1 /*
2 * CDDL HEADER START
3 *
4 * The contents of this file are subject to the terms of the
5 * Common Development and Distribution License (the "License").
6 * You may not use this file except in compliance with the License.
7 *
8 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
9 * or http://www.opensolaris.org/os/licensing.
10 * See the License for the specific language governing permissions
11 * and limitations under the License.
12 *
13 * When distributing Covered Code, include this CDDL HEADER in each
14 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
15 * If applicable, add the following below this CDDL HEADER, with the
16 * fields enclosed by brackets "[]" replaced with your own identifying
17 * information: Portions Copyright [yyyy] [name of copyright owner]
18 *
19 * CDDL HEADER END
20 */
21
22 /*
23 * Copyright 2008 Sun Microsystems, Inc. All rights reserved.
24 * Use is subject to license terms.
25 */
26
27 /*
28 * dns_mt.c
29 *
30 * This file contains all the MT related routines for the DNS backend.
31 */
32
33 #include "dns_common.h"
34 #include <dlfcn.h>
35
36 /*
37 * If the DNS name service switch routines are used in a binary that depends
38 * on an older libresolv (libresolv.so.1, say), then having nss_dns.so.1 or
39 * libnss_dns.a depend on a newer libresolv (libresolv.so.2) will cause
40 * relocation problems. In particular, copy relocation of the _res structure
41 * (which changes in size from libresolv.so.1 to libresolv.so.2) could
42 * cause corruption, and result in a number of strange problems, including
43 * core dumps. Hence, we check if a libresolv is already loaded.
44 */
45
46 #pragma init(_nss_dns_init)
47 static void _nss_dns_init(void);
48
49 extern struct hostent *res_gethostbyname(const char *);
50 #pragma weak res_gethostbyname
51
52 #define RES_SET_NO_HOSTS_FALLBACK "__res_set_no_hosts_fallback"
53 extern void __res_set_no_hosts_fallback(void);
54 #pragma weak __res_set_no_hosts_fallback
55
56 #define RES_UNSET_NO_HOSTS_FALLBACK "__res_unset_no_hosts_fallback"
57 extern void __res_unset_no_hosts_fallback(void);
58 #pragma weak __res_unset_no_hosts_fallback
59
60 #define RES_GET_RES "__res_get_res"
61 extern struct __res_state *__res_get_res(void);
62 #pragma weak __res_get_res
63
64 #define RES_ENABLE_MT "__res_enable_mt"
65 extern int __res_enable_mt(void);
66 #pragma weak __res_enable_mt
67
68 #define RES_DISABLE_MT "__res_disable_mt"
69 extern int __res_disable_mt(void);
70 #pragma weak __res_disable_mt
71
72 #define RES_GET_H_ERRNO "__res_get_h_errno"
73 extern int *__res_get_h_errno();
74 #pragma weak __res_get_h_errno
75
76 #define __H_ERRNO "__h_errno"
77 extern int *__h_errno(void);
78 #pragma weak __h_errno
79
80 #define RES_OVERRIDE_RETRY "__res_override_retry"
81 extern int __res_override_retry(int);
82 #pragma weak __res_override_retry
83
84 static void __fallback_set_no_hosts(void);
85 static int *__fallback_h_errno(void);
86 static int __fallback_override_retry(int);
87 static int __is_mt_safe(void);
88
89 void (*set_no_hosts_fallback)(void) = __fallback_set_no_hosts;
90 void (*unset_no_hosts_fallback)(void) = __fallback_set_no_hosts;
91 struct __res_state *(*set_res_retry)() = 0;
92 int (*enable_mt)() = 0;
93 int (*disable_mt)() = 0;
