1 /*- 2 * SPDX-License-Identifier: BSD-2-Clause 3 * 4 * Copyright (C) 1997 John D. Polstra. All rights reserved. 5 * 6 * Redistribution and use in source and binary forms, with or without 7 * modification, are permitted provided that the following conditions 8 * are met: 9 * 1. Redistributions of source code must retain the above copyright 10 * notice, this list of conditions and the following disclaimer. 11 * 2. Redistributions in binary form must reproduce the above copyright 12 * notice, this list of conditions and the following disclaimer in the 13 * documentation and/or other materials provided with the distribution. 14 * 15 * THIS SOFTWARE IS PROVIDED BY JOHN D. POLSTRA AND CONTRIBUTORS ``AS IS'' AND 16 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 17 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 18 * ARE DISCLAIMED. IN NO EVENT SHALL JOHN D. POLSTRA OR CONTRIBUTORS BE LIABLE 19 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 20 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 21 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 22 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 23 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 24 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 25 * SUCH DAMAGE. 26 */ 27 28 #include <sys/types.h> 29 #include <sys/wait.h> 30 31 #include <assert.h> 32 #include <err.h> 33 #include <errno.h> 34 #include <fcntl.h> 35 #include <limits.h> 36 #include <signal.h> 37 #include <stdio.h> 38 #include <stdlib.h> 39 #include <string.h> 40 #include <sysexits.h> 41 #include <unistd.h> 42 43 #define FDLOCK_PREFIX "/dev/fd/" 44 45 union lock_subject { 46 long subj_fd; 47 const char *subj_name; 48 }; 49 50 static int acquire_lock(union lock_subject *subj, int flags, int silent); 51 static void cleanup(void); 52 static void killed(int sig); 53 static void timeout(int sig); 54 static void usage(void) __dead2; 55 static void wait_for_lock(const char *name); 56 57 static const char *lockname; 58 static int lockfd = -1; 59 static int keep; 60 static int fdlock; 61 static volatile sig_atomic_t timed_out; 62 63 /* 64 * Check if fdlock is implied by the given `lockname`. We'll write the fd that 65 * is represented by it out to ofd, and the caller is expected to do any 66 * necessary validation on it. 67 */ 68 static int 69 fdlock_implied(const char *name, long *ofd) 70 { 71 char *endp; 72 long fd; 73 74 if (strncmp(name, FDLOCK_PREFIX, sizeof(FDLOCK_PREFIX) - 1) != 0) 75 return (0); 76 77 /* Skip past the prefix. */ 78 name += sizeof(FDLOCK_PREFIX) - 1; 79 errno = 0; 80 fd = strtol(name, &endp, 10); 81 if (errno != 0 || *endp != '\0') 82 return (0); 83 84 *ofd = fd; 85 return (1); 86 } 87 88 /* 89 * Execute an arbitrary command while holding a file lock. 90 */ 91 int 92 main(int argc, char **argv) 93 { 94 int ch, flags, silent, status; 95 long long waitsec; 96 pid_t child; 97 union lock_subject subj; 98 99 silent = keep = 0; 100 flags = O_CREAT | O_RDONLY; 101 waitsec = -1; /* Infinite. */ 102 while ((ch = getopt(argc, argv, "knst:w")) != -1) { 103 switch (ch) { 104 case 'k': 105 keep = 1; 106 break; 107 case 'n': 108 flags &= ~O_CREAT; 109 break; 110 case 's': 111 silent = 1; 112 break; 113 case 't': 114 { 115 const char *errstr; 116 117 waitsec = strtonum(optarg, 0, UINT_MAX, &errstr); 118 if (errstr != NULL) 119 errx(EX_USAGE, 120 "invalid timeout \"%s\"", optarg); 121 } 122 break; 123 case 'w': 124 flags = (flags & ~O_RDONLY) | O_WRONLY; 125 break; 126 default: 127 usage(); 128 } 129 } 130 131 argc -= optind; 132 argv += optind; 133 134 if (argc == 0) 135 usage(); 136 137 lockname = argv[0]; 138 139 argc--; 140 argv++; 141 142 /* 143 * If there aren't any arguments left, then we must be in fdlock mode. 144 */ 145 if (argc == 0 && *lockname != '/') { 146 fdlock = 1; 147 subj.subj_fd = -1; 148 } else { 149 fdlock = fdlock_implied(lockname, &subj.subj_fd); 150 if (argc == 0 && !fdlock) { 151 fprintf(stderr, "Expected fd, got '%s'\n", lockname); 152 usage(); 153 } 154 } 155 156 if (fdlock) { 157 if (subj.subj_fd < 0) { 158 char *endp; 159 160 errno = 0; 161 subj.subj_fd = strtol(lockname, &endp, 10); 162 if (errno != 0 || *endp != '\0') { 163 fprintf(stderr, "Expected fd, got '%s'\n", 164 lockname); 165 usage(); 166 } 167 } 168 169 if (subj.subj_fd < 0 || subj.subj_fd > INT_MAX) { 170 fprintf(stderr, "fd '%ld' out of range\n", 171 subj.subj_fd); 172 usage(); 173 } 174 } else { 175 subj.subj_name = lockname; 176 } 177 178 if (waitsec > 0) { /* Set up a timeout. */ 179 struct sigaction act; 180 181 act.sa_handler = timeout; 182 sigemptyset(&act.sa_mask); 183 act.sa_flags = 0; /* Note that we do not set SA_RESTART. */ 184 sigaction(SIGALRM, &act, NULL); 