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 super_block *sb;
 132         struct dentry *ptmx_dentry;
 133 };
 134
 135 static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
 136 {
 137         return sb->s_fs_info;
 138 }
 139
 140 static inline struct super_block *pts_sb_from_inode(struct inode *inode)
 141 {
 142 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 143         if (inode->i_sb->s_magic == DEVPTS_SUPER_MAGIC)
 144                 return inode->i_sb;
 145 #endif
 146         if (!devpts_mnt)
 147                 return NULL;
 148         return devpts_mnt->mnt_sb;
 149 }
 150
 151 #define PARSE_MOUNT     0
 152 #define PARSE_REMOUNT   1
 153
 154 /*
 155  * parse_mount_options():
 156  *      Set @opts to mount options specified in @data. If an option is not
 157  *      specified in @data, set it to its default value. The exception is
 158  *      'newinstance' option which can only be set/cleared on a mount (i.e.
 159  *      cannot be changed during remount).
 160  *
 161  * Note: @data may be NULL (in which case all options are set to default).
 162  */
 163 static int parse_mount_options(char *data, int op, struct pts_mount_opts *opts)
 164 {
 165         char *p;
 166         kuid_t uid;
 167         kgid_t gid;
 168
 169         opts->setuid  = 0;
 170         opts->setgid  = 0;
 171         opts->uid     = GLOBAL_ROOT_UID;
 172         opts->gid     = GLOBAL_ROOT_GID;
 173         opts->mode    = DEVPTS_DEFAULT_MODE;
 174         opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
 175         opts->max     = NR_UNIX98_PTY_MAX;
 176
 177         /* newinstance makes sense only on initial mount */
 178         if (op == PARSE_MOUNT)
 179                 opts->newinstance = 0;
 180
 181         while ((p = strsep(&data, ",")) != NULL) {
 182                 substring_t args[MAX_OPT_ARGS];
 183                 int token;
 184                 int option;
 185
 186                 if (!*p)
 187                         continue;
 188
 189                 token = match_token(p, tokens, args);
 190                 switch (token) {
 191                 case Opt_uid:
 192                         if (match_int(&args[0], &option))
 193                                 return -EINVAL;
 194                         uid = make_kuid(current_user_ns(), option);
 195                         if (!uid_valid(uid))
 196                                 return -EINVAL;
 197                         opts->uid = uid;
 198                         opts->setuid = 1;
 199                         break;
 200                 case Opt_gid:
 201                         if (match_int(&args[0], &option))
 202                                 return -EINVAL;
 203                         gid = make_kgid(current_user_ns(), option);
 204                         if (!gid_valid(gid))
 205                                 return -EINVAL;
 206                         opts->gid = gid;
 207                         opts->setgid = 1;
 208                         break;
 209                 case Opt_mode:
 210                         if (match_octal(&args[0], &option))
 211                                 return -EINVAL;
 212                         opts->mode = option & S_IALLUGO;
 213                         break;
 214 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 215                 case Opt_ptmxmode:
 216                         if (match_octal(&args[0], &option))
 217                                 return -EINVAL;
 218                         opts->ptmxmode = option & S_IALLUGO;
 219                         break;
 220                 case Opt_newinstance:
 221                         /* newinstance makes sense only on initial mount */
 222                         if (op == PARSE_MOUNT)
 223                                 opts->newinstance = 1;
 224                         break;
 225                 case Opt_max:
 226                         if (match_int(&args[0], &option) ||
 227                             option < 0 || option > NR_UNIX98_PTY_MAX)
 228                                 return -EINVAL;
 229                         opts->max = option;
 230                         break;
 231 #endif
 232                 default:
 233                         pr_err("called with bogus options\n");
 234                         return -EINVAL;
 235                 }
 236         }
 237
 238         return 0;
 239 }
 240
 241 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 242 static int mknod_ptmx(struct super_block *sb)
 243 {
 244         int mode;
 245         int rc = -ENOMEM;
 246         struct dentry *dentry;
 247         struct inode *inode;
 248         struct dentry *root = sb->s_root;
 249         struct pts_fs_info *fsi = DEVPTS_SB(sb);
 250         struct pts_mount_opts *opts = &fsi->mount_opts;
 251         kuid_t root_uid;
 252         kgid_t root_gid;
 253
 254         root_uid = make_kuid(current_user_ns(), 0);
 255         root_gid = make_kgid(current_user_ns(), 0);
 256         if (!uid_valid(root_uid) || !gid_valid(root_gid))
 257                 return -EINVAL;
 258
 259         mutex_lock(&d_inode(root)->i_mutex);
 260
 261         /* If we have already created ptmx node, return */
 262         if (fsi->ptmx_dentry) {
 263                 rc = 0;
 264                 goto out;
 265         }
 266
 267         dentry = d_alloc_name(root, "ptmx");
 268         if (!dentry) {
 269                 pr_err("Unable to alloc dentry for ptmx node\n");
 270                 goto out;
 271         }
 272
 273         /*
 274          * Create a new 'ptmx' node in this mount of devpts.
