nss/nss_module.c

   1 /* Global list of NSS service modules.
   2    Copyright (c) 2020-2023 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    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <nsswitch.h>
  20 #include <nscd/nscd.h>
  21 #include <nscd/nscd_proto.h>
  22
  23 #include <array_length.h>
  24 #include <assert.h>
  25 #include <atomic.h>
  26 #include <dlfcn.h>
  27 #include <gnu/lib-names.h>
  28 #include <libc-lock.h>
  29 #include <nss_dns.h>
  30 #include <nss_files.h>
  31 #include <stddef.h>
  32 #include <stdio.h>
  33 #include <stdlib.h>
  34 #include <string.h>
  35 #include <pointer_guard.h>
  36
  37 /* Suffix after .so of NSS service modules.  This is a bit of magic,
  38    but we assume LIBNSS_FILES_SO looks like "libnss_files.so.2" and we
  39    want a pointer to the ".2" part.  We have no API to extract this
  40    except through the auto-generated lib-names.h and some static
  41    pointer manipulation.  The "-1" accounts for the trailing NUL
  42    included in the sizeof.  */
  43 static const char *const __nss_shlib_revision
  44         = LIBNSS_FILES_SO + sizeof("libnss_files.so") - 1;
  45
  46 /* A single-linked list used to implement a mapping from service names
  47    to NSS modules.  (Most systems only use five or so modules, so a
  48    list is sufficient here.)  Elements of this list are never freed
  49    during normal operation.  */
  50 static struct nss_module *nss_module_list;
  51
  52 /* Covers the list and also loading of individual NSS service
  53    modules.  */
  54 __libc_lock_define (static, nss_module_list_lock);
  55
  56 #if defined SHARED && defined USE_NSCD
  57 /* Nonzero if this is the nscd process.  */
  58 static bool is_nscd;
  59 /* The callback passed to the init functions when nscd is used.  */
  60 static void (*nscd_init_cb) (size_t, struct traced_file *);
  61 #endif
  62
  63 /* Allocate the service NAME with length NAME_LENGTH.  If the service
  64    is already allocated in the nss_module_list cache then we return a
  65    pointer to the struct nss_module, otherwise we try to allocate a
  66    new struct nss_module entry and add it to the global
  67    nss_modules_list cache.  If we fail to allocate the entry we return
  68    NULL.  Failure to allocate the entry is always transient.  */
  69 struct nss_module *
  70 __nss_module_allocate (const char *name, size_t name_length)
  71 {
  72   __libc_lock_lock (nss_module_list_lock);
  73
  74   struct nss_module *result = NULL;
  75   for (struct nss_module *p = nss_module_list; p != NULL; p = p->next)
  76     if (strncmp (p->name, name, name_length) == 0
  77         && p->name[name_length] == '\0')
  78       {
  79         /* Return the previously existing object.  */
  80         result = p;
  81         break;
  82       }
  83
  84   if (result == NULL)
  85     {
  86       /* Allocate a new list entry if the name was not found in the
  87          list.  */
  88       result = malloc (sizeof (*result) + name_length + 1);
  89       if (result != NULL)
  90         {
  91           result->state = nss_module_uninitialized;
  92           memcpy (result->name, name, name_length);
  93           result->name[name_length] = '\0';
  94           result->handle = NULL;
  95           result->next = nss_module_list;
  96           nss_module_list = result;
  97         }
  98     }
  99
 100   __libc_lock_unlock (nss_module_list_lock);
 101   return result;
 102 }
 103
 104 /* Long enough to store the name of any function in the
 105    nss_function_name_array list below, as getprotobynumber_r is the
 106    longest entry in that list.  */
 107 typedef char function_name[sizeof("getprotobynumber_r")];
 108
 109 static const function_name nss_function_name_array[] =
 110   {
 111 #undef DEFINE_NSS_FUNCTION
 112 #define DEFINE_NSS_FUNCTION(x) #x,
 113 #include "function.def"
 114   };
