1 /* Read a symlink relative to an open directory.
2 Copyright (C) 2009-2021 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 Eric Blake */
18
19 #include <config.h>
20
21 /* Specification. */
22 #include <unistd.h>
23
24 #include <errno.h>
25 #include <stdlib.h>
26 #include <string.h>
27 #include <sys/stat.h>
28
29 #if HAVE_READLINKAT
30
31 # undef fstatat
32 # undef readlinkat
33
34 ssize_t
rpl_readlinkat(int fd,char const * file,char * buf,size_t bufsize)35 rpl_readlinkat (int fd, char const *file, char *buf, size_t bufsize)
36 {
37 # if READLINK_TRAILING_SLASH_BUG
38 size_t file_len = strlen (file);
39 if (file_len && file[file_len - 1] == '/')
40 {
41 /* Even if FILE without the slash is a symlink to a directory,
42 both lstat() and stat() must resolve the trailing slash to
43 the directory rather than the symlink. We can therefore
44 safely use fstatat(..., 0) to distinguish between EINVAL and
45 ENOTDIR/ENOENT, avoiding extra overhead of rpl_fstatat(). */
46 struct stat st;
47 if (fstatat (fd, file, &st, 0) == 0 || errno == EOVERFLOW)
48 errno = EINVAL;
49 return -1;
50 }
51 # endif /* READLINK_TRAILING_SLASH_BUG */
52
53 ssize_t r = readlinkat (fd, file, buf, bufsize);
54
55 # if READLINK_TRUNCATE_BUG
56 if (r < 0 && errno == ERANGE)
57 {
58 /* Try again with a bigger buffer. This is just for test cases;
59 real code invariably discards short reads. */
60 char stackbuf[4032];
61 r = readlinkat (fd, file, stackbuf, sizeof stackbuf);
62 if (r < 0)
63 {
64 if (errno == ERANGE)
65 {
66 /* Clear the buffer, which is good enough for real code.
67 Thankfully, no test cases try short reads of enormous
68 symlinks and what would be the point anyway? */
69 r = bufsize;
70 memset (buf, 0, r);
71 }
72 }
73 else
74 {
75 if (bufsize < r)
76 r = bufsize;
77 memcpy (buf, stackbuf, r);
78 }
79 }
80 # endif
81
82 return r;
83 }
84
85 #else
86
87 /* Gnulib provides a readlink stub for mingw; use it for distinction
88 between EINVAL and ENOENT, rather than always failing with ENOSYS. */
89
90 /* POSIX 2008 says that unlike readlink, readlinkat returns 0 for
91 success instead of the buffer length. But this would render
92 readlinkat worthless since readlink does not guarantee a
93 NUL-terminated buffer. Assume this was a bug in POSIX. */
94
95 /* Read the contents of symlink FILE into buffer BUF of size BUFSIZE, in the
96 directory open on descriptor FD. If possible, do it without changing
97 the working directory. Otherwise, resort to using save_cwd/fchdir,
98 then readlink/restore_cwd. If either the save_cwd or the restore_cwd
99 fails, then give a diagnostic and exit nonzero. */
100
101 # define AT_FUNC_NAME readlinkat
102 # define AT_FUNC_F1 readlink
103 # define AT_FUNC_POST_FILE_PARAM_DECLS , char *buf, size_t bufsize
104 # define AT_FUNC_POST_FILE_ARGS , buf, bufsize
105 # define AT_FUNC_RESULT ssize_t
106 # include "at-func.c"
107 # undef AT_FUNC_NAME
108 # undef AT_FUNC_F1
109 # undef AT_FUNC_POST_FILE_PARAM_DECLS
110 # undef AT_FUNC_POST_FILE_ARGS
111 # undef AT_FUNC_RESULT
112
113 #endif
114