1 /* provide a chdir function that tries not to fail due to ENAMETOOLONG
2 Copyright (C) 2004-2022 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 */