1 /* $NetBSD: pthread_atfork.c,v 1.17 2022/09/13 10:18:47 riastradh Exp $ */ 2 3 /*- 4 * Copyright (c) 2002 The NetBSD Foundation, Inc. 5 * All rights reserved. 6 * 7 * This code is derived from software contributed to The NetBSD Foundation 8 * by Nathan J. Williams. 9 * 10 * Redistribution and use in source and binary forms, with or without 11 * modification, are permitted provided that the following conditions 12 * are met: 13 * 1. Redistributions of source code must retain the above copyright 14 * notice, this list of conditions and the following disclaimer. 15 * 2. Redistributions in binary form must reproduce the above copyright 16 * notice, this list of conditions and the following disclaimer in the 17 * documentation and/or other materials provided with the distribution. 18 * 19 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 20 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 21 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 22 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 23 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 24 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 25 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 26 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 27 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 28 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 29 * POSSIBILITY OF SUCH DAMAGE. 30 */ 31 32 #include <sys/cdefs.h> 33 #if defined(LIBC_SCCS) && !defined(lint) 34 __RCSID("$NetBSD: pthread_atfork.c,v 1.17 2022/09/13 10:18:47 riastradh Exp $"); 35 #endif /* LIBC_SCCS and not lint */ 36 37 #include "namespace.h" 38 39 #include <errno.h> 40 #include <stdlib.h> 41 #include <unistd.h> 42 #include <sys/queue.h> 43 #include "extern.h" 44 #include "reentrant.h" 45 46 #ifdef __weak_alias 47 __weak_alias(pthread_atfork, _pthread_atfork) 48 __weak_alias(fork, _fork) 49 #endif /* __weak_alias */ 50 51 pid_t __fork(void); /* XXX */ 52 pid_t __locked_fork(int *) __weak; /* XXX */ 53 54 pid_t 55 __locked_fork(int *my_errno) 56 { 57 return __fork(); 58 } 59 60 struct atfork_callback { 61 SIMPLEQ_ENTRY(atfork_callback) next; 62 void (*fn)(void); 63 }; 64 65 /* 66 * Hypothetically, we could protect the queues with a rwlock which is 67 * write-locked by pthread_atfork() and read-locked by fork(), but 68 * since the intended use of the functions is obtaining locks to hold 69 * across the fork, forking is going to be serialized anyway. 70 */ 71 static struct atfork_callback atfork_builtin; 72 #ifdef _REENTRANT 73 static mutex_t atfork_lock = MUTEX_INITIALIZER; 74 #endif 75 SIMPLEQ_HEAD(atfork_callback_q, atfork_callback); 76 77 static struct atfork_callback_q prepareq = SIMPLEQ_HEAD_INITIALIZER(prepareq); 78 static struct atfork_callback_q parentq = SIMPLEQ_HEAD_INITIALIZER(parentq); 79 static struct atfork_callback_q childq = SIMPLEQ_HEAD_INITIALIZER(childq); 80 81 static struct atfork_callback * 82 af_alloc(void) 83 { 84 85 if (atfork_builtin.fn == NULL) 86 return &atfork_builtin; 87 88 return malloc(sizeof(atfork_builtin)); 89 } 90 91 static void 92 af_free(struct atfork_callback *af) 93 { 94 95 if (af != &atfork_builtin) 96 free(af); 97 } 98 99 int 100 pthread_atfork(void (*prepare)(void), void (*parent)(void), 101 void (*child)(void)) 102 { 103 struct atfork_callback *newprepare, *newparent, *newchild; 104 sigset_t mask, omask; 105 int error; 106 107 newprepare = newparent = newchild = NULL; 108 109 sigfillset(&mask); 110 thr_sigsetmask(SIG_SETMASK, &mask, &omask); 111 112 mutex_lock(&atfork_lock); 113 if (prepare != NULL) { 114 newprepare = af_alloc(); 115 if (newprepare == NULL) { 116 error = ENOMEM; 117 goto out; 118 } 119 newprepare->fn = prepare; 120 } 121 122 if (parent != NULL) { 123 newparent = af_alloc(); 124 if (newparent == NULL) { 125 if (newprepare != NULL) 126 af_free(newprepare); 127 error = ENOMEM; 128 goto out; 129 } 130 newparent->fn = parent; 131 } 132 133 if (child != NULL) { 134 newchild = af_alloc(); 135 if (newchild == NULL) { 136 if (newprepare != NULL) 137 af_free(newprepare); 138 if (newparent != NULL) 139 af_free(newparent); 140 error = ENOMEM; 141 goto out; 142 } 143 newchild->fn = child; 144 } 145 146 /* 147 * The order in which the functions are called is specified as 148 * LIFO for the prepare handler and FIFO for the others; insert 149 * at the head and tail as appropriate so that SIMPLEQ_FOREACH() 150 * produces the right order. 151 */ 152 if (prepare) 153 SIMPLEQ_INSERT_HEAD(&prepareq, newprepare, next); 154 if (parent) 155 SIMPLEQ_INSERT_TAIL(&parentq, newparent, next); 156 if (child) 157 SIMPLEQ_INSERT_TAIL(&childq, newchild, next); 158 error = 0; 159 160 out: mutex_unlock(&atfork_lock); 161 thr_sigsetmask(SIG_SETMASK, &omask, NULL); 162 return error; 163 } 164 165 pid_t 166 fork(void) 167 { 168 struct atfork_callback *iter; 169 pid_t ret; 170 171 mutex_lock(&atfork_lock); 172 SIMPLEQ_FOREACH(iter, &prepareq, next) 173 (*iter->fn)(); 174 _malloc_prefork(); 175 176 ret = __locked_fork(&errno); 177 178 if (ret != 0) { 179 /* 180 * We are the parent. It doesn't matter here whether 181 * the fork call succeeded or failed. 182 */ 183 _malloc_postfork(); 184 SIMPLEQ_FOREACH(iter, &parentq, next) 185 (*iter->fn)(); 186 mutex_unlock(&atfork_lock); 187 } else { 188 /* We are the child */ 189 _malloc_postfork_child(); 190 SIMPLEQ_FOREACH(iter, &childq, next) 191 (*iter->fn)(); 192 /* 193 * Note: We are explicitly *not* unlocking 194 * atfork_lock. Unlocking atfork_lock is problematic, 195 * because if any threads in the parent blocked on it 196 * between the initial lock and the fork() syscall, 197 * unlocking in the child will try to schedule 198 * threads, and either the internal mutex interlock or 199 * the runqueue spinlock could have been held at the 200 * moment of fork(). Since the other threads do not 201 * exist in this process, the spinlock will never be 202 * unlocked, and we would wedge. 203 * Instead, we reinitialize atfork_lock, since we know 204 * that the state of the atfork lists is consistent here, 205 * and that there are no other threads to be affected by 206 * the forcible cleaning of the queue. 207 * This permits double-forking to work, although 208 * it requires knowing that it's "safe" to initialize 209 * a locked mutex in this context. 210 * 211 * The problem exists for users of this interface, 212 * too, since the intended use of pthread_atfork() is 213 * to acquire locks across the fork call to ensure 214 * that the child sees consistent state. There's not 215 * much that can usefully be done in a child handler, 216 * and conventional wisdom discourages using them, but 217 * they're part of the interface, so here we are... 218 */ 219 mutex_init(&atfork_lock, NULL); 220 } 221 222 return ret; 223 } 224