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 Lesser General Public 7 License as published by the Free Software Foundation; either 8 version 2.1 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 Lesser General Public License for more details. 14 15 You should have received a copy of the GNU Lesser 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 */