resolv/resolv_context.h

   1 /* Temporary, thread-local resolver state.
   2    Copyright (C) 2017-2018 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 /* 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, and __resolv_context_get_no_inet6 may
  30    also interpose another context object if RES_USE_INET6 needs to be
  31    disabled.)
  32
  33    In contrast to struct __res_state, struct resolv_context is not
  34    affected by ABI compatibility concerns.
  35
  36    For the benefit of the res_n* functions, a struct __res_state
  37    pointer is included in the context object, and a separate
  38    initialization function is provided.  */
  39
  40 #ifndef _RESOLV_CONTEXT_H
  41 #define _RESOLV_CONTEXT_H
  42
  43 #include <bits/types/res_state.h>
  44 #include <resolv/resolv_conf.h>
  45 #include <stdbool.h>
  46 #include <stddef.h>
  47
  48 /* Temporary resolver state.  */
  49 struct resolv_context
  50 {
  51   struct __res_state *resp;     /* Backing resolver state.   */
  52
  53   /* Extended resolver state.  This is set to NULL if the
  54      __resolv_context_get functions are unable to locate an associated
  55      extended state.  In this case, the configuration data in *resp
  56      has to be used; otherwise, the data from *conf should be
  57      preferred (because it is a superset).  */
  58   struct resolv_conf *conf;
  59
  60   /* The following fields are for internal use within the
  61      resolv_context module.  */
  62   size_t __refcount;            /* Count of reusages by the get functions.  */
  63   bool __from_res;              /* True if created from _res.  */
  64
  65   /* If RES_USE_INET6 was disabled at this level, this field points to
  66      the previous context.  */
  67   struct resolv_context *__next;
  68 };
  69
  70 /* Return the current temporary resolver context, or NULL if there was
  71    an error (indicated by errno).  A call to this function must be
  72    paired with a call to __resolv_context_put.  */
  73 struct resolv_context *__resolv_context_get (void)
  74   __attribute__ ((warn_unused_result));
  75 libc_hidden_proto (__resolv_context_get)
  76
  77 /* Deallocate the temporary resolver context.  Converse of
  78    __resolv_context_get.  Restore the RES_USE_INET6 flag if necessary.
  79    Do nothing if CTX is NULL.  */
  80 void __resolv_context_put (struct resolv_context *ctx);
  81 libc_hidden_proto (__resolv_context_put)
  82
  83 /* Like __resolv_context_get, but the _res structure can be partially
  84    initialzed and those changes will not be overwritten.  */
  85 struct resolv_context *__resolv_context_get_preinit (void)
  86   __attribute__ ((warn_unused_result));
  87 libc_hidden_proto (__resolv_context_get_preinit)
  88
  89 /* Wrap a struct __res_state object in a struct resolv_context object.
  90    A call to this function must be paired with a call to
  91    __resolv_context_put.  */
  92 struct resolv_context *__resolv_context_get_override (struct __res_state *)
  93   __attribute__ ((nonnull (1), warn_unused_result));
  94 libc_hidden_proto (__resolv_context_get_override)
  95
  96 /* Return the search path entry at INDEX, or NULL if there are fewer
  97    than INDEX entries.  */
  98 static __attribute__ ((nonnull (1), unused)) const char *
  99 __resolv_context_search_list (const struct resolv_context *ctx, size_t index)
 100 {
 101   if (ctx->conf != NULL)
 102     {
 103       if (index < ctx->conf->search_list_size)
 104         return ctx->conf->search_list[index];
 105       else
 106         return NULL;
 107     }
 108   /* Fallback.  ctx->resp->dnsrch is a NULL-terminated array.  */
 109   for (size_t i = 0; ctx->resp->dnsrch[i] != NULL && i < MAXDNSRCH; ++i)
 110     if (i == index)
 111       return ctx->resp->dnsrch[i];
 112   return NULL;
 113 }
 114
 115 /* Return the number of name servers.  */
 116 static __attribute__ ((nonnull (1), unused)) size_t
 117 __resolv_context_nameserver_count (const struct resolv_context *ctx)
 118 {
 119   if (ctx->conf != NULL)
 120     return ctx->conf->nameserver_list_size;
 121   else
 122     return ctx->resp->nscount;
 123 }
 124
 125 /* Return a pointer to the socket address of the name server INDEX, or
 126    NULL if the index is out of bounds.  */
 127 static __attribute__ ((nonnull (1), unused)) const struct sockaddr *
 128 __resolv_context_nameserver (const struct resolv_context *ctx, size_t index)
 129 {
 130   if (ctx->conf != NULL)
 131     {
 132       if (index < ctx->conf->nameserver_list_size)
 133         return ctx->conf->nameserver_list[index];
 134     }
 135   else
 136     if (index < ctx->resp->nscount)
 137       {
 138         if (ctx->resp->nsaddr_list[index].sin_family != 0)
 139           return (const struct sockaddr *) &ctx->resp->nsaddr_list[index];
 140         else
 141           return (const struct sockaddr *) &ctx->resp->_u._ext.nsaddrs[index];
 142       }
 143   return NULL;
 144 }
 145
 146 /* Return the number of sort list entries.  */
 147 static __attribute__ ((nonnull (1), unused)) size_t
 148 __resolv_context_sort_count (const struct resolv_context *ctx)
 149 {
 150   if (ctx->conf != NULL)
 151     return ctx->conf->sort_list_size;
 152   else
 153     return ctx->resp->nsort;
 154 }
 155
 156 /* Return the sort list entry at INDEX.  */
 157 static __attribute__ ((nonnull (1), unused)) struct resolv_sortlist_entry
 158 __resolv_context_sort_entry (const struct resolv_context *ctx, size_t index)
 159 {
 160   if (ctx->conf != NULL)
 161     {
 162       if (index < ctx->conf->sort_list_size)
 163         return ctx->conf->sort_list[index];
 164       /* Fall through.  */
 165     }
 166   else if (index < ctx->resp->nsort)
 167     return (struct resolv_sortlist_entry)
 168       {
 169         .addr = ctx->resp->sort_list[index].addr,
 170         .mask = ctx->resp->sort_list[index].mask,
 171       };
 172
 173   return (struct resolv_sortlist_entry) { .mask = 0, };
 174 }
 175
 176 /* Called during thread shutdown to free the associated resolver
 177    context (mostly in response to cancellation, otherwise the
 178    __resolv_context_get/__resolv_context_put pairing will already have
 179    deallocated the context object).  */
 180 void __resolv_context_freeres (void) attribute_hidden;
 181
 182 #endif /* _RESOLV_CONTEXT_H */