root/fs/pnode.c

DEFINITIONS

1. next_peer
2. first_slave
3. last_slave
4. next_slave
5. get_peer_under_root
6. get_dominating_id
7. do_make_slave
8. change_mnt_propagation
9. propagation_next
10. skip_propagation_subtree
11. next_group
12. peers
13. propagate_one
14. propagate_mnt
15. find_topper
16. do_refcount_check
17. propagate_mount_busy
18. propagate_mount_unlock
19. umount_one
20. __propagate_umount
21. umount_list
22. restore_mounts
23. cleanup_umount_visitations
24. propagate_umount

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  *  linux/fs/pnode.c
   4  *
   5  * (C) Copyright IBM Corporation 2005.
   6  *      Author : Ram Pai (linuxram@us.ibm.com)
   7  */
   8 #include <linux/mnt_namespace.h>
   9 #include <linux/mount.h>
  10 #include <linux/fs.h>
  11 #include <linux/nsproxy.h>
  12 #include <uapi/linux/mount.h>
  13 #include "internal.h"
  14 #include "pnode.h"
  15 
  16 /* return the next shared peer mount of @p */
  17 static inline struct mount *next_peer(struct mount *p)
  18 {
  19         return list_entry(p->mnt_share.next, struct mount, mnt_share);
  20 }
  21 
  22 static inline struct mount *first_slave(struct mount *p)
  23 {
  24         return list_entry(p->mnt_slave_list.next, struct mount, mnt_slave);
  25 }
  26 
  27 static inline struct mount *last_slave(struct mount *p)
  28 {
  29         return list_entry(p->mnt_slave_list.prev, struct mount, mnt_slave);
  30 }
  31 
  32 static inline struct mount *next_slave(struct mount *p)
  33 {
  34         return list_entry(p->mnt_slave.next, struct mount, mnt_slave);
  35 }
  36 
  37 static struct mount *get_peer_under_root(struct mount *mnt,
  38                                          struct mnt_namespace *ns,
  39                                          const struct path *root)
  40 {
  41         struct mount *m = mnt;
  42 
  43         do {
  44                 /* Check the namespace first for optimization */
  45                 if (m->mnt_ns == ns && is_path_reachable(m, m->mnt.mnt_root, root))
  46                         return m;
  47 
  48                 m = next_peer(m);
  49         } while (m != mnt);
  50 
  51         return NULL;
  52 }
  53 
  54 /*
  55  * Get ID of closest dominating peer group having a representative
  56  * under the given root.
  57  *
  58  * Caller must hold namespace_sem
  59  */
  60 int get_dominating_id(struct mount *mnt, const struct path *root)
  61 {
  62         struct mount *m;
  63 
  64         for (m = mnt->mnt_master; m != NULL; m = m->mnt_master) {
  65                 struct mount *d = get_peer_under_root(m, mnt->mnt_ns, root);
  66                 if (d)
  67                         return d->mnt_group_id;
  68         }
  69 
  70         return 0;
  71 }
  72 
  73 static int do_make_slave(struct mount *mnt)
  74 {
  75         struct mount *master, *slave_mnt;
  76 
  77         if (list_empty(&mnt->mnt_share)) {
  78                 if (IS_MNT_SHARED(mnt)) {
  79                         mnt_release_group_id(mnt);
  80                         CLEAR_MNT_SHARED(mnt);
  81                 }
  82                 master = mnt->mnt_master;
  83                 if (!master) {
  84                         struct list_head *p = &mnt->mnt_slave_list;
  85                         while (!list_empty(p)) {
  86                                 slave_mnt = list_first_entry(p,
  87                                                 struct mount, mnt_slave);
  88                                 list_del_init(&slave_mnt->mnt_slave);
  89                                 slave_mnt->mnt_master = NULL;
  90                         }
  91                         return 0;
  92                 }
  93         } else {
  94                 struct mount *m;
  95                 /*
  96                  * slave 'mnt' to a peer mount that has the
  97                  * same root dentry. If none is available then
  98                  * slave it to anything that is available.
