lib/eloop-threshold.h

   1 /* Threshold at which to diagnose ELOOP.  Generic version.
   2    Copyright (C) 2012-2024 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 /* This file uses _GL_ATTRIBUTE_CONST.  */
  23 #if !_LIBC && !_GL_CONFIG_H_INCLUDED
  24  #error "Please include config.h first."
  25 #endif
  26
  27 #include <limits.h>
  28 #ifdef _LIBC
  29 # include <sys/param.h>
  30 # define _GL_ATTRIBUTE_CONST __attribute__ ((const))
  31 #else
  32 # include <unistd.h>
  33 # include "minmax.h"
  34 # define __sysconf sysconf
  35 # if (!defined SYMLOOP_MAX \
  36       && ! (defined _SC_SYMLOOP_MAX && defined _POSIX_SYMLOOP_MAX))
  37 #  define SYMLOOP_MAX 8
  38 # endif
  39 #endif
  40
  41 /* POSIX specifies SYMLOOP_MAX as the "Maximum number of symbolic
  42    links that can be reliably traversed in the resolution of a
  43    pathname in the absence of a loop."  This makes it a minimum that
  44    we should certainly accept.  But it leaves open the possibility
  45    that more might sometimes work--just not "reliably".
  46
  47    For example, Linux implements a complex policy whereby there is a
  48    small limit on the number of direct symlink traversals (a symlink
  49    to a symlink to a symlink), but larger limit on the total number of
  50    symlink traversals overall.  Hence the SYMLOOP_MAX number should be
  51    the small one, but the limit library functions enforce on users
  52    should be the larger one.
  53
  54    So, we use the larger of the reported SYMLOOP_MAX (if any) and our
  55    own constant MIN_ELOOP_THRESHOLD, below.  This constant should be
  56    large enough that it never rules out a file name and directory tree
  57    that the underlying system (i.e. calls to 'open' et al) would
  58    resolve successfully.  It should be small enough that actual loops
  59    are detected without a huge number of iterations.  */
  60
  61 #ifndef MIN_ELOOP_THRESHOLD
  62 # define MIN_ELOOP_THRESHOLD    40
  63 #endif
  64
  65 /* Return the maximum number of symlink traversals to permit
  66    before diagnosing ELOOP.  */
  67 static inline unsigned int _GL_ATTRIBUTE_CONST
  68 __eloop_threshold (void)
  69 {
  70 #ifdef SYMLOOP_MAX
  71   const int symloop_max = SYMLOOP_MAX;
  72 #else
  73   /* The function is marked 'const' even though we use memory and
  74      call a function, because sysconf is required to return the
  75      same value in every call and so it must always be safe to
  76      call __eloop_threshold exactly once and reuse the value.  */
  77   static long int sysconf_symloop_max;
  78   if (sysconf_symloop_max == 0)
  79     sysconf_symloop_max = __sysconf (_SC_SYMLOOP_MAX);
  80   const unsigned int symloop_max = (sysconf_symloop_max <= 0
  81                                     ? _POSIX_SYMLOOP_MAX
  82                                     : sysconf_symloop_max);
  83 #endif
  84
  85   return MAX (symloop_max, MIN_ELOOP_THRESHOLD);
  86 }
  87
  88 #endif  /* eloop-threshold.h */