   4  *
   5  *  Copyright 1998-2004 H. Peter Anvin -- All Rights Reserved
   6  *
   7  * This file is part of the Linux kernel and is made available under
   8  * the terms of the GNU General Public License, version 2, or at your
   9  * option, any later version, incorporated herein by reference.
  10  *
  11  * ------------------------------------------------------------------------- */
  12
  13 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  14
  15 #include <linux/module.h>
  16 #include <linux/init.h>
  17 #include <linux/fs.h>
  18 #include <linux/sched.h>
  19 #include <linux/namei.h>
  20 #include <linux/slab.h>
  21 #include <linux/mount.h>
  22 #include <linux/tty.h>
  23 #include <linux/mutex.h>
  24 #include <linux/magic.h>
  25 #include <linux/idr.h>
  26 #include <linux/devpts_fs.h>
  27 #include <linux/parser.h>
  28 #include <linux/fsnotify.h>
  29 #include <linux/seq_file.h>
  30
  31 #define DEVPTS_DEFAULT_MODE 0600
  32 /*
  33  * ptmx is a new node in /dev/pts and will be unused in legacy (single-
  34  * instance) mode. To prevent surprises in user space, set permissions of
  35  * ptmx to 0. Use 'chmod' or remount with '-o ptmxmode' to set meaningful
  36  * permissions.
  37  */
  38 #define DEVPTS_DEFAULT_PTMX_MODE 0000
  39 #define PTMX_MINOR      2
  40
  41 /*
  42  * sysctl support for setting limits on the number of Unix98 ptys allocated.
  43  * Otherwise one can eat up all kernel memory by opening /dev/ptmx repeatedly.
  44  */
  45 static int pty_limit = NR_UNIX98_PTY_DEFAULT;
  46 static int pty_reserve = NR_UNIX98_PTY_RESERVE;
  47 static int pty_limit_min;
  48 static int pty_limit_max = INT_MAX;
  49 static int pty_count;
  50
  51 static struct ctl_table pty_table[] = {
  52         {
  53                 .procname       = "max",
  54                 .maxlen         = sizeof(int),
  55                 .mode           = 0644,
  56                 .data           = &pty_limit,
  57                 .proc_handler   = proc_dointvec_minmax,
  58                 .extra1         = &pty_limit_min,
  59                 .extra2         = &pty_limit_max,
  60         }, {
  61                 .procname       = "reserve",
  62                 .maxlen         = sizeof(int),
  63                 .mode           = 0644,
  64                 .data           = &pty_reserve,
  65                 .proc_handler   = proc_dointvec_minmax,
  66                 .extra1         = &pty_limit_min,
  67                 .extra2         = &pty_limit_max,
  68         }, {
  69                 .procname       = "nr",
  70                 .maxlen         = sizeof(int),
  71                 .mode           = 0444,
  72                 .data           = &pty_count,
  73                 .proc_handler   = proc_dointvec,
  74         },
  75         {}
  76 };
  77
  78 static struct ctl_table pty_kern_table[] = {
  79         {
  80                 .procname       = "pty",
  81                 .mode           = 0555,
  82                 .child          = pty_table,
  83         },
  84         {}
  85 };
  86
  87 static struct ctl_table pty_root_table[] = {
  88         {
  89                 .procname       = "kernel",
  90                 .mode           = 0555,
  91                 .child          = pty_kern_table,
  92         },
  93         {}
  94 };
  95
  96 static DEFINE_MUTEX(allocated_ptys_lock);
  97
  98 static struct vfsmount *devpts_mnt;
  99
 100 struct pts_mount_opts {
 101         int setuid;
 102         int setgid;
 103         kuid_t   uid;
 104         kgid_t   gid;
 105         umode_t mode;
 106         umode_t ptmxmode;
 107         int newinstance;
 108         int max;
 109 };
 110
 111 enum {
 112         Opt_uid, Opt_gid, Opt_mode, Opt_ptmxmode, Opt_newinstance,  Opt_max,
 113         Opt_err
 114 };
 115
 116 static const match_table_t tokens = {
 117         {Opt_uid, "uid=%u"},
 118         {Opt_gid, "gid=%u"},
 119         {Opt_mode, "mode=%o"},
 120 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 121         {Opt_ptmxmode, "ptmxmode=%o"},
 122         {Opt_newinstance, "newinstance"},
 123         {Opt_max, "max=%d"},
 124 #endif
 125         {Opt_err, NULL}
 126 };
 127
 128 struct pts_fs_info {
 129         struct ida allocated_ptys;
 130         struct pts_mount_opts mount_opts;
 131         struct dentry *ptmx_dentry;
 132 };
 133
 134 static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
 135 {
 136         return sb->s_fs_info;
 137 }
 138
 139 static inline struct super_block *pts_sb_from_inode(struct inode *inode)
 140 {
 141 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 142         if (inode->i_sb->s_magic == DEVPTS_SUPER_MAGIC)
 143                 return inode->i_sb;
 144 #endif
 145         if (!devpts_mnt)
 146                 return NULL;
 147         return devpts_mnt->mnt_sb;
 148 }
 149
 150 #define PARSE_MOUNT     0
 151 #define PARSE_REMOUNT   1
