1 /* provide a chdir function that tries not to fail due to ENAMETOOLONG 2 Copyright (C) 2004-2015 Free Software Foundation, Inc. 3 4 This program is free software: you can redistribute it and/or modify 5 it under the terms of the GNU General Public License as published by 6 the Free Software Foundation; either version 3 of the License, or 7 (at your option) any later version. 8 9 This program is distributed in the hope that it will be useful, 10 but WITHOUT ANY WARRANTY; without even the implied warranty of 11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 12 GNU General Public License for more details. 13 14 You should have received a copy of the GNU General Public License 15 along with this program. If not, see <http://www.gnu.org/licenses/>. */ 16 17 /* written by Jim Meyering */ 18 19 #include <config.h> 20 21 #include "chdir-long.h" 22 23 #include <errno.h> 24 #include <fcntl.h> 25 #include <stdlib.h> 26 #include <stdbool.h> 27 #include <string.h> 28 #include <stdio.h> 29 30 #include "assure.h" 31 32 #ifndef PATH_MAX 33 # error "compile this file only if your system defines PATH_MAX" 34 #endif 35 36 /* The results of openat() in this file are not leaked to any 37 single-threaded code that could use stdio. 38 FIXME - if the kernel ever adds support for multi-thread safety for 39 avoiding standard fds, then we should use openat_safer. */ 40 41 struct cd_buf 42 { 43 int fd; 44 }; 45 46 static void 47 cdb_init (struct cd_buf *cdb) 48 { 49 cdb->fd = AT_FDCWD; 50 } 51 52 static int 53 cdb_fchdir (struct cd_buf const *cdb) 54 { 55 return fchdir (cdb->fd); 56 } 57 58 static void 59 cdb_free (struct cd_buf const *cdb) 60 { 61 if (0 <= cdb->fd) 62 { 63 bool close_fail = close (cdb->fd); 64 assure (! close_fail); 65 } 66 } 67 68 /* Given a file descriptor of an open directory (or AT_FDCWD), CDB->fd, 69 try to open the CDB->fd-relative directory, DIR. If the open succeeds, 70 update CDB->fd with the resulting descriptor, close the incoming file 71 descriptor, and return zero. Upon failure, return -1 and set errno. */ 72 static int 73 cdb_advance_fd (struct cd_buf *cdb, char const *dir) 74 { 75 int new_fd = openat (cdb->fd, dir, 76 O_SEARCH | O_DIRECTORY | O_NOCTTY | O_NONBLOCK); 77 if (new_fd < 0) 78 return -1; 79 80 cdb_free (cdb); 81 cdb->fd = new_fd; 82 83 return 0; 84 } 85 86 /* Return a pointer to the first non-slash in S. */ 87 static char * _GL_ATTRIBUTE_PURE 88 find_non_slash (char const *s) 89 { 90 size_t n_slash = strspn (s, "/"); 91 return (char *) s + n_slash; 92 } 93 94 /* This is a function much like chdir, but without the PATH_MAX limitation 95 on the length of the directory name. A significant difference is that 96 it must be able to modify (albeit only temporarily) the directory 97 name. It handles an arbitrarily long directory name by operating 98 on manageable portions of the name. On systems without the openat 99 syscall, this means changing the working directory to more and more 100 "distant" points along the long directory name and then restoring 101 the working directory. If any of those attempts to save or restore 102 the working directory fails, this function exits nonzero. 