1  /* Threshold at which to diagnose ELOOP.  Generic version.
       2     Copyright (C) 2012-2023 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  #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 */