1 /* help detect directory cycles efficiently
2 
3    Copyright (C) 2003-2006, 2009-2021 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
main()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 }
86