103 104 Note that this function may still fail with errno == ENAMETOOLONG, but 105 only if the specified directory name contains a component that is long 106 enough to provoke such a failure all by itself (e.g. if the component 107 has length PATH_MAX or greater on systems that define PATH_MAX). */ 108 109 int 110 chdir_long (char *dir) 111 { 112 int e = chdir (dir); 113 if (e == 0 || errno != ENAMETOOLONG) 114 return e; 115 116 { 117 size_t len = strlen (dir); 118 char *dir_end = dir + len; 119 struct cd_buf cdb; 120 size_t n_leading_slash; 121 122 cdb_init (&cdb); 123 124 /* If DIR is the empty string, then the chdir above 125 must have failed and set errno to ENOENT. */ 126 assure (0 < len); 127 assure (PATH_MAX <= len); 128 129 /* Count leading slashes. */ 130 n_leading_slash = strspn (dir, "/"); 131 132 /* Handle any leading slashes as well as any name that matches 133 the regular expression, m!^//hostname[/]*! . Handling this 134 prefix separately usually results in a single additional 135 cdb_advance_fd call, but it's worthwhile, since it makes the 136 code in the following loop cleaner. */ 137 if (n_leading_slash == 2) 138 { 139 int err; 140 /* Find next slash. 141 We already know that dir[2] is neither a slash nor '\0'. */ 142 char *slash = memchr (dir + 3, '/', dir_end - (dir + 3)); 143 if (slash == NULL) 144 { 145 errno = ENAMETOOLONG; 146 return -1; 147 } 148 *slash = '\0'; 149 err = cdb_advance_fd (&cdb, dir); 150 *slash = '/'; 151 if (err != 0) 152 goto Fail; 153 dir = find_non_slash (slash + 1); 154 } 155 else if (n_leading_slash) 156 { 157 if (cdb_advance_fd (&cdb, "/") != 0) 158 goto Fail; 159 dir += n_leading_slash; 160 } 161 162 assure (*dir != '/'); 163 assure (dir <= dir_end); 164 165 while (PATH_MAX <= dir_end - dir) 166 { 167 int err; 168 /* Find a slash that is PATH_MAX or fewer bytes away from dir. 169 I.e. see if there is a slash that will give us a name of 170 length PATH_MAX-1 or less. */ 171 char *slash = memrchr (dir, '/', PATH_MAX); 172 if (slash == NULL) 173 { 174 errno = ENAMETOOLONG; 175 return -1; 176 } 177 178 *slash = '\0'; 179 assure (slash - dir < PATH_MAX); 180 err = cdb_advance_fd (&cdb, dir); 181 *slash = '/'; 182 if (err != 0) 183 goto Fail; 184 185 dir = find_non_slash (slash + 1); 186 } 187 188 if (dir < dir_end) 189 { 190 if (cdb_advance_fd (&cdb, dir) != 0) 191 goto Fail; 192 } 193 194 if (cdb_fchdir (&cdb) != 0) 195 goto Fail; 196 197 cdb_free (&cdb); 198 return 0; 199 200 Fail: 201 { 202 int saved_errno = errno; 203 cdb_free (&cdb); 204 errno = saved_errno; 205 return -1; 206 } 207 } 208 } 209 210 #if TEST_CHDIR 211 212 # include "closeout.h" 213 # include "error.h" 214 215 char *program_name; 216 217 int 218 main (int argc, char *argv[]) 219 { 220 char *line = NULL; 221 size_t n = 0; 222 int len; 223 224 program_name = argv[0]; 225 atexit (close_stdout); 226 227 len = getline (&line, &n, stdin); 228 if (len < 0) 229 { 230 int saved_errno = errno; 231 if (feof (stdin)) 232 exit (0); 233 234 error (EXIT_FAILURE, saved_errno, 235 "reading standard input"); 236 } 237 else if (len == 0) 238 exit (0); 239 240 if (line[len-1] == '\n') 241 line[len-1] = '\0'; 242 243 if (chdir_long (line) != 0) 244 error (EXIT_FAILURE, errno, 245 "chdir_long failed: %s", line); 246 247 if (argc <= 1) 248 { 249 /* Using 'pwd' here makes sense only if it is a robust implementation, 250 like the one in coreutils after the 2004-04-19 changes. */ 251 char const *cmd = "pwd"; 252 execlp (cmd, (char *) NULL); 253 error (EXIT_FAILURE, errno, "%s", cmd); 254 } 255 256 fclose (stdin); 257 fclose (stderr); 258 259 exit (EXIT_SUCCESS); 260 } 261 #endif 262 263 /* 264 Local Variables: 265 compile-command: "gcc -DTEST_CHDIR=1 -g -O -W -Wall chdir-long.c libcoreutils.a" 266 End: 267 */ 268