1  /* help detect directory cycles efficiently
       2  
       3     Copyright (C) 2003-2006, 2009-2022 Free Software Foundation, Inc.
       4  
       5     This program is free software: you can redistribute it and/or modify
       6     it under the terms of the GNU General Public License as published by
       7     the Free Software Foundation, either version 3 of the License, or
       8     (at your option) any later version.
       9  
      10     This program 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
      13     GNU General Public License for more details.
      14  
      15     You should have received a copy of the GNU General Public License
      16     along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
      17  
      18  /* Written by Jim Meyering */
      19  
      20  #include <config.h>
      21  
      22  #include "cycle-check.h"
      23  
      24  #include <sys/types.h>
      25  #include <sys/stat.h>
      26  #include <stdio.h>
      27  #include <stdlib.h>
      28  #include <stdbool.h>
      29  
      30  #include "assure.h"
      31  
      32  #define CC_MAGIC 9827862
      33  
      34  /* Return true if I is a power of 2, or is zero.  */
      35  
      36  static bool
      37  is_zero_or_power_of_two (uintmax_t i)
      38  {
      39    return (i & (i - 1)) == 0;
      40  }
      41  
      42  void
      43  cycle_check_init (struct cycle_check_state *state)
      44  {
      45    state->chdir_counter = 0;
      46    state->magic = CC_MAGIC;
      47  }
      48  
      49  /* In traversing a directory hierarchy, call this function once for each
      50     descending chdir call, with SB corresponding to the chdir operand.
      51     If SB corresponds to a directory that has already been seen,
      52     return true to indicate that there is a directory cycle.
      53     Note that this is done "lazily", which means that some of
      54     the directories in the cycle may be processed twice before
      55     the cycle is detected.  */
      56  
      57  bool
      58  cycle_check (struct cycle_check_state *state, struct stat const *sb)
      59  {
      60    assure (state->magic == CC_MAGIC);
      61  
      62    /* If the current directory ever happens to be the same
      63       as the one we last recorded for the cycle detection,
      64       then it's obviously part of a cycle.  */
      65    if (state->chdir_counter && SAME_INODE (*sb, state->dev_ino))
      66      return true;
      67  
      68    /* If the number of "descending" chdir calls is a power of two,
      69       record the dev/ino of the current directory.  */
      70    if (is_zero_or_power_of_two (++(state->chdir_counter)))
      71      {
      72        /* On all architectures that we know about, if the counter
      73           overflows then there is a directory cycle here somewhere,
      74           even if we haven't detected it yet.  Typically this happens
      75           only after the counter is incremented 2**64 times, so it's a
      76           fairly theoretical point.  */
      77        if (state->chdir_counter == 0)
      78          return true;
      79  
      80        state->dev_ino.st_dev = sb->st_dev;
      81        state->dev_ino.st_ino = sb->st_ino;
      82      }
      83  
      84    return false;
      85  }