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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