resolv/resolv_conf.c

   1 /* Extended resolver state separate from struct __res_state.
   2    Copyright (C) 2017 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <resolv_conf.h>
  20
  21 #include <alloc_buffer.h>
  22 #include <assert.h>
  23 #include <libc-lock.h>
  24 #include <resolv-internal.h>
  25 #include <sys/stat.h>
  26
  27 /* _res._u._ext.__glibc_extension_index is used as an index into a
  28    struct resolv_conf_array object.  The intent of this construction
  29    is to make reasonably sure that even if struct __res_state objects
  30    are copied around and patched by applications, we can still detect
  31    accesses to stale extended resolver state.  The array elements are
  32    either struct resolv_conf * pointers (if the LSB is cleared) or
  33    free list entries (if the LSB is set).  The free list is used to
  34    speed up finding available entries in the array.  */
  35 #define DYNARRAY_STRUCT resolv_conf_array
  36 #define DYNARRAY_ELEMENT uintptr_t
  37 #define DYNARRAY_PREFIX resolv_conf_array_
  38 #define DYNARRAY_INITIAL_SIZE 0
  39 #include <malloc/dynarray-skeleton.c>
  40
  41 /* A magic constant for XORing the extension index
  42    (_res._u._ext.__glibc_extension_index).  This makes it less likely
  43    that a valid index is created by accident.  In particular, a zero
  44    value leads to an invalid index.  */
  45 #define INDEX_MAGIC 0x26a8fa5e48af8061ULL
  46
  47 /* Global resolv.conf-related state.  */
  48 struct resolv_conf_global
  49 {
  50   /* struct __res_state objects contain the extension index
  51      (_res._u._ext.__glibc_extension_index ^ INDEX_MAGIC), which
  52      refers to an element of this array.  When a struct resolv_conf
  53      object (extended resolver state) is associated with a struct
  54      __res_state object (legacy resolver state), its reference count
  55      is increased and added to this array.  Conversely, if the
  56      extended state is detached from the basic state (during
  57      reinitialization or deallocation), the index is decremented, and
  58      the array element is overwritten with NULL.  */
  59   struct resolv_conf_array array;
  60
  61   /* Start of the free list in the array.  Zero if the free list is
  62      empty.  Otherwise, free_list_start >> 1 is the first element of
  63      the free list (and the free list entries all have their LSB set
  64      and are shifted one to the left).  */
  65   uintptr_t free_list_start;
  66
  67   /* Cached current configuration object for /etc/resolv.conf.  */
  68   struct resolv_conf *conf_current;
  69
  70   /* These properties of /etc/resolv.conf are used to check if the
  71      configuration needs reloading.  */
  72   struct timespec conf_mtime;
  73   struct timespec conf_ctime;
  74   off64_t conf_size;
  75   ino64_t conf_ino;
  76 };
  77
  78 /* Lazily allocated storage for struct resolv_conf_global.  */
  79 static struct resolv_conf_global *global;
  80
  81 /* The lock synchronizes access to global and *global.  It also
  82    protects the __refcount member of struct resolv_conf.  */
  83 __libc_lock_define_initialized (static, lock);
  84
  85 /* Ensure that GLOBAL is allocated and lock it.  Return NULL if
  86    memory allocation failes.  */
  87 static struct resolv_conf_global *
  88 get_locked_global (void)
  89 {
  90   __libc_lock_lock (lock);
  91   /* Use relaxed MO through because of load outside the lock in
  92      __resolv_conf_detach.  */
  93   struct resolv_conf_global *global_copy = atomic_load_relaxed (&global);
  94   if (global_copy == NULL)
  95     {
  96       global_copy = calloc (1, sizeof (*global));
  97       if (global_copy == NULL)