  99                  */
 100                 for (m = master = next_peer(mnt); m != mnt; m = next_peer(m)) {
 101                         if (m->mnt.mnt_root == mnt->mnt.mnt_root) {
 102                                 master = m;
 103                                 break;
 104                         }
 105                 }
 106                 list_del_init(&mnt->mnt_share);
 107                 mnt->mnt_group_id = 0;
 108                 CLEAR_MNT_SHARED(mnt);
 109         }
 110         list_for_each_entry(slave_mnt, &mnt->mnt_slave_list, mnt_slave)
 111                 slave_mnt->mnt_master = master;
 112         list_move(&mnt->mnt_slave, &master->mnt_slave_list);
 113         list_splice(&mnt->mnt_slave_list, master->mnt_slave_list.prev);
 114         INIT_LIST_HEAD(&mnt->mnt_slave_list);
 115         mnt->mnt_master = master;
 116         return 0;
 117 }
 118 
 119 /*
 120  * vfsmount lock must be held for write
 121  */
 122 void change_mnt_propagation(struct mount *mnt, int type)
 123 {
 124         if (type == MS_SHARED) {
 125                 set_mnt_shared(mnt);
 126                 return;
 127         }
 128         do_make_slave(mnt);
 129         if (type != MS_SLAVE) {
 130                 list_del_init(&mnt->mnt_slave);
 131                 mnt->mnt_master = NULL;
 132                 if (type == MS_UNBINDABLE)
 133                         mnt->mnt.mnt_flags |= MNT_UNBINDABLE;
 134                 else
 135                         mnt->mnt.mnt_flags &= ~MNT_UNBINDABLE;
 136         }
 137 }
 138 
 139 /*
 140  * get the next mount in the propagation tree.
 141  * @m: the mount seen last
 142  * @origin: the original mount from where the tree walk initiated
 143  *
 144  * Note that peer groups form contiguous segments of slave lists.
 145  * We rely on that in get_source() to be able to find out if
 146  * vfsmount found while iterating with propagation_next() is
 147  * a peer of one we'd found earlier.
 148  */
 149 static struct mount *propagation_next(struct mount *m,
 150                                          struct mount *origin)
 151 {
 152         /* are there any slaves of this mount? */
 153         if (!IS_MNT_NEW(m) && !list_empty(&m->mnt_slave_list))
 154                 return first_slave(m);
 155 
 156         while (1) {
 157                 struct mount *master = m->mnt_master;
 158 
 159                 if (master == origin->mnt_master) {
 160                         struct mount *next = next_peer(m);
 161                         return (next == origin) ? NULL : next;
 162                 } else if (m->mnt_slave.next != &master->mnt_slave_list)
 163                         return next_slave(m);
 164 
 165                 /* back at master */
 166                 m = master;
 167         }
 168 }
 169 
 170 static struct mount *skip_propagation_subtree(struct mount *m,
 171                                                 struct mount *origin)
 172 {
 173         /*
 174          * Advance m such that propagation_next will not return
 175          * the slaves of m.
 176          */
 177         if (!IS_MNT_NEW(m) && !list_empty(&m->mnt_slave_list))
 178                 m = last_slave(m);
 179 
 180         return m;
 181 }
 182 
 183 static struct mount *next_group(struct mount *m, struct mount *origin)
 184 {
 185         while (1) {
 186                 while (1) {
 187                         struct mount *next;
 188                         if (!IS_MNT_NEW(m) && !list_empty(&m->mnt_slave_list))
 189                                 return first_slave(m);
 190                         next = next_peer(m);
 191                         if (m->mnt_group_id == origin->mnt_group_id) {
 192                                 if (next == origin)
 193                                         return NULL;
 194                         } else if (m->mnt_slave.next != &next->mnt_slave)
 195                                 break;
 196                         m = next;
 197                 }
 198                 /* m is the last peer */
 199                 while (1) {
 200                         struct mount *master = m->mnt_master;
 201                         if (m->mnt_slave.next != &master->mnt_slave_list)
 202                                 return next_slave(m);
 203                         m = next_peer(master);
 204                         if (master->mnt_group_id == origin->mnt_group_id)
 205                                 break;
 206                         if (master->mnt_slave.next == &m->mnt_slave)
 207                                 break;
 208                         m = master;
 209                 }
 210                 if (m == origin)
 211                         return NULL;
 212         }
 213 }
 214 
 215 /* all accesses are serialized by namespace_sem */
 216 static struct mount *last_dest, *first_source, *last_source, *dest_master;
 217 static struct mountpoint *mp;
 218 static struct hlist_head *list;
 219 
 220 static inline bool peers(struct mount *m1, struct mount *m2)
 221 {
 222         return m1->mnt_group_id == m2->mnt_group_id && m1->mnt_group_id;
 223 }
 224 
 225 static int propagate_one(struct mount *m)
 226 {
 227         struct mount *child;