185 alarm((unsigned int)waitsec); 186 } 187 /* 188 * If the "-k" option is not given, then we must not block when 189 * acquiring the lock. If we did, then the lock holder would 190 * unlink the file upon releasing the lock, and we would acquire 191 * a lock on a file with no directory entry. Then another 192 * process could come along and acquire the same lock. To avoid 193 * this problem, we separate out the actions of waiting for the 194 * lock to be available and of actually acquiring the lock. 195 * 196 * That approach produces behavior that is technically correct; 197 * however, it causes some performance & ordering problems for 198 * locks that have a lot of contention. First, it is unfair in 199 * the sense that a released lock isn't necessarily granted to 200 * the process that has been waiting the longest. A waiter may 201 * be starved out indefinitely. Second, it creates a thundering 202 * herd situation each time the lock is released. 203 * 204 * When the "-k" option is used, the unlink race no longer 205 * exists. In that case we can block while acquiring the lock, 206 * avoiding the separate step of waiting for the lock. This 207 * yields fairness and improved performance. 208 */ 209 lockfd = acquire_lock(&subj, flags | O_NONBLOCK, silent); 210 while (lockfd == -1 && !timed_out && waitsec != 0) { 211 if (keep || fdlock) 212 lockfd = acquire_lock(&subj, flags, silent); 213 else { 214 wait_for_lock(lockname); 215 lockfd = acquire_lock(&subj, flags | O_NONBLOCK, 216 silent); 217 } 218 } 219 if (waitsec > 0) 220 alarm(0); 221 if (lockfd == -1) { /* We failed to acquire the lock. */ 222 if (silent) 223 exit(EX_TEMPFAIL); 224 errx(EX_TEMPFAIL, "%s: already locked", lockname); 225 } 226 227 /* At this point, we own the lock. */ 228 229 /* Nothing else to do for FD lock, just exit */ 230 if (argc == 0) { 231 assert(fdlock); 232 return 0; 233 } 234 235 if (atexit(cleanup) == -1) 236 err(EX_OSERR, "atexit failed"); 237 if ((child = fork()) == -1) 238 err(EX_OSERR, "cannot fork"); 239 if (child == 0) { /* The child process. */ 240 close(lockfd); 241 execvp(argv[0], argv); 242 warn("%s", argv[0]); 243 _exit(1); 244 } 245 /* This is the parent process. */ 246 signal(SIGINT, SIG_IGN); 247 signal(SIGQUIT, SIG_IGN); 248 signal(SIGTERM, killed); 249 fclose(stdin); 250 fclose(stdout); 251 fclose(stderr); 252 if (waitpid(child, &status, 0) == -1) 253 exit(EX_OSERR); 254 return (WIFEXITED(status) ? WEXITSTATUS(status) : EX_SOFTWARE); 255 } 256 257 /* 258 * Try to acquire a lock on the given file/fd, creating the file if 259 * necessary. The flags argument is O_NONBLOCK or 0, depending on 260 * whether we should wait for the lock. Returns an open file descriptor 261 * on success, or -1 on failure. 262 */ 263 static int 264 acquire_lock(union lock_subject *subj, int flags, int silent) 265 { 266 int fd; 267 268 if (fdlock) { 269 assert(subj->subj_fd >= 0 && subj->subj_fd <= INT_MAX); 270 fd = (int)subj->subj_fd; 271 272 if (flock(fd, LOCK_EX | LOCK_NB) == -1) { 273 if (errno == EAGAIN || errno == EINTR) 274 return (-1); 275 err(EX_CANTCREAT, "cannot lock fd %d", fd); 276 } 277 } else if ((fd = open(subj->subj_name, O_EXLOCK|flags, 0666)) == -1) { 278 if (errno == EAGAIN || errno == EINTR) 279 return (-1); 280 else if (errno == ENOENT && (flags & O_CREAT) == 0) { 281 if (!silent) 282 warn("%s", subj->subj_name); 283 exit(EX_UNAVAILABLE); 284 } 285 err(EX_CANTCREAT, "cannot open %s", subj->subj_name); 286 } 287 return (fd); 288 } 289 290 /* 291 * Remove the lock file. 292 */ 293 static void 294 cleanup(void) 295 { 296 297 if (keep || fdlock) 298 flock(lockfd, LOCK_UN); 299 else 300 unlink(lockname); 301 } 302 303 /* 304 * Signal handler for SIGTERM. Cleans up the lock file, then re-raises 305 * the signal. 306 */ 307 static void 308 killed(int sig) 309 { 310 311 cleanup(); 312 signal(sig, SIG_DFL); 313 if (kill(getpid(), sig) == -1) 314 _Exit(EX_OSERR); 315 } 316 317 /* 318 * Signal handler for SIGALRM. 319 */ 320 static void 321 timeout(int sig __unused) 322 { 323 324 timed_out = 1; 325 } 326 327 static void 328 usage(void) 329 { 330 331 fprintf(stderr, 332 "usage: lockf [-knsw] [-t seconds] file command [arguments]\n" 333 " lockf [-s] [-t seconds] fd\n"); 334 exit(EX_USAGE); 335 } 336 337 /* 338 * Wait until it might be possible to acquire a lock on the given file. 339 * If the file does not exist, return immediately without creating it. 340 */ 341 static void 342 wait_for_lock(const char *name) 343 { 344 int fd; 345 346 if ((fd = open(name, O_RDONLY|O_EXLOCK, 0666)) == -1) { 347 if (errno == ENOENT || errno == EINTR) 348 return; 349 err(EX_CANTCREAT, "cannot open %s", name); 350 } 351 close(fd); 352 } 353