resolv/resolv_context.h

   1 /* Temporary, thread-local resolver state.
   2    Copyright (C) 2017-2024 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 /* struct resolv_context objects are allocated on the heap,
  20    initialized by __resolv_context_get (and its variants), and
  21    destroyed by __resolv_context_put.
  22
  23    A nested call to __resolv_context_get (after another call to
  24    __resolv_context_get without a matching __resolv_context_put call,
  25    on the same thread) returns the original pointer, instead of
  26    allocating a new context.  This prevents unexpected reloading of
  27    the resolver configuration.  Care is taken to keep the context in
  28    sync with the thread-local _res object.  (This does not happen with
  29    __resolv_context_get_override.)
  30
  31    In contrast to struct __res_state, struct resolv_context is not
  32    affected by ABI compatibility concerns.
  33
  34    For the benefit of the res_n* functions, a struct __res_state
  35    pointer is included in the context object, and a separate
  36    initialization function is provided.  */
  37
  38 #ifndef _RESOLV_CONTEXT_H
  39 #define _RESOLV_CONTEXT_H
  40
  41 #include <bits/types/res_state.h>
  42 #include <resolv/resolv_conf.h>
  43 #include <stdbool.h>
  44 #include <stddef.h>
  45
  46 /* Temporary resolver state.  */
  47 struct resolv_context
  48 {
  49   struct __res_state *resp;     /* Backing resolver state.   */
  50
  51   /* Extended resolver state.  This is set to NULL if the
  52      __resolv_context_get functions are unable to locate an associated
  53      extended state.  In this case, the configuration data in *resp
  54      has to be used; otherwise, the data from *conf should be
  55      preferred (because it is a superset).  */
  56   struct resolv_conf *conf;
  57
  58   /* The following fields are for internal use within the
  59      resolv_context module.  */
  60   size_t __refcount;            /* Count of reusages by the get functions.  */
  61   bool __from_res;              /* True if created from _res.  */
  62
  63   /* Single-linked list of resolver contexts.  Used for memory
  64      deallocation on thread cancellation.  */
  65   struct resolv_context *__next;
  66 };
  67
  68 /* Return the current temporary resolver context, or NULL if there was
  69    an error (indicated by errno).  A call to this function must be
  70    paired with a call to __resolv_context_put.  */
  71 struct resolv_context *__resolv_context_get (void)
  72   __attribute__ ((warn_unused_result));
  73 libc_hidden_proto (__resolv_context_get)
  74
  75 /* Deallocate the temporary resolver context.  Converse of
  76    __resolv_context_get.  Do nothing if CTX is NULL.  */
  77 void __resolv_context_put (struct resolv_context *ctx);
  78 libc_hidden_proto (__resolv_context_put)
  79
  80 /* Like __resolv_context_get, but the _res structure can be partially
  81    initialized and those changes will not be overwritten.  */
  82 struct resolv_context *__resolv_context_get_preinit (void)
  83   __attribute__ ((warn_unused_result));
  84 libc_hidden_proto (__resolv_context_get_preinit)
  85
  86 /* Wrap a struct __res_state object in a struct resolv_context object.
  87    A call to this function must be paired with a call to
  88    __resolv_context_put.  */
  89 struct resolv_context *__resolv_context_get_override (struct __res_state *)
  90   __attribute__ ((nonnull (1), warn_unused_result));
  91 libc_hidden_proto (__resolv_context_get_override)
  92
  93 /* Return the search path entry at INDEX, or NULL if there are fewer
  94    than INDEX entries.  */
  95 static __attribute__ ((nonnull (1), unused)) const char *
  96 __resolv_context_search_list (const struct resolv_context *ctx, size_t index)
  97 {
  98   if (ctx->conf != NULL)
  99     {
 100       if (index < ctx->conf->search_list_size)
 101         return ctx->conf->search_list[index];
 102       else
 103         return NULL;
 104     }
 105   /* Fallback.  ctx->resp->dnsrch is a NULL-terminated array.  */
 106   for (size_t i = 0; ctx->resp->dnsrch[i] != NULL && i < MAXDNSRCH; ++i)
 107     if (i == index)
 108       return ctx->resp->dnsrch[i];
 109   return NULL;
 110 }
 111
 112 /* Return the number of name servers.  */
 113 static __attribute__ ((nonnull (1), unused)) size_t
 114 __resolv_context_nameserver_count (const struct resolv_context *ctx)
 115 {
 116   if (ctx->conf != NULL)
 117     return ctx->conf->nameserver_list_size;
 118   else
 119     return ctx->resp->nscount;
 120 }
 121
 122 /* Return a pointer to the socket address of the name server INDEX, or
 123    NULL if the index is out of bounds.  */
 124 static __attribute__ ((nonnull (1), unused)) const struct sockaddr *
 125 __resolv_context_nameserver (const struct resolv_context *ctx, size_t index)
 126 {
 127   if (ctx->conf != NULL)
 128     {
 129       if (index < ctx->conf->nameserver_list_size)
 130         return ctx->conf->nameserver_list[index];
 131     }
 132   else
 133     if (index < ctx->resp->nscount)
 134       {
 135         if (ctx->resp->nsaddr_list[index].sin_family != 0)
 136           return (const struct sockaddr *) &ctx->resp->nsaddr_list[index];
 137         else
 138           return (const struct sockaddr *) &ctx->resp->_u._ext.nsaddrs[index];
 139       }
 140   return NULL;
 141 }
 142
 143 /* Return the number of sort list entries.  */
 144 static __attribute__ ((nonnull (1), unused)) size_t
 145 __resolv_context_sort_count (const struct resolv_context *ctx)
 146 {
 147   if (ctx->conf != NULL)
 148     return ctx->conf->sort_list_size;
 149   else
 150     return ctx->resp->nsort;
 151 }
 152
 153 /* Return the sort list entry at INDEX.  */
 154 static __attribute__ ((nonnull (1), unused)) struct resolv_sortlist_entry
 155 __resolv_context_sort_entry (const struct resolv_context *ctx, size_t index)
 156 {
 157   if (ctx->conf != NULL)
 158     {
 159       if (index < ctx->conf->sort_list_size)
 160         return ctx->conf->sort_list[index];
 161       /* Fall through.  */
 162     }
 163   else if (index < ctx->resp->nsort)
 164     return (struct resolv_sortlist_entry)
 165       {
 166         .addr = ctx->resp->sort_list[index].addr,
 167         .mask = ctx->resp->sort_list[index].mask,
 168       };
 169
 170   return (struct resolv_sortlist_entry) { .mask = 0, };
 171 }
 172
 173 /* Called during thread shutdown to free the associated resolver
 174    context (mostly in response to cancellation, otherwise the
 175    __resolv_context_get/__resolv_context_put pairing will already have
 176    deallocated the context object).  */
 177 void __resolv_context_freeres (void) attribute_hidden;
 178
 179 #endif /* _RESOLV_CONTEXT_H */