1 /* Return the canonical absolute name of a given file inside chroot.
2 Copyright (C) 1996-2023 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 This program is free software; you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published
7 by the Free Software Foundation; version 2 of the License, or
8 (at your option) any later version.
9
10 This program is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
14
15 You should have received a copy of the GNU General Public License
16 along with this program; if not, see <https://www.gnu.org/licenses/>. */
17
18 #include <stdlib.h>
19 #include <string.h>
20 #include <unistd.h>
21 #include <limits.h>
22 #include <sys/stat.h>
23 #include <errno.h>
24 #include <stddef.h>
25 #include <stdint.h>
26
27 #include <eloop-threshold.h>
28 #include <ldconfig.h>
29
30 #ifndef PATH_MAX
31 #define PATH_MAX 1024
32 #endif
33
34 /* Return the canonical absolute name of file NAME as if chroot(CHROOT) was
35 done first. A canonical name does not contain any `.', `..' components
36 nor any repeated path separators ('/') or symlinks. All path components
37 must exist and NAME must be absolute filename. The result is malloc'd.
38 The returned name includes the CHROOT prefix. */
39
40 char *
41 chroot_canon (const char *chroot, const char *name)
42 {
43 char *rpath;
44 char *dest;
45 char *extra_buf = NULL;
46 char *rpath_root;
47 const char *start;
48 const char *end;
49 const char *rpath_limit;
50 int num_links = 0;
51 size_t chroot_len = strlen (chroot);
52
53 if (chroot_len < 1)
54 {
55 __set_errno (EINVAL);
56 return NULL;
57 }
58
59 rpath = xmalloc (chroot_len + PATH_MAX);
60
61 rpath_limit = rpath + chroot_len + PATH_MAX;
62
63 rpath_root = (char *) mempcpy (rpath, chroot, chroot_len) - 1;
64 if (*rpath_root != '/')
65 *++rpath_root = '/';
66 dest = rpath_root + 1;
67
68 for (start = end = name; *start; start = end)
69 {
70 struct stat st;
71
72 /* Skip sequence of multiple path-separators. */
73 while (*start == '/')
74 ++start;
75
76 /* Find end of path component. */
77 for (end = start; *end && *end != '/'; ++end)
78 /* Nothing. */;
79
80 if (end - start == 0)
81 break;
82 else if (end - start == 1 && start[0] == '.')
83 /* nothing */;
84 else if (end - start == 2 && start[0] == '.' && start[1] == '.')
85 {
86 /* Back up to previous component, ignore if at root already. */
87 if (dest > rpath_root + 1)
88 while ((--dest)[-1] != '/');
89 }
90 else
91 {
92 size_t new_size;
93
94 if (dest[-1] != '/')
95 *dest++ = '/';
96
97 if (dest + (end - start) >= rpath_limit)
98 {
99 ptrdiff_t dest_offset = dest - rpath;
100 char *new_rpath;
101
102 new_size = rpath_limit - rpath;
103 if (end - start + 1 > PATH_MAX)
104 new_size += end - start + 1;
105 else
106 new_size += PATH_MAX;
107 new_rpath = (char *) xrealloc (rpath, new_size);
108 rpath = new_rpath;
109 rpath_limit = rpath + new_size;
110
111 dest = rpath + dest_offset;
112 }
113
114 dest = mempcpy (dest, start, end - start);
115 *dest = '\0';
116
117 if (lstat (rpath, &st) < 0)
118 {
119 if (*end == '\0')
120 goto done;
121 goto error;
122 }
123
124 if (S_ISLNK (st.st_mode))
125 {
126 char *buf = alloca (PATH_MAX);
127 size_t len;
128
129 if (++num_links > __eloop_threshold ())
130 {
131 __set_errno (ELOOP);
132 goto error;
133 }
134
135 ssize_t n = readlink (rpath, buf, PATH_MAX - 1);
136 if (n < 0)
137 {
138 if (*end == '\0')
139 goto done;
140 goto error;
141 }
142 buf[n] = '\0';
143
144 if (!extra_buf)
145 extra_buf = alloca (PATH_MAX);
146
147 len = strlen (end);
148 if (len >= PATH_MAX - n)
149 {
150 __set_errno (ENAMETOOLONG);
151 goto error;
152 }
153
154 /* Careful here, end may be a pointer into extra_buf... */
155 memmove (&extra_buf[n], end, len + 1);
156 name = end = memcpy (extra_buf, buf, n);
157
158 if (buf[0] == '/')
159 dest = rpath_root + 1; /* It's an absolute symlink */
160 else
161 /* Back up to previous component, ignore if at root already: */
162 if (dest > rpath_root + 1)
163 while ((--dest)[-1] != '/');
164 }
165 }
166 }
167 done:
168 if (dest > rpath_root + 1 && dest[-1] == '/')
169 --dest;
170 *dest = '\0';
171
172 return rpath;
173
174 error:
175 free (rpath);
176 return NULL;
177 }