 115
 116 /* Loads a built-in module, binding the symbols using the supplied
 117    callback function.  Always returns true.  */
 118 static bool
 119 module_load_builtin (struct nss_module *module,
 120                      void (*bind) (nss_module_functions_untyped))
 121 {
 122   /* Initialize the function pointers, following the double-checked
 123      locking idiom.  */
 124   __libc_lock_lock (nss_module_list_lock);
 125   switch ((enum nss_module_state) atomic_load_acquire (&module->state))
 126     {
 127     case nss_module_uninitialized:
 128     case nss_module_failed:
 129       bind (module->functions.untyped);
 130
 131       for (int i = 0; i < nss_module_functions_count; ++i)
 132         PTR_MANGLE (module->functions.untyped[i]);
 133
 134       module->handle = NULL;
 135       /* Synchronizes with unlocked __nss_module_load atomic_load_acquire.  */
 136       atomic_store_release (&module->state, nss_module_loaded);
 137       break;
 138     case nss_module_loaded:
 139       /* Nothing to clean up.  */
 140       break;
 141     }
 142   __libc_lock_unlock (nss_module_list_lock);
 143   return true;
 144 }
 145
 146 /* Loads the built-in nss_files module.  */
 147 static bool
 148 module_load_nss_files (struct nss_module *module)
 149 {
 150 #if defined SHARED && defined USE_NSCD
 151   if (is_nscd)
 152     {
 153       void (*cb) (size_t, struct traced_file *) = nscd_init_cb;
 154       PTR_DEMANGLE (cb);
 155       _nss_files_init (cb);
 156     }
 157 #endif
 158   return module_load_builtin (module, __nss_files_functions);
 159 }
 160
 161 /* Loads the built-in nss_dns module.  */
 162 static bool
 163 module_load_nss_dns (struct nss_module *module)
 164 {
 165   return module_load_builtin (module, __nss_dns_functions);
 166 }
 167
 168 /* Internal implementation of __nss_module_load.  */
 169 static bool
 170 module_load (struct nss_module *module)
 171 {
 172   if (strcmp (module->name, "files") == 0)
 173     return module_load_nss_files (module);
 174   if (strcmp (module->name, "dns") == 0)
 175     return module_load_nss_dns (module);
 176
 177   void *handle;
 178   {
 179     char *shlib_name;
 180     if (__asprintf (&shlib_name, "libnss_%s.so%s",
 181                     module->name, __nss_shlib_revision) < 0)
 182       /* This is definitely a temporary failure.  Do not update
 183          module->state.  This will trigger another attempt at the next
 184          call.  */
 185       return false;
 186
 187     handle = __libc_dlopen (shlib_name);
 188     free (shlib_name);
 189   }
 190
 191   /* Failing to load the module can be caused by several different
 192      scenarios.  One such scenario is that the module has been removed
 193      from the disk.  In which case the in-memory version is all that
 194      we have, and if the module->state indidates it is loaded then we
 195      can use it.  */
 196   if (handle == NULL)
 197     {
 198       /* dlopen failure.  We do not know if this a temporary or
 199          permanent error.  See bug 22041.  Update the state using the
 200          double-checked locking idiom.  */
 201
 202       __libc_lock_lock (nss_module_list_lock);
 203       bool result = result;
 204       switch ((enum nss_module_state) atomic_load_acquire (&module->state))
 205         {
 206         case nss_module_uninitialized:
 207           atomic_store_release (&module->state, nss_module_failed);
 208           result = false;
 209           break;
 210         case nss_module_loaded:
 211           result = true;
 212           break;
 213         case nss_module_failed:
 214           result = false;
 215           break;
 216         }
 217       __libc_lock_unlock (nss_module_list_lock);
 218       return result;
 219     }
 220
 221   nss_module_functions_untyped pointers;
 222
 223   /* Look up and store locally all the function pointers we may need