  98         return NULL;
  99       atomic_store_relaxed (&global, global_copy);
 100       resolv_conf_array_init (&global_copy->array);
 101     }
 102   return global_copy;
 103 }
 104
 105 /* Relinquish the lock acquired by get_locked_global.  */
 106 static void
 107 put_locked_global (struct resolv_conf_global *global_copy)
 108 {
 109   __libc_lock_unlock (lock);
 110 }
 111
 112 /* Decrement the reference counter.  The caller must acquire the lock
 113    around the function call.  */
 114 static void
 115 conf_decrement (struct resolv_conf *conf)
 116 {
 117   assert (conf->__refcount > 0);
 118   if (--conf->__refcount == 0)
 119     free (conf);
 120 }
 121
 122 struct resolv_conf *
 123 __resolv_conf_get_current (void)
 124 {
 125   struct stat64 st;
 126   if (stat64 (_PATH_RESCONF, &st) != 0)
 127     {
 128     switch (errno)
 129       {
 130       case EACCES:
 131       case EISDIR:
 132       case ELOOP:
 133       case ENOENT:
 134       case ENOTDIR:
 135       case EPERM:
 136         /* Ignore errors due to file system contents.  */
 137         memset (&st, 0, sizeof (st));
 138         break;
 139       default:
 140         /* Other errors are fatal.  */
 141         return NULL;
 142       }
 143     }
 144
 145   struct resolv_conf_global *global_copy = get_locked_global ();
 146   if (global_copy == NULL)
 147     return NULL;
 148   struct resolv_conf *conf;
 149   if (global_copy->conf_current != NULL
 150       && (global_copy->conf_mtime.tv_sec == st.st_mtim.tv_sec
 151           && global_copy->conf_mtime.tv_nsec == st.st_mtim.tv_nsec
 152           && global_copy->conf_ctime.tv_sec == st.st_ctim.tv_sec
 153           && global_copy->conf_ctime.tv_nsec == st.st_ctim.tv_nsec
 154           && global_copy->conf_ino == st.st_ino
 155           && global_copy->conf_size == st.st_size))
 156     /* We can reuse the cached configuration object.  */
 157     conf = global_copy->conf_current;
 158   else
 159     {
 160       /* Parse configuration while holding the lock.  This avoids
 161          duplicate work.  */
 162       conf = __resolv_conf_load (NULL);
 163       if (conf != NULL)
 164         {
 165           if (global_copy->conf_current != NULL)
 166             conf_decrement (global_copy->conf_current);
 167           global_copy->conf_current = conf; /* Takes ownership.  */
 168
 169           /* Update file modification stamps.  The configuration we
 170              read could be a newer version of the file, but this does
 171              not matter because this will lead to an extraneous reload
 172              later.  */
 173           global_copy->conf_mtime = st.st_mtim;
 174           global_copy->conf_ctime = st.st_ctim;
 175           global_copy->conf_ino = st.st_ino;
 176           global_copy->conf_size = st.st_size;
 177         }
 178     }
 179
 180   if (conf != NULL)
 181     {
 182       /* Return an additional reference.  */
 183       assert (conf->__refcount > 0);
 184       ++conf->__refcount;
 185       assert (conf->__refcount > 0);
 186     }
 187   put_locked_global (global_copy);
 188   return conf;
 189 }
 190
 191 /* Internal implementation of __resolv_conf_get, without validation
 192    against *RESP.  */
 193 static struct resolv_conf *
 194 resolv_conf_get_1 (const struct __res_state *resp)
 195 {
 196   /* Not initialized, and therefore no assoicated context.  */
 197   if (!(resp->options & RES_INIT))
 198     return NULL;
 199
 200   struct resolv_conf_global *global_copy = get_locked_global ();
 201   if (global_copy == NULL)
 202     /* A memory allocation failure here means that no associated
 203        contexts exists, so returning NULL is correct.  */
