gnulib/import/eloop-threshold.h

   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 */