1 /* provide a chdir function that tries not to fail due to ENAMETOOLONG
2 Copyright (C) 2004-2023 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 <string.h>
27 #include <stdio.h>
28
29 #include "assure.h"
30
31 #ifndef PATH_MAX
32 # error "compile this file only if your system defines PATH_MAX"
33 #endif
34
35 /* The results of openat() in this file are not leaked to any
36 single-threaded code that could use stdio.
37 FIXME - if the kernel ever adds support for multi-thread safety for
38 avoiding standard fds, then we should use openat_safer. */
39
40 struct cd_buf
41 {
42 int fd;
43 };
44
45 static void
46 cdb_init (struct cd_buf *cdb)
47 {
48 cdb->fd = AT_FDCWD;
49 }
50
51 static int
52 cdb_fchdir (struct cd_buf const *cdb)
53 {
54 return fchdir (cdb->fd);
55 }
56
57 static void
58 cdb_free (struct cd_buf const *cdb)
59 {
60 if (0 <= cdb->fd)
61 {
62 bool close_fail = close (cdb->fd);
63 assure (! close_fail);
64 }
65 }
66
67 /* Given a file descriptor of an open directory (or AT_FDCWD), CDB->fd,
68 try to open the CDB->fd-relative directory, DIR. If the open succeeds,
69 update CDB->fd with the resulting descriptor, close the incoming file
70 descriptor, and return zero. Upon failure, return -1 and set errno. */
71 static int
72 cdb_advance_fd (struct cd_buf *cdb, char const *dir)
73 {
74 int new_fd = openat (cdb->fd, dir,
75 O_SEARCH | O_DIRECTORY | O_NOCTTY | O_NONBLOCK);
76 if (new_fd < 0)
77 return -1;
78
79 cdb_free (cdb);
80 cdb->fd = new_fd;
81
82 return 0;
83 }
84
85 /* Return a pointer to the first non-slash in S. */
86 static char * _GL_ATTRIBUTE_PURE
87 find_non_slash (char const *s)
88 {
89 size_t n_slash = strspn (s, "/");
90 return (char *) s + n_slash;
91 }
92
93 /* This is a function much like chdir, but without the PATH_MAX limitation
94 on the length of the directory name. A significant difference is that
95 it must be able to modify (albeit only temporarily) the directory
96 name. It handles an arbitrarily long directory name by operating
97 on manageable portions of the name. On systems without the openat
98 syscall, this means changing the working directory to more and more
99 "distant" points along the long directory name and then restoring
100 the working directory. If any of those attempts to save or restore
101 the working directory fails, this function exits nonzero.
102
103 Note that this function may still fail with errno == ENAMETOOLONG, but
104 only if the specified directory name contains a component that is long
105 enough to provoke such a failure all by itself (e.g. if the component
106 has length PATH_MAX or greater on systems that define PATH_MAX). */
107
108 int
109 chdir_long (char *dir)
110 {
111 int e = chdir (dir);
112 if (e == 0 || errno != ENAMETOOLONG)
113 return e;
114
115 {
116 size_t len = strlen (dir);
117 char *dir_end = dir + len;
118 struct cd_buf cdb;
119 size_t n_leading_slash;
120
121 cdb_init (&cdb);
122
123 /* If DIR is the empty string, then the chdir above
124 must have failed and set errno to ENOENT. */
125 assure (0 < len);
126 assure (PATH_MAX <= len);
127
128 /* Count leading slashes. */
129 n_leading_slash = strspn (dir, "/");
130
131 /* Handle any leading slashes as well as any name that matches
132 the regular expression, m!^//hostname[/]*! . Handling this
133 prefix separately usually results in a single additional
134 cdb_advance_fd call, but it's worthwhile, since it makes the
135 code in the following loop cleaner. */
136 if (n_leading_slash == 2)
137 {
138 int err;
139 /* Find next slash.
140 We already know that dir[2] is neither a slash nor '\0'. */
141 char *slash = memchr (dir + 3, '/', dir_end - (dir + 3));
142 if (slash == NULL)
143 {
144 errno = ENAMETOOLONG;
145 return -1;
146 }
147 *slash = '\0';
148 err = cdb_advance_fd (&cdb, dir);
149 *slash = '/';
150 if (err != 0)
151 goto Fail;
152 dir = find_non_slash (slash + 1);
153 }
154 else if (n_leading_slash)
155 {
156 if (cdb_advance_fd (&cdb, "/") != 0)
157 goto Fail;
158 dir += n_leading_slash;
159 }
160
161 assure (*dir != '/');
162 assure (dir <= dir_end);
163
164 while (PATH_MAX <= dir_end - dir)
165 {
166 int err;
167 /* Find a slash that is PATH_MAX or fewer bytes away from dir.
168 I.e. see if there is a slash that will give us a name of
169 length PATH_MAX-1 or less. */
170 char *slash = memrchr (dir, '/', PATH_MAX);
171 if (slash == NULL)
172 {
173 errno = ENAMETOOLONG;
174 return -1;
175 }
176
177 *slash = '\0';
178 assure (slash - dir < PATH_MAX);
179 err = cdb_advance_fd (&cdb, dir);
180 *slash = '/';
181 if (err != 0)
182 goto Fail;
183
184 dir = find_non_slash (slash + 1);
185 }
186
187 if (dir < dir_end)
188 {
189 if (cdb_advance_fd (&cdb, dir) != 0)
190 goto Fail;
191 }
192
193 if (cdb_fchdir (&cdb) != 0)
194 goto Fail;
195
196 cdb_free (&cdb);
197 return 0;
198
199 Fail:
200 {
201 int saved_errno = errno;
202 cdb_free (&cdb);
203 errno = saved_errno;
204 return -1;
205 }
206 }
207 }
208
209 #if TEST_CHDIR
210
211 # include "closeout.h"
212 # include "error.h"
213
214 int
215 main (int argc, char *argv[])
216 {
217 char *line = NULL;
218 size_t n = 0;
219 int len;
220
221 atexit (close_stdout);
222
223 len = getline (&line, &n, stdin);
224 if (len < 0)
225 {
226 int saved_errno = errno;
227 if (feof (stdin))
228 exit (0);
229
230 error (EXIT_FAILURE, saved_errno,
231 "reading standard input");
232 }
233 else if (len == 0)
234 exit (0);
235
236 if (line[len-1] == '\n')
237 line[len-1] = '\0';
238
239 if (chdir_long (line) != 0)
240 error (EXIT_FAILURE, errno,
241 "chdir_long failed: %s", line);
242
243 if (argc <= 1)
244 {
245 /* Using 'pwd' here makes sense only if it is a robust implementation,
246 like the one in coreutils after the 2004-04-19 changes. */
247 char const *cmd = "pwd";
248 execlp (cmd, (char *) NULL);
249 error (EXIT_FAILURE, errno, "%s", cmd);
250 }
251
252 fclose (stdin);
253 fclose (stderr);
254
255 exit (EXIT_SUCCESS);
256 }
257 #endif
258
259 /*
260 Local Variables:
261 compile-command: "gcc -DTEST_CHDIR=1 -g -O -W -Wall chdir-long.c libcoreutils.a"
262 End:
263 */