 228         int type;
 229         /* skip ones added by this propagate_mnt() */
 230         if (IS_MNT_NEW(m))
 231                 return 0;
 232         /* skip if mountpoint isn't covered by it */
 233         if (!is_subdir(mp->m_dentry, m->mnt.mnt_root))
 234                 return 0;
 235         if (peers(m, last_dest)) {
 236                 type = CL_MAKE_SHARED;
 237         } else {
 238                 struct mount *n, *p;
 239                 bool done;
 240                 for (n = m; ; n = p) {
 241                         p = n->mnt_master;
 242                         if (p == dest_master || IS_MNT_MARKED(p))
 243                                 break;
 244                 }
 245                 do {
 246                         struct mount *parent = last_source->mnt_parent;
 247                         if (last_source == first_source)
 248                                 break;
 249                         done = parent->mnt_master == p;
 250                         if (done && peers(n, parent))
 251                                 break;
 252                         last_source = last_source->mnt_master;
 253                 } while (!done);
 254 
 255                 type = CL_SLAVE;
 256                 /* beginning of peer group among the slaves? */
 257                 if (IS_MNT_SHARED(m))
 258                         type |= CL_MAKE_SHARED;
 259         }
 260                 
 261         child = copy_tree(last_source, last_source->mnt.mnt_root, type);
 262         if (IS_ERR(child))
 263                 return PTR_ERR(child);
 264         read_seqlock_excl(&mount_lock);
 265         mnt_set_mountpoint(m, mp, child);
 266         if (m->mnt_master != dest_master)
 267                 SET_MNT_MARK(m->mnt_master);
 268         read_sequnlock_excl(&mount_lock);
 269         last_dest = m;
 270         last_source = child;
 271         hlist_add_head(&child->mnt_hash, list);
 272         return count_mounts(m->mnt_ns, child);
 273 }
 274 
 275 /*
 276  * mount 'source_mnt' under the destination 'dest_mnt' at
 277  * dentry 'dest_dentry'. And propagate that mount to
 278  * all the peer and slave mounts of 'dest_mnt'.
 279  * Link all the new mounts into a propagation tree headed at
 280  * source_mnt. Also link all the new mounts using ->mnt_list
 281  * headed at source_mnt's ->mnt_list
 282  *
 283  * @dest_mnt: destination mount.
 284  * @dest_dentry: destination dentry.
 285  * @source_mnt: source mount.
 286  * @tree_list : list of heads of trees to be attached.
 287  */
 288 int propagate_mnt(struct mount *dest_mnt, struct mountpoint *dest_mp,
 289                     struct mount *source_mnt, struct hlist_head *tree_list)
 290 {
 291         struct mount *m, *n;
 292         int ret = 0;
 293 
 294         /*
 295          * we don't want to bother passing tons of arguments to
 296          * propagate_one(); everything is serialized by namespace_sem,
 297          * so globals will do just fine.
 298          */
 299         last_dest = dest_mnt;
 300         first_source = source_mnt;
 301         last_source = source_mnt;
 302         mp = dest_mp;
 303         list = tree_list;
 304         dest_master = dest_mnt->mnt_master;
 305 
 306         /* all peers of dest_mnt, except dest_mnt itself */
 307         for (n = next_peer(dest_mnt); n != dest_mnt; n = next_peer(n)) {
 308                 ret = propagate_one(n);
 309                 if (ret)
 310                         goto out;
 311         }
 312 
 313         /* all slave groups */
 314         for (m = next_group(dest_mnt, dest_mnt); m;
 315                         m = next_group(m, dest_mnt)) {
 316                 /* everything in that slave group */
 317                 n = m;
 318                 do {
 319                         ret = propagate_one(n);
 320                         if (ret)
 321                                 goto out;
 322                         n = next_peer(n);
 323                 } while (n != m);
 324         }
 325 out:
 326         read_seqlock_excl(&mount_lock);
 327         hlist_for_each_entry(n, tree_list, mnt_hash) {
 328                 m = n->mnt_parent;
 329                 if (m->mnt_master != dest_mnt->mnt_master)
 330                         CLEAR_MNT_MARK(m->mnt_master);
 331         }
 332         read_sequnlock_excl(&mount_lock);
 333         return ret;
 334 }
 335 
 336 static struct mount *find_topper(struct mount *mnt)
 337 {
 338         /* If there is exactly one mount covering mnt completely return it. */
 339         struct mount *child;
 340 
 341         if (!list_is_singular(&mnt->mnt_mounts))
 342                 return NULL;
 343 
 344         child = list_first_entry(&mnt->mnt_mounts, struct mount, mnt_child);
 345         if (child->mnt_mountpoint != mnt->mnt.mnt_root)
 346                 return NULL;
 347 
 348         return child;
 349 }
 350 
 351 /*
 352  * return true if the refcount is greater than count
 353  */
 354 static inline int do_refcount_check(struct mount *mnt, int count)
 355 {
 356         return mnt_get_count(mnt) > count;
 357 }
 358 
 359 /*
 360  * check if the mount 'mnt' can be unmounted successfully.
