sysdeps/generic/eloop-threshold.h

   1 /* Threshold at which to diagnose ELOOP.  Generic version.
   2    Copyright (C) 2012-2017 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    <http://www.gnu.org/licenses/>.  */
  18
  19 #ifndef _ELOOP_THRESHOLD_H
  20 #define _ELOOP_THRESHOLD_H      1
  21
  22 #include <limits.h>
  23 #include <sys/param.h>
  24
  25 /* POSIX specifies SYMLOOP_MAX as the "Maximum number of symbolic
  26    links that can be reliably traversed in the resolution of a
  27    pathname in the absence of a loop."  This makes it a minimum that
  28    we should certainly accept.  But it leaves open the possibility
  29    that more might sometimes work--just not "reliably".
  30
  31    For example, Linux implements a complex policy whereby there is a
  32    small limit on the number of direct symlink traversals (a symlink
  33    to a symlink to a symlink), but larger limit on the total number of
  34    symlink traversals overall.  Hence the SYMLOOP_MAX number should be
  35    the small one, but the limit library functions enforce on users
  36    should be the larger one.
  37
  38    So, we use the larger of the reported SYMLOOP_MAX (if any) and our
  39    own constant MIN_ELOOP_THRESHOLD, below.  This constant should be
  40    large enough that it never rules out a file name and directory tree
  41    that the underlying system (i.e. calls to 'open' et al) would
  42    resolve successfully.  It should be small enough that actual loops
  43    are detected without a huge number of iterations.  */
  44
  45 #ifndef MIN_ELOOP_THRESHOLD
  46 # define MIN_ELOOP_THRESHOLD    40
  47 #endif
  48
  49 /* Return the maximum number of symlink traversals to permit
  50    before diagnosing ELOOP.  */
  51 static inline unsigned int __attribute__ ((const))
  52 __eloop_threshold (void)
  53 {
  54 #ifdef SYMLOOP_MAX
  55   const int symloop_max = SYMLOOP_MAX;
  56 #else
  57   /* The function is marked 'const' even though we use memory and
  58      call a function, because sysconf is required to return the
  59      same value in every call and so it must always be safe to
  60      call __eloop_threshold exactly once and reuse the value.  */
  61   static long int sysconf_symloop_max;
  62   if (sysconf_symloop_max == 0)
  63     sysconf_symloop_max = __sysconf (_SC_SYMLOOP_MAX);
  64   const unsigned int symloop_max = (sysconf_symloop_max <= 0
  65                                     ? _POSIX_SYMLOOP_MAX
  66                                     : sysconf_symloop_max);
  67 #endif
  68
  69   return MAX (symloop_max, MIN_ELOOP_THRESHOLD);
  70 }
  71
  72 #endif  /* eloop-threshold.h */