 152
 153 /*
 154  * parse_mount_options():
 155  *      Set @opts to mount options specified in @data. If an option is not
 156  *      specified in @data, set it to its default value. The exception is
 157  *      'newinstance' option which can only be set/cleared on a mount (i.e.
 158  *      cannot be changed during remount).
 159  *
 160  * Note: @data may be NULL (in which case all options are set to default).
 161  */
 162 static int parse_mount_options(char *data, int op, struct pts_mount_opts *opts)
 163 {
 164         char *p;
 165         kuid_t uid;
 166         kgid_t gid;
 167
 168         opts->setuid  = 0;
 169         opts->setgid  = 0;
 170         opts->uid     = GLOBAL_ROOT_UID;
 171         opts->gid     = GLOBAL_ROOT_GID;
 172         opts->mode    = DEVPTS_DEFAULT_MODE;
 173         opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
 174         opts->max     = NR_UNIX98_PTY_MAX;
 175
 176         /* newinstance makes sense only on initial mount */
 177         if (op == PARSE_MOUNT)
 178                 opts->newinstance = 0;
 179
 180         while ((p = strsep(&data, ",")) != NULL) {
 181                 substring_t args[MAX_OPT_ARGS];
 182                 int token;
 183                 int option;
 184
 185                 if (!*p)
 186                         continue;
 187
 188                 token = match_token(p, tokens, args);
 189                 switch (token) {
 190                 case Opt_uid:
 191                         if (match_int(&args[0], &option))
 192                                 return -EINVAL;
 193                         uid = make_kuid(current_user_ns(), option);
 194                         if (!uid_valid(uid))
 195                                 return -EINVAL;
 196                         opts->uid = uid;
 197                         opts->setuid = 1;
 198                         break;
 199                 case Opt_gid:
 200                         if (match_int(&args[0], &option))
 201                                 return -EINVAL;
 202                         gid = make_kgid(current_user_ns(), option);
 203                         if (!gid_valid(gid))
 204                                 return -EINVAL;
 205                         opts->gid = gid;
 206                         opts->setgid = 1;
 207                         break;
 208                 case Opt_mode:
 209                         if (match_octal(&args[0], &option))
 210                                 return -EINVAL;
 211                         opts->mode = option & S_IALLUGO;
 212                         break;
 213 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 214                 case Opt_ptmxmode:
 215                         if (match_octal(&args[0], &option))
 216                                 return -EINVAL;
 217                         opts->ptmxmode = option & S_IALLUGO;
 218                         break;
 219                 case Opt_newinstance:
 220                         /* newinstance makes sense only on initial mount */
 221                         if (op == PARSE_MOUNT)
 222                                 opts->newinstance = 1;
 223                         break;
 224                 case Opt_max:
 225                         if (match_int(&args[0], &option) ||
 226                             option < 0 || option > NR_UNIX98_PTY_MAX)
 227                                 return -EINVAL;
 228                         opts->max = option;
 229                         break;
 230 #endif
 231                 default:
 232                         pr_err("called with bogus options\n");
 233                         return -EINVAL;
 234                 }
 235         }
 236
 237         return 0;
 238 }
 239
 240 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 241 static int mknod_ptmx(struct super_block *sb)
 242 {
 243         int mode;
 244         int rc = -ENOMEM;
 245         struct dentry *dentry;
 246         struct inode *inode;
 247         struct dentry *root = sb->s_root;
 248         struct pts_fs_info *fsi = DEVPTS_SB(sb);
 249         struct pts_mount_opts *opts = &fsi->mount_opts;
 250         kuid_t root_uid;
 251         kgid_t root_gid;
 252
 253         root_uid = make_kuid(current_user_ns(), 0);
 254         root_gid = make_kgid(current_user_ns(), 0);
 255         if (!uid_valid(root_uid) || !gid_valid(root_gid))
 256                 return -EINVAL;
 257
 258         inode_lock(d_inode(root));
 259
 260         /* If we have already created ptmx node, return */
 261         if (fsi->ptmx_dentry) {
 262                 rc = 0;
 263                 goto out;
 264         }
 265
 266         dentry = d_alloc_name(root, "ptmx");
 267         if (!dentry) {
 268                 pr_err("Unable to alloc dentry for ptmx node\n");
 269                 goto out;
 270         }
 271
 272         /*
 273          * Create a new 'ptmx' node in this mount of devpts.