 361  * @mnt: the mount to be checked for unmount
 362  * NOTE: unmounting 'mnt' would naturally propagate to all
 363  * other mounts its parent propagates to.
 364  * Check if any of these mounts that **do not have submounts**
 365  * have more references than 'refcnt'. If so return busy.
 366  *
 367  * vfsmount lock must be held for write
 368  */
 369 int propagate_mount_busy(struct mount *mnt, int refcnt)
 370 {
 371         struct mount *m, *child, *topper;
 372         struct mount *parent = mnt->mnt_parent;
 373 
 374         if (mnt == parent)
 375                 return do_refcount_check(mnt, refcnt);
 376 
 377         /*
 378          * quickly check if the current mount can be unmounted.
 379          * If not, we don't have to go checking for all other
 380          * mounts
 381          */
 382         if (!list_empty(&mnt->mnt_mounts) || do_refcount_check(mnt, refcnt))
 383                 return 1;
 384 
 385         for (m = propagation_next(parent, parent); m;
 386                         m = propagation_next(m, parent)) {
 387                 int count = 1;
 388                 child = __lookup_mnt(&m->mnt, mnt->mnt_mountpoint);
 389                 if (!child)
 390                         continue;
 391 
 392                 /* Is there exactly one mount on the child that covers
 393                  * it completely whose reference should be ignored?
 394                  */
 395                 topper = find_topper(child);
 396                 if (topper)
 397                         count += 1;
 398                 else if (!list_empty(&child->mnt_mounts))
 399                         continue;
 400 
 401                 if (do_refcount_check(child, count))
 402                         return 1;
 403         }
 404         return 0;
 405 }
 406 
 407 /*
 408  * Clear MNT_LOCKED when it can be shown to be safe.
 409  *
 410  * mount_lock lock must be held for write
 411  */
 412 void propagate_mount_unlock(struct mount *mnt)
 413 {
 414         struct mount *parent = mnt->mnt_parent;
 415         struct mount *m, *child;
 416 
 417         BUG_ON(parent == mnt);
 418 
 419         for (m = propagation_next(parent, parent); m;
 420                         m = propagation_next(m, parent)) {
 421                 child = __lookup_mnt(&m->mnt, mnt->mnt_mountpoint);
 422                 if (child)
 423                         child->mnt.mnt_flags &= ~MNT_LOCKED;
 424         }
 425 }
 426 
 427 static void umount_one(struct mount *mnt, struct list_head *to_umount)
 428 {
 429         CLEAR_MNT_MARK(mnt);
 430         mnt->mnt.mnt_flags |= MNT_UMOUNT;
 431         list_del_init(&mnt->mnt_child);
 432         list_del_init(&mnt->mnt_umounting);
 433         list_move_tail(&mnt->mnt_list, to_umount);
 434 }
 435 
 436 /*
 437  * NOTE: unmounting 'mnt' naturally propagates to all other mounts its
 438  * parent propagates to.
 439  */
 440 static bool __propagate_umount(struct mount *mnt,
 441                                struct list_head *to_umount,
 442                                struct list_head *to_restore)
 443 {
 444         bool progress = false;
 445         struct mount *child;
 446 
 447         /*
 448          * The state of the parent won't change if this mount is
 449          * already unmounted or marked as without children.
 450          */
 451         if (mnt->mnt.mnt_flags & (MNT_UMOUNT | MNT_MARKED))
 452                 goto out;
 453 
 454         /* Verify topper is the only grandchild that has not been
 455          * speculatively unmounted.