 275          */
 276         inode = new_inode(sb);
 277         if (!inode) {
 278                 pr_err("Unable to alloc inode for ptmx node\n");
 279                 dput(dentry);
 280                 goto out;
 281         }
 282
 283         inode->i_ino = 2;
 284         inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
 285
 286         mode = S_IFCHR|opts->ptmxmode;
 287         init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
 288         inode->i_uid = root_uid;
 289         inode->i_gid = root_gid;
 290
 291         d_add(dentry, inode);
 292
 293         fsi->ptmx_dentry = dentry;
 294         rc = 0;
 295 out:
 296         mutex_unlock(&d_inode(root)->i_mutex);
 297         return rc;
 298 }
 299
 300 static void update_ptmx_mode(struct pts_fs_info *fsi)
 301 {
 302         struct inode *inode;
 303         if (fsi->ptmx_dentry) {
 304                 inode = d_inode(fsi->ptmx_dentry);
 305                 inode->i_mode = S_IFCHR|fsi->mount_opts.ptmxmode;
 306         }
 307 }
 308 #else
 309 static inline void update_ptmx_mode(struct pts_fs_info *fsi)
 310 {
 311         return;
 312 }
 313 #endif
 314
 315 static int devpts_remount(struct super_block *sb, int *flags, char *data)
 316 {
 317         int err;
 318         struct pts_fs_info *fsi = DEVPTS_SB(sb);
 319         struct pts_mount_opts *opts = &fsi->mount_opts;
 320
 321         sync_filesystem(sb);
 322         err = parse_mount_options(data, PARSE_REMOUNT, opts);
 323
 324         /*
 325          * parse_mount_options() restores options to default values
 326          * before parsing and may have changed ptmxmode. So, update the
 327          * mode in the inode too. Bogus options don't fail the remount,
 328          * so do this even on error return.
 329          */
 330         update_ptmx_mode(fsi);
 331
 332         return err;
 333 }
 334
 335 static int devpts_show_options(struct seq_file *seq, struct dentry *root)
 336 {
 337         struct pts_fs_info *fsi = DEVPTS_SB(root->d_sb);
 338         struct pts_mount_opts *opts = &fsi->mount_opts;
 339
 340         if (opts->setuid)
 341                 seq_printf(seq, ",uid=%u",
 342                            from_kuid_munged(&init_user_ns, opts->uid));
 343         if (opts->setgid)
 344                 seq_printf(seq, ",gid=%u",
 345                            from_kgid_munged(&init_user_ns, opts->gid));
 346         seq_printf(seq, ",mode=%03o", opts->mode);
 347 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 348         seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
 349         if (opts->max < NR_UNIX98_PTY_MAX)
 350                 seq_printf(seq, ",max=%d", opts->max);
 351 #endif
 352
 353         return 0;
 354 }
 355
 356 static const struct super_operations devpts_sops = {
 357         .statfs         = simple_statfs,
 358         .remount_fs     = devpts_remount,
 359         .show_options   = devpts_show_options,
 360 };
 361
 362 static void *new_pts_fs_info(struct super_block *sb)
 363 {
 364         struct pts_fs_info *fsi;
 365
 366         fsi = kzalloc(sizeof(struct pts_fs_info), GFP_KERNEL);
 367         if (!fsi)
 368                 return NULL;
 369
 370         ida_init(&fsi->allocated_ptys);
 371         fsi->mount_opts.mode = DEVPTS_DEFAULT_MODE;