 274          */
 275         inode = new_inode(sb);
 276         if (!inode) {
 277                 pr_err("Unable to alloc inode for ptmx node\n");
 278                 dput(dentry);
 279                 goto out;
 280         }
 281
 282         inode->i_ino = 2;
 283         inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
 284
 285         mode = S_IFCHR|opts->ptmxmode;
 286         init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
 287         inode->i_uid = root_uid;
 288         inode->i_gid = root_gid;
 289
 290         d_add(dentry, inode);
 291
 292         fsi->ptmx_dentry = dentry;
 293         rc = 0;
 294 out:
 295         inode_unlock(d_inode(root));
 296         return rc;
 297 }
 298
 299 static void update_ptmx_mode(struct pts_fs_info *fsi)
 300 {
 301         struct inode *inode;
 302         if (fsi->ptmx_dentry) {
 303                 inode = d_inode(fsi->ptmx_dentry);
 304                 inode->i_mode = S_IFCHR|fsi->mount_opts.ptmxmode;
 305         }
 306 }
 307 #else
 308 static inline void update_ptmx_mode(struct pts_fs_info *fsi)
 309 {
 310         return;
 311 }
 312 #endif
 313
 314 static int devpts_remount(struct super_block *sb, int *flags, char *data)
 315 {
 316         int err;
 317         struct pts_fs_info *fsi = DEVPTS_SB(sb);
 318         struct pts_mount_opts *opts = &fsi->mount_opts;
 319
 320         sync_filesystem(sb);
 321         err = parse_mount_options(data, PARSE_REMOUNT, opts);
 322
 323         /*
 324          * parse_mount_options() restores options to default values
 325          * before parsing and may have changed ptmxmode. So, update the
 326          * mode in the inode too. Bogus options don't fail the remount,
 327          * so do this even on error return.
