1  /* Threshold at which to diagnose ELOOP.  Generic version.
       2     Copyright (C) 2012-2022 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 */