 204     return NULL;
 205   size_t index = resp->_u._ext.__glibc_extension_index ^ INDEX_MAGIC;
 206   struct resolv_conf *conf = NULL;
 207   if (index < resolv_conf_array_size (&global_copy->array))
 208     {
 209       uintptr_t *slot = resolv_conf_array_at (&global_copy->array, index);
 210       if (!(*slot & 1))
 211         {
 212           conf = (struct resolv_conf *) *slot;
 213           assert (conf->__refcount > 0);
 214           ++conf->__refcount;
 215         }
 216     }
 217   put_locked_global (global_copy);
 218   return conf;
 219 }
 220
 221 /* Return true if both IPv4 addresses are equal.  */
 222 static bool
 223 same_address_v4 (const struct sockaddr_in *left,
 224                  const struct sockaddr_in *right)
 225 {
 226   return left->sin_addr.s_addr == right->sin_addr.s_addr
 227     && left->sin_port == right->sin_port;
 228 }
 229
 230 /* Return true if both IPv6 addresses are equal.  This ignores the
 231    flow label.  */
 232 static bool
 233 same_address_v6 (const struct sockaddr_in6 *left,
 234                  const struct sockaddr_in6 *right)
 235 {
 236   return memcmp (&left->sin6_addr, &right->sin6_addr,
 237                  sizeof (left->sin6_addr)) == 0
 238     && left->sin6_port == right->sin6_port
 239     && left->sin6_scope_id == right->sin6_scope_id;
 240 }
 241
 242 static bool
 243 same_address (const struct sockaddr *left, const struct sockaddr *right)
 244 {
 245   if (left->sa_family != right->sa_family)
 246     return false;
 247   switch (left->sa_family)
 248     {
 249     case AF_INET:
 250       return same_address_v4 ((const struct sockaddr_in *) left,
 251                               (const struct sockaddr_in *) right);
 252     case AF_INET6:
 253       return same_address_v6 ((const struct sockaddr_in6 *) left,
 254                               (const struct sockaddr_in6 *) right);
 255     }
 256   return false;
 257 }
 258
 259 /* Check that *RESP and CONF match.  Used by __resolv_conf_get.  */
 260 static bool
 261 resolv_conf_matches (const struct __res_state *resp,
 262                      const struct resolv_conf *conf)
 263 {
 264   /* NB: Do not compare the options, retrans, retry, ndots.  These can
 265      be changed by applicaiton.  */
 266
 267   /* Check that the name servers in *RESP have not been modified by
 268      the application.  */
 269   {
 270     size_t nserv = conf->nameserver_list_size;
 271     if (nserv > MAXNS)
 272       nserv = MAXNS;
 273     /* _ext.nscount is 0 until initialized by res_send.c.  */
 274     if (resp->nscount != nserv
 275         || (resp->_u._ext.nscount != 0 && resp->_u._ext.nscount != nserv))
 276       return false;
 277     for (size_t i = 0; i < nserv; ++i)
 278       {
 279         if (resp->nsaddr_list[i].sin_family == 0)
 280           {
 281             if (resp->_u._ext.nsaddrs[i]->sin6_family != AF_INET6)
 282               return false;
 283             if (!same_address ((struct sockaddr *) resp->_u._ext.nsaddrs[i],
 284                                conf->nameserver_list[i]))
 285               return false;
 286           }
 287         else if (resp->nsaddr_list[i].sin_family != AF_INET)
 288           return false;
 289         else if (!same_address ((struct sockaddr *) &resp->nsaddr_list[i],
 290                                 conf->nameserver_list[i]))
 291           return false;
 292       }
 293   }
 294
 295   /* Check that the search list in *RESP has not been modified by the
 296      application.  */
 297   {
 298     if (resp->dnsrch[0] == NULL)
 299       {
 300         /* Empty search list.  No default domain name.  */
 301         return conf->search_list_size == 0 && resp->defdname[0] == '\0';