94 int *(*get_h_errno)(void) = 0;
95 int (*override_retry)(int) = 0;
96
97 /* Usually set from the Makefile */
98 #ifndef NSS_DNS_LIBRESOLV
99 #define NSS_DNS_LIBRESOLV "libresolv.so.2"
100 #endif
101
102 /* From libresolv */
103 extern int h_errno;
104
105 mutex_t one_lane = DEFAULTMUTEX;
106
107 void
_nss_dns_init(void)108 _nss_dns_init(void)
109 {
110 void *reslib, (*f_void_ptr)();
111
112 /* If no libresolv library, then load one */
113 if (res_gethostbyname == 0) {
114 if ((reslib =
115 dlopen(NSS_DNS_LIBRESOLV, RTLD_LAZY|RTLD_GLOBAL)) != 0) {
116 /* Turn off /etc/hosts fall back in libresolv */
117 if ((f_void_ptr = (void (*)(void))dlsym(reslib,
118 RES_SET_NO_HOSTS_FALLBACK)) != 0) {
119 set_no_hosts_fallback = f_void_ptr;
120 }
121 if ((f_void_ptr = (void (*)(void))dlsym(reslib,
122 RES_SET_NO_HOSTS_FALLBACK)) != 0) {
123 unset_no_hosts_fallback = f_void_ptr;
124 }
125 /* Set number of resolver retries */
126 if ((override_retry = (int (*)(int))dlsym(reslib,
127 RES_OVERRIDE_RETRY)) == 0) {
128 set_res_retry =
129 (struct __res_state *(*)(void))dlsym(reslib,
130 RES_GET_RES);
131 override_retry = __fallback_override_retry;
132 }
133 /*
134 * Select h_errno retrieval function. A BIND 8.2.2
135 * libresolv.so.2 will have __h_errno, a BIND 8.1.2
136 * one will have __res_get_h_errno, and other
137 * versions may have nothing at all.
138 *
139 * Also try to bind to the relevant MT enable/disable
140 * functions which are also dependent on the version
141 * of the BIND libresolv.so.2 being used.
142 */
143 if ((get_h_errno = (int *(*)(void))dlsym(reslib,
144 __H_ERRNO)) != 0) {
145 /* BIND 8.2.2 libresolv.so.2 is MT safe. */
146 enable_mt = __is_mt_safe;
147 disable_mt = __is_mt_safe;
148 } else {
149 if ((get_h_errno =
150 (int *(*)(void))dlsym(reslib,
151 RES_GET_H_ERRNO)) == 0) {
152 get_h_errno = __fallback_h_errno;
153 }
154 /*
155 * Pre-BIND 8.2.2 was not MT safe. Try to
156 * bind the MT enable/disable functions.
157 */
158 if ((enable_mt = (int (*)(void))dlsym(reslib,
159 RES_ENABLE_MT)) != 0 &&
160 (disable_mt = (int (*)(void))dlsym(reslib,
161 RES_DISABLE_MT)) == 0) {
162 enable_mt = 0;
163 }
164 }
165 }
166 } else {
167 /* Libresolv already loaded */
168 if ((f_void_ptr = __res_set_no_hosts_fallback) != 0) {
169 set_no_hosts_fallback = f_void_ptr;
170 }
171 if ((f_void_ptr = __res_unset_no_hosts_fallback) != 0) {
172 unset_no_hosts_fallback = f_void_ptr;
173 }
174 if ((override_retry = __res_override_retry) == 0) {
175 set_res_retry = __res_get_res;
176 override_retry = __fallback_override_retry;
177 }
178 if ((get_h_errno = __h_errno) == 0 &&
179 (get_h_errno = __res_get_h_errno) == 0) {
180 get_h_errno = __fallback_h_errno;
181 }
182 if (get_h_errno == __h_errno) {
183 enable_mt = __is_mt_safe;
184 disable_mt = __is_mt_safe;
185 } else {
186 if ((enable_mt = __res_enable_mt) != 0 &&
187 (disable_mt = __res_disable_mt) == 0) {
188 enable_mt = 0;
189 }
190 }
191 }
192 }
193
194
195 /*
196 *
197 * Integration of BIND 8.1.2 introduced two new Sun private functions,
198 * __res_enable_mt() and __res_disable_mt(), that enabled and disabled
199 * MT mode per-thread. These functions are in the private libresolv.so.2
200 * interface, and intended for use by nss_dns.so.1.