 456          */
 457         list_for_each_entry(child, &mnt->mnt_mounts, mnt_child) {
 458                 if (child->mnt_mountpoint == mnt->mnt.mnt_root)
 459                         continue;
 460                 if (!list_empty(&child->mnt_umounting) && IS_MNT_MARKED(child))
 461                         continue;
 462                 /* Found a mounted child */
 463                 goto children;
 464         }
 465 
 466         /* Mark mounts that can be unmounted if not locked */
 467         SET_MNT_MARK(mnt);
 468         progress = true;
 469 
 470         /* If a mount is without children and not locked umount it. */
 471         if (!IS_MNT_LOCKED(mnt)) {
 472                 umount_one(mnt, to_umount);
 473         } else {
 474 children:
 475                 list_move_tail(&mnt->mnt_umounting, to_restore);
 476         }
 477 out:
 478         return progress;
 479 }
 480 
 481 static void umount_list(struct list_head *to_umount,
 482                         struct list_head *to_restore)
 483 {
 484         struct mount *mnt, *child, *tmp;
 485         list_for_each_entry(mnt, to_umount, mnt_list) {
 486                 list_for_each_entry_safe(child, tmp, &mnt->mnt_mounts, mnt_child) {
 487                         /* topper? */
 488                         if (child->mnt_mountpoint == mnt->mnt.mnt_root)
 489                                 list_move_tail(&child->mnt_umounting, to_restore);
 490                         else
 491                                 umount_one(child, to_umount);
 492                 }
 493         }
 494 }
 495 
 496 static void restore_mounts(struct list_head *to_restore)
 497 {
 498         /* Restore mounts to a clean working state */
 499         while (!list_empty(to_restore)) {
 500                 struct mount *mnt, *parent;
 501                 struct mountpoint *mp;
 502 
 503                 mnt = list_first_entry(to_restore, struct mount, mnt_umounting);
 504                 CLEAR_MNT_MARK(mnt);
 505                 list_del_init(&mnt->mnt_umounting);
 506 
 507                 /* Should this mount be reparented? */
 508                 mp = mnt->mnt_mp;
 509                 parent = mnt->mnt_parent;
 510                 while (parent->mnt.mnt_flags & MNT_UMOUNT) {
 511                         mp = parent->mnt_mp;
 512                         parent = parent->mnt_parent;
 513                 }
 514                 if (parent != mnt->mnt_parent)
 515                         mnt_change_mountpoint(parent, mp, mnt);
 516         }
 517 }
 518 
 519 static void cleanup_umount_visitations(struct list_head *visited)
 520 {
 521         while (!list_empty(visited)) {
 522                 struct mount *mnt =
 523                         list_first_entry(visited, struct mount, mnt_umounting);
 524                 list_del_init(&mnt->mnt_umounting);
 525         }
 526 }
 527 
 528 /*
 529  * collect all mounts that receive propagation from the mount in @list,
 530  * and return these additional mounts in the same list.
 531  * @list: the list of mounts to be unmounted.
 532  *
 533  * vfsmount lock must be held for write
 534  */
 535 int propagate_umount(struct list_head *list)
 536 {
 537         struct mount *mnt;
 538         LIST_HEAD(to_restore);
 539         LIST_HEAD(to_umount);
 540         LIST_HEAD(visited);
 541 
 542         /* Find candidates for unmounting */
 543         list_for_each_entry_reverse(mnt, list, mnt_list) {
 544                 struct mount *parent = mnt->mnt_parent;
 545                 struct mount *m;
 546 
 547                 /*
 548                  * If this mount has already been visited it is known that it's
 549                  * entire peer group and all of their slaves in the propagation
 550                  * tree for the mountpoint has already been visited and there is
 551                  * no need to visit them again.
 552                  */
 553                 if (!list_empty(&mnt->mnt_umounting))
 554                         continue;
 555 
 556                 list_add_tail(&mnt->mnt_umounting, &visited);
 557                 for (m = propagation_next(parent, parent); m;
 558                      m = propagation_next(m, parent)) {
 559                         struct mount *child = __lookup_mnt(&m->mnt,
 560                                                            mnt->mnt_mountpoint);
 561                         if (!child)
 562                                 continue;
 563 
 564                         if (!list_empty(&child->mnt_umounting)) {
 565                                 /*
 566                                  * If the child has already been visited it is
 567                                  * know that it's entire peer group and all of
 568                                  * their slaves in the propgation tree for the
 569                                  * mountpoint has already been visited and there
 570                                  * is no need to visit this subtree again.
 571                                  */
 572                                 m = skip_propagation_subtree(m, parent);
 573                                 continue;
 574                         } else if (child->mnt.mnt_flags & MNT_UMOUNT) {
 575                                 /*
 576                                  * We have come accross an partially unmounted
 577                                  * mount in list that has not been visited yet.
 578                                  * Remember it has been visited and continue
 579                                  * about our merry way.
 580                                  */
 581                                 list_add_tail(&child->mnt_umounting, &visited);
 582                                 continue;
 583                         }
 584 
 585                         /* Check the child and parents while progress is made */
 586                         while (__propagate_umount(child,
 587                                                   &to_umount, &to_restore)) {
 588                                 /* Is the parent a umount candidate? */
 589                                 child = child->mnt_parent;
 590                                 if (list_empty(&child->mnt_umounting))
 591                                         break;
 592                         }
 593                 }
 594         }
 595 
 596         umount_list(&to_umount, &to_restore);
 597         restore_mounts(&to_restore);
 598         cleanup_umount_visitations(&visited);
 599         list_splice_tail(&to_umount, list);
 600 
 601         return 0;
 602 }