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1 | /* Threshold at which to diagnose ELOOP. Generic version. |
2 | Copyright (C) 2012-2021 Free Software Foundation, Inc. | |
3 | This file is part of the GNU C Library. | |
4 | ||
5 | The GNU C Library is free software; you can redistribute it and/or | |
6 | modify it under the terms of the GNU General Public | |
7 | License as published by the Free Software Foundation; either | |
8 | version 3 of the License, or (at your option) any later version. | |
9 | ||
10 | The GNU C Library is distributed in the hope that it will be useful, | |
11 | but WITHOUT ANY WARRANTY; without even the implied warranty of | |
12 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
13 | General Public License for more details. | |
14 | ||
15 | You should have received a copy of the GNU General Public | |
16 | License along with the GNU C Library; if not, see | |
17 | <https://www.gnu.org/licenses/>. */ | |
18 | ||
19 | #ifndef _ELOOP_THRESHOLD_H | |
20 | #define _ELOOP_THRESHOLD_H 1 | |
21 | ||
22 | #include <limits.h> | |
23 | #ifdef _LIBC | |
24 | # include <sys/param.h> | |
25 | # define _GL_ATTRIBUTE_CONST __attribute__ ((const)) | |
26 | #else | |
27 | # include <unistd.h> | |
28 | # include "minmax.h" | |
29 | # define __sysconf sysconf | |
30 | # if (!defined SYMLOOP_MAX \ | |
31 | && ! (defined _SC_SYMLOOP_MAX && defined _POSIX_SYMLOOP_MAX)) | |
32 | # define SYMLOOP_MAX 8 | |
33 | # endif | |
34 | #endif | |
35 | ||
36 | /* POSIX specifies SYMLOOP_MAX as the "Maximum number of symbolic | |
37 | links that can be reliably traversed in the resolution of a | |
38 | pathname in the absence of a loop." This makes it a minimum that | |
39 | we should certainly accept. But it leaves open the possibility | |
40 | that more might sometimes work--just not "reliably". | |
41 | ||
42 | For example, Linux implements a complex policy whereby there is a | |
43 | small limit on the number of direct symlink traversals (a symlink | |
44 | to a symlink to a symlink), but larger limit on the total number of | |
45 | symlink traversals overall. Hence the SYMLOOP_MAX number should be | |
46 | the small one, but the limit library functions enforce on users | |
47 | should be the larger one. | |
48 | ||
49 | So, we use the larger of the reported SYMLOOP_MAX (if any) and our | |
50 | own constant MIN_ELOOP_THRESHOLD, below. This constant should be | |
51 | large enough that it never rules out a file name and directory tree | |
52 | that the underlying system (i.e. calls to 'open' et al) would | |
53 | resolve successfully. It should be small enough that actual loops | |
54 | are detected without a huge number of iterations. */ | |
55 | ||
56 | #ifndef MIN_ELOOP_THRESHOLD | |
57 | # define MIN_ELOOP_THRESHOLD 40 | |
58 | #endif | |
59 | ||
60 | /* Return the maximum number of symlink traversals to permit | |
61 | before diagnosing ELOOP. */ | |
62 | static inline unsigned int _GL_ATTRIBUTE_CONST | |
63 | __eloop_threshold (void) | |
64 | { | |
65 | #ifdef SYMLOOP_MAX | |
66 | const int symloop_max = SYMLOOP_MAX; | |
67 | #else | |
68 | /* The function is marked 'const' even though we use memory and | |
69 | call a function, because sysconf is required to return the | |
70 | same value in every call and so it must always be safe to | |
71 | call __eloop_threshold exactly once and reuse the value. */ | |
72 | static long int sysconf_symloop_max; | |
73 | if (sysconf_symloop_max == 0) | |
74 | sysconf_symloop_max = __sysconf (_SC_SYMLOOP_MAX); | |
75 | const unsigned int symloop_max = (sysconf_symloop_max <= 0 | |
76 | ? _POSIX_SYMLOOP_MAX | |
77 | : sysconf_symloop_max); | |
78 | #endif | |
79 | ||
80 | return MAX (symloop_max, MIN_ELOOP_THRESHOLD); | |
81 | } | |
82 | ||
83 | #endif /* eloop-threshold.h */ |