201 *
202 * BIND 8.2.2 removed the need for those two functions. As similar
203 * functionality was provided in BIND further up the stack. However the
204 * functions remain to satisfy any application that directly called upon
205 * them. Only, __res_enable_mt() was modified to return failure.
206 * Indicated by a non-zero return value. So that those unconventional
207 * applications would not then presume that res_send() and friends are
208 * MT-safe, when in fact they are not.
209 *
210 * To prevent nss_dns from locking inappropriately __is_mt_safe() is
211 * called in place of __res_enable_mt() and __res_disable_mt() if BIND
212 * 8.2.2 libresolv.so.2 being used. __is_mt_safe() returns success
213 * indicated by a return code of zero. Signifying that no locking is
214 * necessary.
215 *
216 * MT applications making calls to gethostby*_r() or getipnodeby*()
217 * linked to libresolv.so.1 or linked statically with pre-BIND 8.2.2
218 * libresolv.a, doubtful as we don't ship a static version, would require
219 * locking within the nsswitch back-end. Hence the mechanism can not
220 * simply be removed.
221 *
222 */
223 static int
__is_mt_safe(void)224 __is_mt_safe(void) {
225 return (0);
226 }
227
228
229 /*
230 * Return pointer to the global h_errno variable
231 */
232 static int *
__fallback_h_errno(void)233 __fallback_h_errno(void) {
234 return (&h_errno);
235 }
236
237
238 /*
239 * This function is called when the resolver library doesn't provide its
240 * own function to establish an override retry. If we can get a pointer
241 * to the per-thread _res (i.e., set_res_retry != 0), we set the retries
242 * directly, and return the previous number of retries. Otherwise, there's
243 * nothing to do.
244 */
245 static int
__fallback_override_retry(int retry)246 __fallback_override_retry(int retry) {
247 struct __res_state *res;
248 int old_retry = 0;
249
250 if (set_res_retry != 0) {
251 res = set_res_retry();
252 old_retry = res->retry;
253 res->retry = retry;
254 }
255 return (old_retry);
256 }
257
258
259 static void
__fallback_set_no_hosts(void)260 __fallback_set_no_hosts(void) {
261 }
262
263
264 /*
265 * Common code to enable/disable MT mode, set/unset no-/etc/hosts fallback,
266 * and to set the number of retries.
267 */
268 void
switch_resolver_setup(int * mt_disabled,sigset_t * oldmask,int * old_retry)269 switch_resolver_setup(int *mt_disabled, sigset_t *oldmask, int *old_retry) {
270
271 /*
272 * Try to enable MT mode. If that isn't possible, mask signals,
273 * and mutex_lock.
274 */
275 *mt_disabled = 1;
276 if (enable_mt == 0 || (*mt_disabled = (*enable_mt)()) != 0) {
277 sigset_t newmask;
278 (void) sigfillset(&newmask);
279 (void) thr_sigsetmask(SIG_SETMASK, &newmask, oldmask);
280 (void) mutex_lock(&one_lane);
281 }
282
283 /*
284 * Disable any fallback to /etc/hosts (or /etc/inet/ipnodes, when
285 * libresolv knows about that file).
286 */
287 (*set_no_hosts_fallback)();
288
289 /*
290 * The NS switch wants to handle retries on its own.
291 */
292 *old_retry = (*override_retry)(1);
293 }
294
295
296 void
switch_resolver_reset(int mt_disabled,sigset_t oldmask,int old_retry)297 switch_resolver_reset(int mt_disabled, sigset_t oldmask, int old_retry) {
298
299 if (mt_disabled) {
300 (void) mutex_unlock(&one_lane);
301 (void) thr_sigsetmask(SIG_SETMASK, &oldmask, NULL);
302 } else {
303 (void) (*disable_mt)();
304 }
305
306 (*unset_no_hosts_fallback)();
307
308 (void) (*override_retry)(old_retry);
309 }
310