| 1 | /* provide a chdir function that tries not to fail due to ENAMETOOLONG |
| 2 | Copyright (C) 2004-2020 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 <https://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 | int |
| 216 | main (int argc, char *argv[]) |
| 217 | { |
| 218 | char *line = NULL; |
| 219 | size_t n = 0; |
| 220 | int len; |
| 221 | |
| 222 | atexit (close_stdout); |
| 223 | |
| 224 | len = getline (&line, &n, stdin); |
| 225 | if (len < 0) |
| 226 | { |
| 227 | int saved_errno = errno; |
| 228 | if (feof (stdin)) |
| 229 | exit (0); |
| 230 | |
| 231 | error (EXIT_FAILURE, saved_errno, |
| 232 | "reading standard input"); |
| 233 | } |
| 234 | else if (len == 0) |
| 235 | exit (0); |
| 236 | |
| 237 | if (line[len-1] == '\n') |
| 238 | line[len-1] = '\0'; |
| 239 | |
| 240 | if (chdir_long (line) != 0) |
| 241 | error (EXIT_FAILURE, errno, |
| 242 | "chdir_long failed: %s", line); |
| 243 | |
| 244 | if (argc <= 1) |
| 245 | { |
| 246 | /* Using 'pwd' here makes sense only if it is a robust implementation, |
| 247 | like the one in coreutils after the 2004-04-19 changes. */ |
| 248 | char const *cmd = "pwd"; |
| 249 | execlp (cmd, (char *) NULL); |
| 250 | error (EXIT_FAILURE, errno, "%s", cmd); |
| 251 | } |
| 252 | |
| 253 | fclose (stdin); |
| 254 | fclose (stderr); |
| 255 | |
| 256 | exit (EXIT_SUCCESS); |
| 257 | } |
| 258 | #endif |
| 259 | |
| 260 | /* |
| 261 | Local Variables: |
| 262 | compile-command: "gcc -DTEST_CHDIR=1 -g -O -W -Wall chdir-long.c libcoreutils.a" |
| 263 | End: |
| 264 | */ |