 372         fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
 373         fsi->sb = sb;
 374
 375         return fsi;
 376 }
 377
 378 static int
 379 devpts_fill_super(struct super_block *s, void *data, int silent)
 380 {
 381         struct inode *inode;
 382
 383         s->s_blocksize = 1024;
 384         s->s_blocksize_bits = 10;
 385         s->s_magic = DEVPTS_SUPER_MAGIC;
 386         s->s_op = &devpts_sops;
 387         s->s_time_gran = 1;
 388
 389         s->s_fs_info = new_pts_fs_info(s);
 390         if (!s->s_fs_info)
 391                 goto fail;
 392
 393         inode = new_inode(s);
 394         if (!inode)
 395                 goto fail;
 396         inode->i_ino = 1;
 397         inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
 398         inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR;
 399         inode->i_op = &simple_dir_inode_operations;
 400         inode->i_fop = &simple_dir_operations;
 401         set_nlink(inode, 2);
 402
 403         s->s_root = d_make_root(inode);
 404         if (s->s_root)
 405                 return 0;
 406
 407         pr_err("get root dentry failed\n");
 408
 409 fail:
 410         return -ENOMEM;
 411 }
 412
 413 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 414 static int compare_init_pts_sb(struct super_block *s, void *p)
 415 {
 416         if (devpts_mnt)
 417                 return devpts_mnt->mnt_sb == s;
 418         return 0;
 419 }
 420
 421 /*
 422  * devpts_mount()
 423  *
 424  *     If the '-o newinstance' mount option was specified, mount a new
 425  *     (private) instance of devpts.  PTYs created in this instance are
 426  *     independent of the PTYs in other devpts instances.
 427  *
 428  *     If the '-o newinstance' option was not specified, mount/remount the
 429  *     initial kernel mount of devpts.  This type of mount gives the
 430  *     legacy, single-instance semantics.
 431  *
 432  *     The 'newinstance' option is needed to support multiple namespace
 433  *     semantics in devpts while preserving backward compatibility of the
 434  *     current 'single-namespace' semantics. i.e all mounts of devpts
 435  *     without the 'newinstance' mount option should bind to the initial
 436  *     kernel mount, like mount_single().
 437  *
 438  *     Mounts with 'newinstance' option create a new, private namespace.
 439  *
 440  *     NOTE:
 441  *
 442  *     For single-mount semantics, devpts cannot use mount_single(),
 443  *     because mount_single()/sget() find and use the super-block from
 444  *     the most recent mount of devpts. But that recent mount may be a
 445  *     'newinstance' mount and mount_single() would pick the newinstance
 446  *     super-block instead of the initial super-block.
 447  */
 448 static struct dentry *devpts_mount(struct file_system_type *fs_type,
 449         int flags, const char *dev_name, void *data)
 450 {
 451         int error;
 452         struct pts_mount_opts opts;
 453         struct super_block *s;
 454
 455         error = parse_mount_options(data, PARSE_MOUNT, &opts);
 456         if (error)
 457                 return ERR_PTR(error);
 458
 459         /* Require newinstance for all user namespace mounts to ensure
 460          * the mount options are not changed.
