1 /*
2 * Copyright (C) 2019 The Android Open Source Project
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * * Redistributions of source code must retain the above copyright
9 * notice, this list of conditions and the following disclaimer.
10 * * Redistributions in binary form must reproduce the above copyright
11 * notice, this list of conditions and the following disclaimer in
12 * the documentation and/or other materials provided with the
13 * distribution.
14 *
15 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
16 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
17 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
18 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
19 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
20 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
21 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
22 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
23 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
24 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
25 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 * SUCH DAMAGE.
27 */
28
29 #include <stdlib.h>
30
31 #include <errno.h>
32 #include <fcntl.h>
33 #include <string.h>
34 #include <sys/stat.h>
35
36 #include "private/FdPath.h"
37 #include "private/ScopedFd.h"
38
39 // This function needs a 4KiB (PATH_MAX) buffer.
40 // The alternative is to heap allocate and then trim, but that's 2x the code.
41 // (Remember that readlink(2) won't tell you the needed size, so the multi-pass
42 // algorithm isn't even an option unless you want to just guess, in which case
43 // you're back needing to trim again.)
44 #pragma GCC diagnostic ignored "-Wframe-larger-than="
45
realpath(const char * path,char * result)46 char* realpath(const char* path, char* result) {
47 // Weird special case.
48 if (!path) {
49 errno = EINVAL;
50 return nullptr;
51 }
52
53 // Get an O_PATH fd, and...
54 ScopedFd fd(open(path, O_PATH | O_CLOEXEC));
55 if (fd.get() == -1) return nullptr;
56
57 // (...remember the device/inode that we're talking about and...)
58 struct stat sb;
59 if (fstat(fd.get(), &sb) == -1) return nullptr;
60 dev_t st_dev = sb.st_dev;
61 ino_t st_ino = sb.st_ino;
62
63 // ...ask the kernel to do the hard work for us.
64 FdPath fd_path(fd.get());
65 char dst[PATH_MAX];
66 ssize_t l = readlink(fd_path.c_str(), dst, sizeof(dst) - 1);
67 if (l == -1) return nullptr;
68 dst[l] = '\0';
69
70 // What if the file was removed in the meantime? readlink(2) will have
71 // returned "/a/b/c (deleted)", and we want to return ENOENT instead.
72 if (stat(dst, &sb) == -1 || st_dev != sb.st_dev || st_ino != sb.st_ino) {
73 errno = ENOENT;
74 return nullptr;
75 }
76
77 return result ? strcpy(result, dst) : strdup(dst);
78 }
79