 302       }
 303
 304     if (resp->dnsrch[0] != resp->defdname)
 305       /* If the search list is not empty, it must start with the
 306          default domain name.  */
 307       return false;
 308
 309     size_t nsearch;
 310     for (nsearch = 0; nsearch < MAXDNSRCH; ++nsearch)
 311       if (resp->dnsrch[nsearch] == NULL)
 312         break;
 313     if (nsearch > MAXDNSRCH)
 314       /* Search list is not null-terminated.  */
 315       return false;
 316
 317     size_t search_list_size = 0;
 318     for (size_t i = 0; i < conf->search_list_size; ++i)
 319       {
 320         if (resp->dnsrch[i] != NULL)
 321           {
 322             search_list_size += strlen (resp->dnsrch[i]) + 1;
 323             if (strcmp (resp->dnsrch[i], conf->search_list[i]) != 0)
 324               return false;
 325           }
 326         else
 327           {
 328             /* resp->dnsrch is truncated if the number of elements
 329                exceeds MAXDNSRCH, or if the combined storage space for
 330                the search list exceeds what can be stored in
 331                resp->defdname.  */
 332             if (i == MAXDNSRCH || search_list_size > sizeof (resp->dnsrch))
 333               break;
 334             /* Otherwise, a mismatch indicates a match failure.  */
 335             return false;
 336           }
 337       }
 338   }
 339
 340   /* Check that the sort list has not been modified.  */
 341   {
 342     size_t nsort = conf->sort_list_size;
 343     if (nsort > MAXRESOLVSORT)
 344       nsort = MAXRESOLVSORT;
 345     if (resp->nsort != nsort)
 346       return false;
 347     for (size_t i = 0; i < nsort; ++i)
 348       if (resp->sort_list[i].addr.s_addr != conf->sort_list[i].addr.s_addr
 349           || resp->sort_list[i].mask != conf->sort_list[i].mask)
 350         return false;
 351   }
 352
 353   return true;
 354 }
 355
 356 struct resolv_conf *
 357 __resolv_conf_get (struct __res_state *resp)
 358 {
 359   struct resolv_conf *conf = resolv_conf_get_1 (resp);
 360   if (conf == NULL)
 361     return NULL;
 362   if (resolv_conf_matches (resp, conf))
 363     return conf;
 364   __resolv_conf_put (conf);
 365   return NULL;
 366 }
 367
 368 void
 369 __resolv_conf_put (struct resolv_conf *conf)
 370 {
 371   if (conf == NULL)
 372     return;
 373
 374   __libc_lock_lock (lock);
 375   conf_decrement (conf);
 376   __libc_lock_unlock (lock);
 377 }
 378
 379 struct resolv_conf *
 380 __resolv_conf_allocate (const struct resolv_conf *init)
 381 {
 382   /* Allocate in decreasing order of alignment.  */
 383   _Static_assert (__alignof__ (const char *const *)
 384                   <= __alignof__ (struct resolv_conf), "alignment");
 385   _Static_assert (__alignof__ (struct sockaddr_in6)
 386                   <= __alignof__ (const char *const *), "alignment");
 387   _Static_assert (__alignof__ (struct sockaddr_in)
 388                   ==  __alignof__ (struct sockaddr_in6), "alignment");
 389   _Static_assert (__alignof__ (struct resolv_sortlist_entry)
 390                   <= __alignof__ (struct sockaddr_in), "alignment");
 391
 392   /* Space needed by the nameserver addresses.  */
 393   size_t address_space = 0;
 394   for (size_t i = 0; i < init->nameserver_list_size; ++i)
 395     if (init->nameserver_list[i]->sa_family == AF_INET)
 396       address_space += sizeof (struct sockaddr_in);
 397     else
 398       {
 399         assert (init->nameserver_list[i]->sa_family == AF_INET6);
 400         address_space += sizeof (struct sockaddr_in6);
 401       }
 402
 403   /* Space needed by the search list strings.  */
 404   size_t string_space = 0;
