1 /* readlink wrapper to return the link name in malloc'd storage.
2 Unlike xreadlink and xreadlink_with_size, don't ever call exit.
3
4 Copyright (C) 2001, 2003-2007, 2009-2020 Free Software Foundation, Inc.
5
6 This program is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 3 of the License, or
9 (at your option) any later version.
10
11 This program is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
15
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <https://www.gnu.org/licenses/>. */
18
19 /* Written by Jim Meyering <jim@meyering.net> */
20
21 #include <config.h>
22
23 #include "areadlink.h"
24
25 #include <errno.h>
26 #include <limits.h>
27 #include <stdint.h>
28 #include <stdlib.h>
29 #include <string.h>
30 #include <unistd.h>
31
32 #ifndef SSIZE_MAX
33 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
34 #endif
35
36 /* SYMLINK_MAX is used only for an initial memory-allocation sanity
37 check, so it's OK to guess too small on hosts where there is no
38 arbitrary limit to symbolic link length. */
39 #ifndef SYMLINK_MAX
40 # define SYMLINK_MAX 1024
41 #endif
42
43 #define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
44
45 /* Call readlink to get the symbolic link value of FILE.
46 SIZE is a hint as to how long the link is expected to be;
47 typically it is taken from st_size. It need not be correct.
48 Return a pointer to that NUL-terminated string in malloc'd storage.
49 If readlink fails, malloc fails, or if the link value is longer
50 than SSIZE_MAX, return NULL (caller may use errno to diagnose). */
51
52 char *
areadlink_with_size(char const * file,size_t size)53 areadlink_with_size (char const *file, size_t size)
54 {
55 /* Some buggy file systems report garbage in st_size. Defend
56 against them by ignoring outlandish st_size values in the initial
57 memory allocation. */
58 size_t symlink_max = SYMLINK_MAX;
59 size_t INITIAL_LIMIT_BOUND = 8 * 1024;
60 size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND
61 ? symlink_max + 1
62 : INITIAL_LIMIT_BOUND);
63
64 enum { stackbuf_size = 128 };
65
66 /* The initial buffer size for the link value. */
67 size_t buf_size = (size == 0 ? stackbuf_size
68 : size < initial_limit ? size + 1 : initial_limit);
69
70 while (1)
71 {
72 ssize_t r;
73 size_t link_length;
74 char stackbuf[stackbuf_size];
75 char *buf = stackbuf;
76 char *buffer = NULL;
77
78 if (! (size == 0 && buf_size == stackbuf_size))
79 {
80 buf = buffer = malloc (buf_size);
81 if (!buffer)
82 {
83 errno = ENOMEM;
84 return NULL;
85 }
86 }
87
88 r = readlink (file, buf, buf_size);
89 link_length = r;
90
91 /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
92 with errno == ERANGE if the buffer is too small. */
93 if (r < 0 && errno != ERANGE)
94 {
95 int saved_errno = errno;
96 free (buffer);
97 errno = saved_errno;
98 return NULL;
99 }
100
101 if (link_length < buf_size)
102 {
103 buf[link_length] = 0;
104 if (!buffer)
105 {
106 buffer = malloc (link_length + 1);
107 if (buffer)
108 return memcpy (buffer, buf, link_length + 1);
109 }
110 else if (link_length + 1 < buf_size)
111 {
112 /* Shrink BUFFER before returning it. */
113 char *shrinked_buffer = realloc (buffer, link_length + 1);
114 if (shrinked_buffer != NULL)
115 buffer = shrinked_buffer;
116 }
117 return buffer;
118 }
119
120 free (buffer);
121 if (buf_size <= MAXSIZE / 2)
122 buf_size *= 2;
123 else if (buf_size < MAXSIZE)
124 buf_size = MAXSIZE;
125 else
126 {
127 errno = ENOMEM;
128 return NULL;
129 }
130 }
131 }
132