 461          */
 462         if ((current_user_ns() != &init_user_ns) && !opts.newinstance)
 463                 return ERR_PTR(-EINVAL);
 464
 465         if (opts.newinstance)
 466                 s = sget(fs_type, NULL, set_anon_super, flags, NULL);
 467         else
 468                 s = sget(fs_type, compare_init_pts_sb, set_anon_super, flags,
 469                          NULL);
 470
 471         if (IS_ERR(s))
 472                 return ERR_CAST(s);
 473
 474         if (!s->s_root) {
 475                 error = devpts_fill_super(s, data, flags & MS_SILENT ? 1 : 0);
 476                 if (error)
 477                         goto out_undo_sget;
 478                 s->s_flags |= MS_ACTIVE;
 479         }
 480
 481         memcpy(&(DEVPTS_SB(s))->mount_opts, &opts, sizeof(opts));
 482
 483         error = mknod_ptmx(s);
 484         if (error)
 485                 goto out_undo_sget;
 486
 487         return dget(s->s_root);
 488
 489 out_undo_sget:
 490         deactivate_locked_super(s);
 491         return ERR_PTR(error);
 492 }
 493
 494 #else
 495 /*
 496  * This supports only the legacy single-instance semantics (no
 497  * multiple-instance semantics)
 498  */
 499 static struct dentry *devpts_mount(struct file_system_type *fs_type, int flags,
 500                 const char *dev_name, void *data)
 501 {
 502         return mount_single(fs_type, flags, data, devpts_fill_super);
 503 }
 504 #endif
 505
 506 static void devpts_kill_sb(struct super_block *sb)
 507 {
 508         struct pts_fs_info *fsi = DEVPTS_SB(sb);
 509
 510         ida_destroy(&fsi->allocated_ptys);
 511         kfree(fsi);
 512         kill_litter_super(sb);
 513 }
 514
 515 static struct file_system_type devpts_fs_type = {
 516         .name           = "devpts",
 517         .mount          = devpts_mount,
 518         .kill_sb        = devpts_kill_sb,
 519 #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
 520         .fs_flags       = FS_USERNS_MOUNT | FS_USERNS_DEV_MOUNT,
 521 #endif
 522 };
 523
 524 /*
 525  * The normal naming convention is simply /dev/pts/<number>; this conforms
 526  * to the System V naming convention
 527  */
 528
 529 int devpts_new_index(struct pts_fs_info *fsi)
 530 {
 531         int index;
 532         int ida_ret;
 533
 534         if (!fsi)
 535                 return -ENODEV;
 536
 537 retry:
 538         if (!ida_pre_get(&fsi->allocated_ptys, GFP_KERNEL))
 539                 return -ENOMEM;
 540
 541         mutex_lock(&allocated_ptys_lock);
 542         if (pty_count >= pty_limit -
 543                         (fsi->mount_opts.newinstance ? pty_reserve : 0)) {
 544                 mutex_unlock(&allocated_ptys_lock);
 545                 return -ENOSPC;
 546         }
 547
 548         ida_ret = ida_get_new(&fsi->allocated_ptys, &index);
 549         if (ida_ret < 0) {
 550                 mutex_unlock(&allocated_ptys_lock);
 551                 if (ida_ret == -EAGAIN)
 552                         goto retry;
 553                 return -EIO;
 554         }
 555
 556         if (index >= fsi->mount_opts.max) {
 557                 ida_remove(&fsi->allocated_ptys, index);
 558                 mutex_unlock(&allocated_ptys_lock);
 559                 return -ENOSPC;
 560         }
 561         pty_count++;
 562         mutex_unlock(&allocated_ptys_lock);
 563         return index;
 564 }
 565
 566 void devpts_kill_index(struct pts_fs_info *fsi, int idx)
 567 {
 568         mutex_lock(&allocated_ptys_lock);
 569         ida_remove(&fsi->allocated_ptys, idx);
 570         pty_count--;
 571         mutex_unlock(&allocated_ptys_lock);
 572 }
 573
 574 /*
 575  * pty code needs to hold extra references in case of last /dev/tty close
 576  */
 577 struct pts_fs_info *devpts_get_ref(struct inode *ptmx_inode, struct file *file)
 578 {
 579         struct super_block *sb;
 580         struct pts_fs_info *fsi;
 581
 582         sb = pts_sb_from_inode(ptmx_inode);
 583         if (!sb)
 584                 return NULL;
 585         fsi = DEVPTS_SB(sb);
 586         if (!fsi)
 587                 return NULL;
 588
 589         atomic_inc(&sb->s_active);
 590         return fsi;
 591 }
 592
 593 void devpts_put_ref(struct pts_fs_info *fsi)
 594 {
 595         deactivate_super(fsi->sb);
 596 }
 597
 598 /**
 599  * devpts_pty_new -- create a new inode in /dev/pts/
 600  * @ptmx_inode: inode of the master
 601  * @device: major+minor of the node to be created
 602  * @index: used as a name of the node
 603  * @priv: what's given back by devpts_get_priv
 604  *
 605  * The created inode is returned. Remove it from /dev/pts/ by devpts_pty_kill.