 405   for (size_t i = 0; i < init->search_list_size; ++i)
 406     string_space += strlen (init->search_list[i]) + 1;
 407
 408   /* Allocate the buffer.  */
 409   void *ptr;
 410   struct alloc_buffer buffer = alloc_buffer_allocate
 411     (sizeof (struct resolv_conf)
 412      + init->nameserver_list_size * sizeof (init->nameserver_list[0])
 413      + address_space
 414      + init->search_list_size * sizeof (init->search_list[0])
 415      + init->sort_list_size * sizeof (init->sort_list[0])
 416      + string_space,
 417      &ptr);
 418   struct resolv_conf *conf
 419     = alloc_buffer_alloc (&buffer, struct resolv_conf);
 420   if (conf == NULL)
 421     /* Memory allocation failure.  */
 422     return NULL;
 423   assert (conf == ptr);
 424
 425   /* Initialize the contents.  */
 426   conf->__refcount = 1;
 427   conf->retrans = init->retrans;
 428   conf->retry = init->retry;
 429   conf->options = init->options;
 430   conf->ndots = init->ndots;
 431
 432   /* Allocate the arrays with pointers.  These must come first because
 433      they have the highets alignment.  */
 434   conf->nameserver_list_size = init->nameserver_list_size;
 435   const struct sockaddr **nameserver_array = alloc_buffer_alloc_array
 436     (&buffer, const struct sockaddr *, init->nameserver_list_size);
 437   conf->nameserver_list = nameserver_array;
 438
 439   conf->search_list_size = init->search_list_size;
 440   const char **search_array = alloc_buffer_alloc_array
 441     (&buffer, const char *, init->search_list_size);
 442   conf->search_list = search_array;
 443
 444   /* Fill the name server list array.  */
 445   for (size_t i = 0; i < init->nameserver_list_size; ++i)
 446     if (init->nameserver_list[i]->sa_family == AF_INET)
 447       {
 448         struct sockaddr_in *sa = alloc_buffer_alloc
 449           (&buffer, struct sockaddr_in);
 450         *sa = *(struct sockaddr_in *) init->nameserver_list[i];
 451         nameserver_array[i] = (struct sockaddr *) sa;
 452       }
 453     else
 454       {
 455         struct sockaddr_in6 *sa = alloc_buffer_alloc
 456           (&buffer, struct sockaddr_in6);
 457         *sa = *(struct sockaddr_in6 *) init->nameserver_list[i];
 458         nameserver_array[i] = (struct sockaddr *) sa;
 459       }
 460
 461   /* Allocate and fill the sort list array.  */
 462   {
 463     conf->sort_list_size = init->sort_list_size;
 464     struct resolv_sortlist_entry *array = alloc_buffer_alloc_array
 465       (&buffer, struct resolv_sortlist_entry, init->sort_list_size);
 466     conf->sort_list = array;
 467     for (size_t i = 0; i < init->sort_list_size; ++i)
 468       array[i] = init->sort_list[i];
 469   }
 470
 471   /* Fill the search list array.  This must come last because the
 472      strings are the least aligned part of the allocation.  */
 473   {
 474     for (size_t i = 0; i < init->search_list_size; ++i)
 475       search_array[i] = alloc_buffer_copy_string
 476         (&buffer, init->search_list[i]);
 477   }
 478
 479   assert (!alloc_buffer_has_failed (&buffer));
 480   return conf;
 481 }
 482
 483 /* Update *RESP from the extended state.  */
 484 static __attribute__ ((nonnull (1, 2), warn_unused_result)) bool
 485 update_from_conf (struct __res_state *resp, const struct resolv_conf *conf)
 486 {
 487   resp->defdname[0] = '\0';
 488   resp->pfcode = 0;
 489   resp->_vcsock = -1;
 490   resp->_flags = 0;
 491   resp->ipv6_unavail = false;
 492   resp->__glibc_unused_qhook = NULL;
 493   resp->__glibc_unused_rhook = NULL;
 494
 495   resp->retrans = conf->retrans;
 496   resp->retry = conf->retry;
 497   resp->options = conf->options;
 498   resp->ndots = conf->ndots;
 499