 328          */
 329         update_ptmx_mode(fsi);
 330
 331         return err;
 332 }
 333
 334 static int devpts_show_options(struct seq_file *seq, struct dentry *root)
 335 {
 336         struct pts_fs_info *fsi = DEVPTS_SB(root->d_sb);
 337         struct pts_mount_opts *opts = &fsi->mount_opts;
 338
 339         if (opts->setuid)
 340                 seq_printf(seq, ",uid=%u",
 341                            from_kuid_munged(&init_user_ns, opts->uid));
 342         if (opts->setgid)
 343                 seq_printf(seq, ",gid=%u",
 344                            from_kgid_munged(&init_user_ns, opts->gid));
 345         seq_printf(seq, ",mode=%03o", opts->mode);
 346 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 347         seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
 348         if (opts->max < NR_UNIX98_PTY_MAX)
 349                 seq_printf(seq, ",max=%d", opts->max);
 350 #endif
 351
 352         return 0;
 353 }
 354
 355 static const struct super_operations devpts_sops = {
 356         .statfs         = simple_statfs,
 357         .remount_fs     = devpts_remount,
 358         .show_options   = devpts_show_options,
 359 };
 360
 361 static void *new_pts_fs_info(void)
 362 {
 363         struct pts_fs_info *fsi;
 364
 365         fsi = kzalloc(sizeof(struct pts_fs_info), GFP_KERNEL);
 366         if (!fsi)
 367                 return NULL;
 368
 369         ida_init(&fsi->allocated_ptys);
 370         fsi->mount_opts.mode = DEVPTS_DEFAULT_MODE;
 371         fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
 372
 373         return fsi;
 374 }
 375
 376 static int
 377 devpts_fill_super(struct super_block *s, void *data, int silent)
 378 {
 379         struct inode *inode;
 380
 381         s->s_blocksize = 1024;
 382         s->s_blocksize_bits = 10;
 383         s->s_magic = DEVPTS_SUPER_MAGIC;
 384         s->s_op = &devpts_sops;
 385         s->s_time_gran = 1;
 386
 387         s->s_fs_info = new_pts_fs_info();
 388         if (!s->s_fs_info)
 389                 goto fail;
 390
 391         inode = new_inode(s);
 392         if (!inode)
 393                 goto fail;
 394         inode->i_ino = 1;
 395         inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
 396         inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR;
 397         inode->i_op = &simple_dir_inode_operations;
 398         inode->i_fop = &simple_dir_operations;
 399         set_nlink(inode, 2);
 400
 401         s->s_root = d_make_root(inode);
 402         if (s->s_root)
 403                 return 0;
 404
 405         pr_err("get root dentry failed\n");
 406
 407 fail:
 408         return -ENOMEM;
 409 }
 410
 411 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 412 static int compare_init_pts_sb(struct super_block *s, void *p)
 413 {
 414         if (devpts_mnt)
 415                 return devpts_mnt->mnt_sb == s;
 416         return 0;
 417 }
 418
 419 /*
 420  * devpts_mount()
 421  *
 422  *     If the '-o newinstance' mount option was specified, mount a new
 423  *     (private) instance of devpts.  PTYs created in this instance are
 424  *     independent of the PTYs in other devpts instances.
 425  *
 426  *     If the '-o newinstance' option was not specified, mount/remount the
 427  *     initial kernel mount of devpts.  This type of mount gives the
 428  *     legacy, single-instance semantics.
 429  *
 430  *     The 'newinstance' option is needed to support multiple namespace
 431  *     semantics in devpts while preserving backward compatibility of the
 432  *     current 'single-namespace' semantics. i.e all mounts of devpts
 433  *     without the 'newinstance' mount option should bind to the initial
 434  *     kernel mount, like mount_single().
 435  *
 436  *     Mounts with 'newinstance' option create a new, private namespace.
 437  *
 438  *     NOTE:
 439  *
 440  *     For single-mount semantics, devpts cannot use mount_single(),
 441  *     because mount_single()/sget() find and use the super-block from
 442  *     the most recent mount of devpts. But that recent mount may be a
 443  *     'newinstance' mount and mount_single() would pick the newinstance
 444  *     super-block instead of the initial super-block.
 445  */
 446 static struct dentry *devpts_mount(struct file_system_type *fs_type,
 447         int flags, const char *dev_name, void *data)
 448 {
 449         int error;
 450         struct pts_mount_opts opts;
 451         struct super_block *s;
 452
 453         error = parse_mount_options(data, PARSE_MOUNT, &opts);
 454         if (error)
 455                 return ERR_PTR(error);
 456
 457         /* Require newinstance for all user namespace mounts to ensure
 458          * the mount options are not changed.