 224      later.  Doing this now means the data will not change in the
 225      future.  */
 226   for (size_t idx = 0; idx < array_length (nss_function_name_array); ++idx)
 227     {
 228       char *function_name;
 229       if (__asprintf (&function_name, "_nss_%s_%s",
 230                       module->name, nss_function_name_array[idx]) < 0)
 231         {
 232           /* Definitely a temporary error.  */
 233           __libc_dlclose (handle);
 234           return false;
 235         }
 236       pointers[idx] = __libc_dlsym (handle, function_name);
 237       free (function_name);
 238       PTR_MANGLE (pointers[idx]);
 239     }
 240
 241 # if defined SHARED && defined USE_NSCD
 242   if (is_nscd)
 243     {
 244       /* Call the init function when nscd is used.  */
 245       size_t initlen = (5 + strlen (module->name)
 246                         + strlen ("_init") + 1);
 247       char init_name[initlen];
 248
 249       /* Construct the init function name.  */
 250       __stpcpy (__stpcpy (__stpcpy (init_name,
 251                                     "_nss_"),
 252                           module->name),
 253                 "_init");
 254
 255       /* Find the optional init function.  */
 256       void (*ifct) (void (*) (size_t, struct traced_file *))
 257         = __libc_dlsym (handle, init_name);
 258       if (ifct != NULL)
 259         {
 260           void (*cb) (size_t, struct traced_file *) = nscd_init_cb;
 261           PTR_DEMANGLE (cb);
 262           ifct (cb);
 263         }
 264     }
 265 # endif
 266
 267   /* Install the function pointers, following the double-checked
 268      locking idiom.  Delay this after all processing, in case loading
 269      the module triggers unwinding.  */
 270   __libc_lock_lock (nss_module_list_lock);
 271   switch ((enum nss_module_state) atomic_load_acquire (&module->state))
 272     {
 273     case nss_module_uninitialized:
 274     case nss_module_failed:
 275       memcpy (module->functions.untyped, pointers,
 276               sizeof (module->functions.untyped));
 277       module->handle = handle;
 278       /* Synchronizes with unlocked __nss_module_load atomic_load_acquire.  */
 279       atomic_store_release (&module->state, nss_module_loaded);
 280       break;
 281     case nss_module_loaded:
 282       /* If the module was already loaded, close our own handle.  This
 283          does not actually unload the modules, only the reference
 284          counter is decremented for the loaded module.  */
 285       __libc_dlclose (handle);
 286       break;
 287     }
 288   __libc_lock_unlock (nss_module_list_lock);
 289   return true;
 290 }
 291
 292 /* Force the module identified by MODULE to be loaded.  We return
 293    false if the module could not be loaded, true otherwise.  Loading
 294    the module requires looking up all the possible interface APIs and
 295    caching the results.  */
 296 bool
 297 __nss_module_load (struct nss_module *module)
 298 {
 299   switch ((enum nss_module_state) atomic_load_acquire (&module->state))
 300     {
 301     case nss_module_uninitialized:
 302       return module_load (module);
 303     case nss_module_loaded:
 304       /* Loading has already succeeded.  */
 305       return true;
 306     case nss_module_failed:
 307       /* Loading previously failed.  */
 308       return false;
 309     }
 310   __builtin_unreachable ();
 311 }
 312
 313 static int
 314 name_search (const void *left, const void *right)
 315 {
 316   return strcmp (left, right);
 317 }
 318
 319 /* Load module MODULE (if it isn't already) and return a pointer to
 320    the module's implementation of NAME, otherwise return NULL on
 321    failure or error.  */
 322 void *
 323 __nss_module_get_function (struct nss_module *module, const char *name)
 324 {
 325   /* A successful dlopen might clobber errno.   */