 500   /* Copy the name server addresses.  */
 501   {
 502     resp->nscount = 0;
 503     resp->_u._ext.nscount = 0;
 504     size_t nserv = conf->nameserver_list_size;
 505     if (nserv > MAXNS)
 506       nserv = MAXNS;
 507     for (size_t i = 0; i < nserv; i++)
 508       {
 509         if (conf->nameserver_list[i]->sa_family == AF_INET)
 510           {
 511             resp->nsaddr_list[i]
 512               = *(struct sockaddr_in *)conf->nameserver_list[i];
 513             resp->_u._ext.nsaddrs[i] = NULL;
 514           }
 515         else
 516           {
 517             assert (conf->nameserver_list[i]->sa_family == AF_INET6);
 518             resp->nsaddr_list[i].sin_family = 0;
 519             /* Make a defensive copy of the name server address, in
 520                case the application overwrites it.  */
 521             struct sockaddr_in6 *sa = malloc (sizeof (*sa));
 522             if (sa == NULL)
 523               {
 524                 for (size_t j = 0; j < i; ++j)
 525                   free (resp->_u._ext.nsaddrs[j]);
 526                 return false;
 527               }
 528             *sa = *(struct sockaddr_in6 *)conf->nameserver_list[i];
 529             resp->_u._ext.nsaddrs[i] = sa;
 530           }
 531         resp->_u._ext.nssocks[i] = -1;
 532       }
 533     resp->nscount = nserv;
 534     /* Leave resp->_u._ext.nscount at 0.  res_send.c handles this.  */
 535   }
 536
 537   /* Fill in the prefix of the search list.  It is truncated either at
 538      MAXDNSRCH, or if reps->defdname has insufficient space.  */
 539   {
 540     struct alloc_buffer buffer
 541       = alloc_buffer_create (resp->defdname, sizeof (resp->defdname));
 542     size_t size = conf->search_list_size;
 543     size_t i;
 544     for (i = 0; i < size && i < MAXDNSRCH; ++i)
 545       {
 546         resp->dnsrch[i] = alloc_buffer_copy_string
 547           (&buffer, conf->search_list[i]);
 548         if (resp->dnsrch[i] == NULL)
 549           /* No more space in resp->defdname.  Truncate.  */
 550           break;
 551       }
 552     resp->dnsrch[i] = NULL;
 553   }
 554
 555   /* Copy the sort list.  */
 556   {
 557     size_t nsort = conf->sort_list_size;
 558     if (nsort > MAXRESOLVSORT)
 559       nsort = MAXRESOLVSORT;
 560     for (size_t i = 0; i < nsort; ++i)
 561       {
 562         resp->sort_list[i].addr = conf->sort_list[i].addr;
 563         resp->sort_list[i].mask = conf->sort_list[i].mask;
 564       }
 565     resp->nsort = nsort;
 566   }
 567
 568   /* The overlapping parts of both configurations should agree after
 569      initialization.  */
 570   assert (resolv_conf_matches (resp, conf));
 571   return true;
 572 }
 573
 574 /* Decrement the configuration object at INDEX and free it if the
 575    reference counter reaches 0.  *GLOBAL_COPY must be locked and
 576    remains so.  */
 577 static void
 578 decrement_at_index (struct resolv_conf_global *global_copy, size_t index)
 579 {
 580   if (index < resolv_conf_array_size (&global_copy->array))
 581     {
 582       /* Index found.  */
 583       uintptr_t *slot = resolv_conf_array_at (&global_copy->array, index);
 584       /* Check that the slot is not already part of the free list.  */
 585       if (!(*slot & 1))
 586         {
 587           struct resolv_conf *conf = (struct resolv_conf *) *slot;
 588           conf_decrement (conf);
 589           /* Put the slot onto the free list.  */
 590           *slot = global_copy->free_list_start;
 591           global_copy->free_list_start = (index << 1) | 1;
 592         }
 593     }
 594 }
 595
 596 bool
 597 __resolv_conf_attach (struct __res_state *resp, struct resolv_conf *conf)