 459          */
 460         if ((current_user_ns() != &init_user_ns) && !opts.newinstance)
 461                 return ERR_PTR(-EINVAL);
 462
 463         if (opts.newinstance)
 464                 s = sget(fs_type, NULL, set_anon_super, flags, NULL);
 465         else
 466                 s = sget(fs_type, compare_init_pts_sb, set_anon_super, flags,
 467                          NULL);
 468
 469         if (IS_ERR(s))
 470                 return ERR_CAST(s);
 471
 472         if (!s->s_root) {
 473                 error = devpts_fill_super(s, data, flags & MS_SILENT ? 1 : 0);
 474                 if (error)
 475                         goto out_undo_sget;
 476                 s->s_flags |= MS_ACTIVE;
 477         }
 478
 479         memcpy(&(DEVPTS_SB(s))->mount_opts, &opts, sizeof(opts));
 480
 481         error = mknod_ptmx(s);
 482         if (error)
 483                 goto out_undo_sget;
 484
 485         return dget(s->s_root);
 486
 487 out_undo_sget:
 488         deactivate_locked_super(s);
 489         return ERR_PTR(error);
 490 }
 491
 492 #else
 493 /*
 494  * This supports only the legacy single-instance semantics (no
 495  * multiple-instance semantics)
 496  */
 497 static struct dentry *devpts_mount(struct file_system_type *fs_type, int flags,
 498                 const char *dev_name, void *data)
 499 {
 500         return mount_single(fs_type, flags, data, devpts_fill_super);
 501 }
 502 #endif
 503
 504 static void devpts_kill_sb(struct super_block *sb)
 505 {
 506         struct pts_fs_info *fsi = DEVPTS_SB(sb);
 507
 508         ida_destroy(&fsi->allocated_ptys);
 509         kfree(fsi);
 510         kill_litter_super(sb);
 511 }
 512
 513 static struct file_system_type devpts_fs_type = {
 514         .name           = "devpts",
 515         .mount          = devpts_mount,
 516         .kill_sb        = devpts_kill_sb,
 517 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 518         .fs_flags       = FS_USERNS_MOUNT | FS_USERNS_DEV_MOUNT,
 519 #endif
 520 };
 521
 522 /*
 523  * The normal naming convention is simply /dev/pts/<number>; this conforms
 524  * to the System V naming convention
 525  */
 526
 527 int devpts_new_index(struct inode *ptmx_inode)
 528 {
 529         struct super_block *sb = pts_sb_from_inode(ptmx_inode);
 530         struct pts_fs_info *fsi;
 531         int index;
 532         int ida_ret;
 533
 534         if (!sb)
 535                 return -ENODEV;
 536
 537         fsi = DEVPTS_SB(sb);
 538 retry:
 539         if (!ida_pre_get(&fsi->allocated_ptys, GFP_KERNEL))
 540                 return -ENOMEM;
 541
 542         mutex_lock(&allocated_ptys_lock);
 543         if (pty_count >= pty_limit -
 544                         (fsi->mount_opts.newinstance ? pty_reserve : 0)) {
 545                 mutex_unlock(&allocated_ptys_lock);
 546                 return -ENOSPC;
 547         }
 548
 549         ida_ret = ida_get_new(&fsi->allocated_ptys, &index);
 550         if (ida_ret < 0) {
 551                 mutex_unlock(&allocated_ptys_lock);
 552                 if (ida_ret == -EAGAIN)
 553                         goto retry;
 554                 return -EIO;
 555         }
 556
 557         if (index >= fsi->mount_opts.max) {
 558                 ida_remove(&fsi->allocated_ptys, index);
 559                 mutex_unlock(&allocated_ptys_lock);
 560                 return -ENOSPC;
 561         }
 562         pty_count++;
 563         mutex_unlock(&allocated_ptys_lock);
 564         return index;
 565 }
 566
 567 void devpts_kill_index(struct inode *ptmx_inode, int idx)
 568 {
 569         struct super_block *sb = pts_sb_from_inode(ptmx_inode);
 570         struct pts_fs_info *fsi = DEVPTS_SB(sb);
 571
 572         mutex_lock(&allocated_ptys_lock);
 573         ida_remove(&fsi->allocated_ptys, idx);
 574         pty_count--;
 575         mutex_unlock(&allocated_ptys_lock);
 576 }
 577
 578 /**
 579  * devpts_pty_new -- create a new inode in /dev/pts/
 580  * @ptmx_inode: inode of the master
 581  * @device: major+minor of the node to be created
 582  * @index: used as a name of the node
 583  * @priv: what's given back by devpts_get_priv
 584  *
 585  * The created inode is returned. Remove it from /dev/pts/ by devpts_pty_kill.
