1 | /* Read symbolic links into a buffer without size limitation, relative to fd. |
2 | |
3 | Copyright (C) 2001, 2003-2004, 2007, 2009-2019 Free Software Foundation, |
4 | 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 Paul Eggert, Bruno Haible, and Jim Meyering. */ |
20 | |
21 | #include <config.h> |
22 | |
23 | #include "careadlinkat.h" |
24 | |
25 | #include <errno.h> |
26 | #include <limits.h> |
27 | #include <string.h> |
28 | #include <unistd.h> |
29 | |
30 | /* Define this independently so that stdint.h is not a prerequisite. */ |
31 | #ifndef SIZE_MAX |
32 | # define SIZE_MAX ((size_t) -1) |
33 | #endif |
34 | |
35 | #ifndef SSIZE_MAX |
36 | # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2)) |
37 | #endif |
38 | |
39 | #include "allocator.h" |
40 | |
41 | /* Assuming the current directory is FD, get the symbolic link value |
42 | of FILENAME as a null-terminated string and put it into a buffer. |
43 | If FD is AT_FDCWD, FILENAME is interpreted relative to the current |
44 | working directory, as in openat. |
45 | |
46 | If the link is small enough to fit into BUFFER put it there. |
47 | BUFFER's size is BUFFER_SIZE, and BUFFER can be null |
48 | if BUFFER_SIZE is zero. |
49 | |
50 | If the link is not small, put it into a dynamically allocated |
51 | buffer managed by ALLOC. It is the caller's responsibility to free |
52 | the returned value if it is nonnull and is not BUFFER. A null |
53 | ALLOC stands for the standard allocator. |
54 | |
55 | The PREADLINKAT function specifies how to read links. It operates |
56 | like POSIX readlinkat() |
57 | <http://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html> |
58 | but can assume that its first argument is the same as FD. |
59 | |
60 | If successful, return the buffer address; otherwise return NULL and |
61 | set errno. */ |
62 | |
63 | char * |
64 | careadlinkat (int fd, char const *filename, |
65 | char *buffer, size_t buffer_size, |
66 | struct allocator const *alloc, |
67 | ssize_t (*preadlinkat) (int, char const *, char *, size_t)) |
68 | { |
69 | char *buf; |
70 | size_t buf_size; |
71 | size_t buf_size_max = |
72 | SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX; |
73 | char stack_buf[1024]; |
74 | |
75 | if (! alloc) |
76 | alloc = &stdlib_allocator; |
77 | |
78 | if (! buffer_size) |
79 | { |
80 | /* Allocate the initial buffer on the stack. This way, in the |
81 | common case of a symlink of small size, we get away with a |
82 | single small malloc() instead of a big malloc() followed by a |
83 | shrinking realloc(). */ |
84 | buffer = stack_buf; |
85 | buffer_size = sizeof stack_buf; |
86 | } |
87 | |
88 | buf = buffer; |
89 | buf_size = buffer_size; |
90 | |
91 | do |
92 | { |
93 | /* Attempt to read the link into the current buffer. */ |
94 | ssize_t link_length = preadlinkat (fd, filename, buf, buf_size); |
95 | size_t link_size; |
96 | if (link_length < 0) |
97 | { |
98 | /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1 |
99 | with errno == ERANGE if the buffer is too small. */ |
100 | int readlinkat_errno = errno; |
101 | if (readlinkat_errno != ERANGE) |
102 | { |
103 | if (buf != buffer) |
104 | { |
105 | alloc->free (buf); |
106 | errno = readlinkat_errno; |
107 | } |
108 | return NULL; |
109 | } |
110 | } |
111 | |
112 | link_size = link_length; |
113 | |
114 | if (link_size < buf_size) |
115 | { |
116 | buf[link_size++] = '\0'; |
117 | |
118 | if (buf == stack_buf) |
119 | { |
120 | char *b = (char *) alloc->allocate (link_size); |
121 | buf_size = link_size; |
122 | if (! b) |
123 | break; |
124 | memcpy (b, buf, link_size); |
125 | buf = b; |
126 | } |
127 | else if (link_size < buf_size && buf != buffer && alloc->reallocate) |
128 | { |
129 | /* Shrink BUF before returning it. */ |
130 | char *b = (char *) alloc->reallocate (buf, link_size); |
131 | if (b) |
132 | buf = b; |
133 | } |
134 | |
135 | return buf; |
136 | } |
137 | |
138 | if (buf != buffer) |
139 | alloc->free (buf); |
140 | |
141 | if (buf_size <= buf_size_max / 2) |
142 | buf_size *= 2; |
143 | else if (buf_size < buf_size_max) |
144 | buf_size = buf_size_max; |
145 | else if (buf_size_max < SIZE_MAX) |
146 | { |
147 | errno = ENAMETOOLONG; |
148 | return NULL; |
149 | } |
150 | else |
151 | break; |
152 | buf = (char *) alloc->allocate (buf_size); |
153 | } |
154 | while (buf); |
155 | |
156 | if (alloc->die) |
157 | alloc->die (buf_size); |
158 | errno = ENOMEM; |
159 | return NULL; |
160 | } |
161 | |