 326   int saved_errno = errno;
 327
 328   if (!__nss_module_load (module))
 329     {
 330       /* Reporting module load failure is currently inaccurate.  See
 331          bug 22041.  Not changing errno is the conservative choice.  */
 332       __set_errno (saved_errno);
 333       return NULL;
 334     }
 335
 336   __set_errno (saved_errno);
 337
 338   function_name *name_entry = bsearch (name, nss_function_name_array,
 339                                        array_length (nss_function_name_array),
 340                                        sizeof (function_name), name_search);
 341   assert (name_entry != NULL);
 342   size_t idx = name_entry - nss_function_name_array;
 343   void *fptr = module->functions.untyped[idx];
 344   PTR_DEMANGLE (fptr);
 345   return fptr;
 346 }
 347
 348 #if defined SHARED && defined USE_NSCD
 349 /* Load all libraries for the service.  */
 350 static void
 351 nss_load_all_libraries (enum nss_database service)
 352 {
 353   nss_action_list ni = NULL;
 354
 355   if (__nss_database_get (service, &ni))
 356     while (ni->module != NULL)
 357       {
 358         __nss_module_load (ni->module);
 359         ++ni;
 360       }
 361 }
 362
 363 define_traced_file (pwd, _PATH_NSSWITCH_CONF);
 364 define_traced_file (grp, _PATH_NSSWITCH_CONF);
 365 define_traced_file (hst, _PATH_NSSWITCH_CONF);
 366 define_traced_file (serv, _PATH_NSSWITCH_CONF);
 367 define_traced_file (netgr, _PATH_NSSWITCH_CONF);
 368
 369 /* Called by nscd and nscd alone.  */
 370 void
 371 __nss_disable_nscd (void (*cb) (size_t, struct traced_file *))
 372 {
 373   void (*cb1) (size_t, struct traced_file *);
 374   cb1 = cb;
 375   PTR_MANGLE (cb);
 376   nscd_init_cb = cb;
 377   is_nscd = true;
 378
 379   /* Find all the relevant modules so that the init functions are called.  */
 380   nss_load_all_libraries (nss_database_passwd);
 381   nss_load_all_libraries (nss_database_group);
 382   nss_load_all_libraries (nss_database_hosts);
 383   nss_load_all_libraries (nss_database_services);
 384
 385   /* Make sure NSCD purges its cache if nsswitch.conf changes.  */
 386   init_traced_file (&pwd_traced_file.file, _PATH_NSSWITCH_CONF, 0);
 387   cb1 (pwddb, &pwd_traced_file.file);
 388   init_traced_file (&grp_traced_file.file, _PATH_NSSWITCH_CONF, 0);
 389   cb1 (grpdb, &grp_traced_file.file);
 390   init_traced_file (&hst_traced_file.file, _PATH_NSSWITCH_CONF, 0);
 391   cb1 (hstdb, &hst_traced_file.file);
 392   init_traced_file (&serv_traced_file.file, _PATH_NSSWITCH_CONF, 0);
 393   cb1 (servdb, &serv_traced_file.file);
 394   init_traced_file (&netgr_traced_file.file, _PATH_NSSWITCH_CONF, 0);
 395   cb1 (netgrdb, &netgr_traced_file.file);
 396
 397   /* Disable all uses of NSCD.  */
 398   __nss_not_use_nscd_passwd = -1;
 399   __nss_not_use_nscd_group = -1;
 400   __nss_not_use_nscd_hosts = -1;
 401   __nss_not_use_nscd_services = -1;
 402   __nss_not_use_nscd_netgroup = -1;
 403 }
 404 #endif
 405
 406 /* Block attempts to dlopen any module we haven't already opened.  */
 407 void
 408 __nss_module_disable_loading (void)
 409 {
 410   __libc_lock_lock (nss_module_list_lock);
 411
 412   for (struct nss_module *p = nss_module_list; p != NULL; p = p->next)
 413     if (p->state == nss_module_uninitialized)
 414       p->state = nss_module_failed;
 415
 416   __libc_lock_unlock (nss_module_list_lock);
 417 }
 418
 419 void
 420 __nss_module_freeres (void)
 421 {
 422   struct nss_module *current = nss_module_list;
 423   while (current != NULL)
 424     {
 425       /* Ignore built-in modules (which have a NULL handle).  */
 426       if (current->state == nss_module_loaded
 427           && current->handle != NULL)
 428         __libc_dlclose (current->handle);
 429
 430       struct nss_module *next = current->next;
 431       free (current);
 432       current = next;
 433     }
 434   nss_module_list = NULL;
 435 }