 598 {
 599   assert (conf->__refcount > 0);
 600
 601   struct resolv_conf_global *global_copy = get_locked_global ();
 602   if (global_copy == NULL)
 603     return false;
 604
 605   /* Try to find an unused index in the array.  */
 606   size_t index;
 607   {
 608     if (global_copy->free_list_start & 1)
 609       {
 610         /* Unlink from the free list.  */
 611         index = global_copy->free_list_start >> 1;
 612         uintptr_t *slot = resolv_conf_array_at (&global_copy->array, index);
 613         global_copy->free_list_start = *slot;
 614         assert (global_copy->free_list_start == 0
 615                 || global_copy->free_list_start & 1);
 616         /* Install the configuration pointer.  */
 617         *slot = (uintptr_t) conf;
 618       }
 619     else
 620       {
 621         size_t size = resolv_conf_array_size (&global_copy->array);
 622         /* No usable index found.  Increase the array size.  */
 623         resolv_conf_array_add (&global_copy->array, (uintptr_t) conf);
 624         if (resolv_conf_array_has_failed (&global_copy->array))
 625           {
 626             put_locked_global (global_copy);
 627             __set_errno (ENOMEM);
 628             return false;
 629           }
 630         /* The new array element was added at the end.  */
 631         index = size;
 632       }
 633   }
 634
 635   /* We have added a new reference to the object.  */
 636   ++conf->__refcount;
 637   assert (conf->__refcount > 0);
 638   put_locked_global (global_copy);
 639
 640   if (!update_from_conf (resp, conf))
 641     {
 642       /* Drop the reference we acquired.  Reacquire the lock.  The
 643          object has already been allocated, so it cannot be NULL this
 644          time.  */
 645       global_copy = get_locked_global ();
 646       decrement_at_index (global_copy, index);
 647       put_locked_global (global_copy);
 648       return false;
 649     }
 650   resp->_u._ext.__glibc_extension_index = index ^ INDEX_MAGIC;
 651
 652   return true;
 653 }
 654
 655 void
 656 __resolv_conf_detach (struct __res_state *resp)
 657 {
 658   if (atomic_load_relaxed (&global) == NULL)
 659     /* Detach operation after a shutdown, or without any prior
 660        attachment.  We cannot free the data (and there might not be
 661        anything to free anyway).  */
 662     return;
 663
 664   struct resolv_conf_global *global_copy = get_locked_global ();
 665   size_t index = resp->_u._ext.__glibc_extension_index ^ INDEX_MAGIC;
 666   decrement_at_index (global_copy, index);
 667
 668   /* Clear the index field, so that accidental reuse is less
 669      likely.  */
 670   resp->_u._ext.__glibc_extension_index = 0;
 671
 672   put_locked_global (global_copy);
 673 }
 674
 675 /* Deallocate the global data.  */
 676 static void __attribute__ ((section ("__libc_thread_freeres_fn")))
 677 freeres (void)
 678 {
 679   /* No locking because this function is supposed to be called when
 680      the process has turned single-threaded.  */
 681   if (global == NULL)
 682     return;
 683
 684   if (global->conf_current != NULL)
 685     {
 686       conf_decrement (global->conf_current);
 687       global->conf_current = NULL;
 688     }
 689
 690   /* Note that this frees only the array itself.  The pointed-to
 691      configuration objects should have been deallocated by res_nclose
 692      and per-thread cleanup functions.  */
 693   resolv_conf_array_free (&global->array);
 694
 695   free (global);
 696
 697   /* Stop potential future __resolv_conf_detach calls from accessing
 698      deallocated memory.  */
 699   global = NULL;
 700 }
 701 text_set_element (__libc_subfreeres, freeres);