 606  */
 607 struct inode *devpts_pty_new(struct pts_fs_info *fsi, dev_t device, int index,
 608                 void *priv)
 609 {
 610         struct dentry *dentry;
 611         struct super_block *sb;
 612         struct inode *inode;
 613         struct dentry *root;
 614         struct pts_mount_opts *opts;
 615         char s[12];
 616
 617         if (!fsi)
 618                 return ERR_PTR(-ENODEV);
 619
 620         sb = fsi->sb;
 621         root = sb->s_root;
 622         opts = &fsi->mount_opts;
 623
 624         inode = new_inode(sb);
 625         if (!inode)
 626                 return ERR_PTR(-ENOMEM);
 627
 628         inode->i_ino = index + 3;
 629         inode->i_uid = opts->setuid ? opts->uid : current_fsuid();
 630         inode->i_gid = opts->setgid ? opts->gid : current_fsgid();
 631         inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
 632         init_special_inode(inode, S_IFCHR|opts->mode, device);
 633         inode->i_private = priv;
 634
 635         sprintf(s, "%d", index);
 636
 637         mutex_lock(&d_inode(root)->i_mutex);
 638
 639         dentry = d_alloc_name(root, s);
 640         if (dentry) {
 641                 d_add(dentry, inode);
 642                 fsnotify_create(d_inode(root), dentry);
 643         } else {
 644                 iput(inode);
 645                 inode = ERR_PTR(-ENOMEM);
 646         }
 647
 648         mutex_unlock(&d_inode(root)->i_mutex);
 649
 650         return inode;
 651 }
 652
 653 /**
 654  * devpts_get_priv -- get private data for a slave
 655  * @pts_inode: inode of the slave
 656  *
 657  * Returns whatever was passed as priv in devpts_pty_new for a given inode.
 658  */
 659 void *devpts_get_priv(struct inode *pts_inode)
 660 {
 661         struct dentry *dentry;
 662         void *priv = NULL;
 663
 664         BUG_ON(pts_inode->i_rdev == MKDEV(TTYAUX_MAJOR, PTMX_MINOR));
 665
 666         /* Ensure dentry has not been deleted by devpts_pty_kill() */
 667         dentry = d_find_alias(pts_inode);
 668         if (!dentry)
 669                 return NULL;
 670
 671         if (pts_inode->i_sb->s_magic == DEVPTS_SUPER_MAGIC)
 672                 priv = pts_inode->i_private;
 673
 674         dput(dentry);
 675
 676         return priv;
 677 }
 678
 679 /**
 680  * devpts_pty_kill -- remove inode form /dev/pts/
 681  * @inode: inode of the slave to be removed
 682  *
 683  * This is an inverse operation of devpts_pty_new.
 684  */
 685 void devpts_pty_kill(struct inode *inode)
 686 {
 687         struct super_block *sb = pts_sb_from_inode(inode);
 688         struct dentry *root = sb->s_root;
 689         struct dentry *dentry;
 690
 691         BUG_ON(inode->i_rdev == MKDEV(TTYAUX_MAJOR, PTMX_MINOR));
 692
 693         mutex_lock(&d_inode(root)->i_mutex);
 694
 695         dentry = d_find_alias(inode);
 696
 697         drop_nlink(inode);
 698         d_delete(dentry);
 699         dput(dentry);   /* d_alloc_name() in devpts_pty_new() */
 700         dput(dentry);           /* d_find_alias above */
 701
 702         mutex_unlock(&d_inode(root)->i_mutex);
 703 }
 704
 705 static int __init init_devpts_fs(void)
 706 {
 707         int err = register_filesystem(&devpts_fs_type);
 708         struct ctl_table_header *table;
 709
 710         if (!err) {
 711                 struct vfsmount *mnt;
 712
 713                 table = register_sysctl_table(pty_root_table);
 714                 mnt = kern_mount(&devpts_fs_type);
 715                 if (IS_ERR(mnt)) {
 716                         err = PTR_ERR(mnt);
 717                         unregister_filesystem(&devpts_fs_type);
 718                         unregister_sysctl_table(table);
 719                 } else {
 720                         devpts_mnt = mnt;
 721                 }
 722         }
 723         return err;
 724 }
 725 module_init(init_devpts_fs)