1 /* Temporary, thread-local resolver state.
2    Copyright (C) 2017-2022 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    initialzed 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));
libc_hidden_proto(__resolv_context_get_override)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
__resolv_context_nameserver_count(const struct resolv_context * ctx)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 *
__resolv_context_nameserver(const struct resolv_context * ctx,size_t index)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
__resolv_context_sort_count(const struct resolv_context * ctx)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
__resolv_context_sort_entry(const struct resolv_context * ctx,size_t index)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 */
180