 586  */
 587 struct inode *devpts_pty_new(struct inode *ptmx_inode, dev_t device, int index,
 588                 void *priv)
 589 {
 590         struct dentry *dentry;
 591         struct super_block *sb = pts_sb_from_inode(ptmx_inode);
 592         struct inode *inode;
 593         struct dentry *root;
 594         struct pts_fs_info *fsi;
 595         struct pts_mount_opts *opts;
 596         char s[12];
 597
 598         if (!sb)
 599                 return ERR_PTR(-ENODEV);
 600
 601         root = sb->s_root;
 602         fsi = DEVPTS_SB(sb);
 603         opts = &fsi->mount_opts;
 604
 605         inode = new_inode(sb);
 606         if (!inode)
 607                 return ERR_PTR(-ENOMEM);
 608
 609         inode->i_ino = index + 3;
 610         inode->i_uid = opts->setuid ? opts->uid : current_fsuid();
 611         inode->i_gid = opts->setgid ? opts->gid : current_fsgid();
 612         inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
 613         init_special_inode(inode, S_IFCHR|opts->mode, device);
 614         inode->i_private = priv;
 615
 616         sprintf(s, "%d", index);
 617
 618         inode_lock(d_inode(root));
 619
 620         dentry = d_alloc_name(root, s);
 621         if (dentry) {
 622                 d_add(dentry, inode);
 623                 fsnotify_create(d_inode(root), dentry);
 624         } else {
 625                 iput(inode);
 626                 inode = ERR_PTR(-ENOMEM);
 627         }
 628
 629         inode_unlock(d_inode(root));
 630
 631         return inode;
 632 }
 633
 634 /**
 635  * devpts_get_priv -- get private data for a slave
 636  * @pts_inode: inode of the slave
 637  *
 638  * Returns whatever was passed as priv in devpts_pty_new for a given inode.
 639  */
 640 void *devpts_get_priv(struct inode *pts_inode)
 641 {
 642         struct dentry *dentry;
 643         void *priv = NULL;
 644
 645         BUG_ON(pts_inode->i_rdev == MKDEV(TTYAUX_MAJOR, PTMX_MINOR));
 646
 647         /* Ensure dentry has not been deleted by devpts_pty_kill() */
 648         dentry = d_find_alias(pts_inode);
 649         if (!dentry)
 650                 return NULL;
 651
 652         if (pts_inode->i_sb->s_magic == DEVPTS_SUPER_MAGIC)
 653                 priv = pts_inode->i_private;
 654
 655         dput(dentry);
 656
 657         return priv;
 658 }
 659
 660 /**
 661  * devpts_pty_kill -- remove inode form /dev/pts/
 662  * @inode: inode of the slave to be removed
 663  *
 664  * This is an inverse operation of devpts_pty_new.
 665  */
 666 void devpts_pty_kill(struct inode *inode)
 667 {
 668         struct super_block *sb = pts_sb_from_inode(inode);
 669         struct dentry *root = sb->s_root;
 670         struct dentry *dentry;
 671
 672         BUG_ON(inode->i_rdev == MKDEV(TTYAUX_MAJOR, PTMX_MINOR));
 673
 674         inode_lock(d_inode(root));
 675
 676         dentry = d_find_alias(inode);
 677
 678         drop_nlink(inode);
 679         d_delete(dentry);
 680         dput(dentry);   /* d_alloc_name() in devpts_pty_new() */
 681         dput(dentry);           /* d_find_alias above */
 682
 683         inode_unlock(d_inode(root));
 684 }
 685
 686 static int __init init_devpts_fs(void)
 687 {
 688         int err = register_filesystem(&devpts_fs_type);
 689         struct ctl_table_header *table;
 690
 691         if (!err) {
 692                 struct vfsmount *mnt;
 693
 694                 table = register_sysctl_table(pty_root_table);
 695                 mnt = kern_mount(&devpts_fs_type);
 696                 if (IS_ERR(mnt)) {
 697                         err = PTR_ERR(mnt);
 698                         unregister_filesystem(&devpts_fs_type);
 699                         unregister_sysctl_table(table);
 700                 } else {
 701                         devpts_mnt = mnt;
 702                 }
 703         }
 704         return err;
 705 }
 706 module_init(init_devpts_fs)