1 /* help detect directory cycles efficiently
2
3 Copyright (C) 2003, 2004, 2005 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 2, or (at your option)
8 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; see the file COPYING.
17 If not, write to the Free Software Foundation,
18 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
19 #include <sys/cdefs.h>
20 __RCSID("$NetBSD: cycle-check.c,v 1.2 2016/05/17 14:00:09 christos Exp $");
21
22
23 /* Written by Jim Meyering */
24
25 #ifdef HAVE_CONFIG_H
26 # include <config.h>
27 #endif
28
29 #include <sys/types.h>
30 #include <sys/stat.h>
31 #include <stdio.h>
32 #include <assert.h>
33 #include <stdlib.h>
34
35 #include <stdbool.h>
36
37 #include "cycle-check.h"
38
39 #define SAME_INODE(Stat_buf_1, Stat_buf_2) \
40 ((Stat_buf_1).st_ino == (Stat_buf_2).st_ino \
41 && (Stat_buf_1).st_dev == (Stat_buf_2).st_dev)
42
43 #define CC_MAGIC 9827862
44
45 /* Return true if I is a power of 2, or is zero. */
46
47 static inline bool
is_zero_or_power_of_two(uintmax_t i)48 is_zero_or_power_of_two (uintmax_t i)
49 {
50 return (i & (i - 1)) == 0;
51 }
52
53 void
cycle_check_init(struct cycle_check_state * state)54 cycle_check_init (struct cycle_check_state *state)
55 {
56 state->chdir_counter = 0;
57 state->magic = CC_MAGIC;
58 }
59
60 /* In traversing a directory hierarchy, call this function once for each
61 descending chdir call, with SB corresponding to the chdir operand.
62 If SB corresponds to a directory that has already been seen,
63 return true to indicate that there is a directory cycle.
64 Note that this is done `lazily', which means that some of
65 the directories in the cycle may be processed twice before
66 the cycle is detected. */
67
68 bool
cycle_check(struct cycle_check_state * state,struct stat const * sb)69 cycle_check (struct cycle_check_state *state, struct stat const *sb)
70 {
71 assert (state->magic == CC_MAGIC);
72
73 /* If the current directory ever happens to be the same
74 as the one we last recorded for the cycle detection,
75 then it's obviously part of a cycle. */
76 if (state->chdir_counter && SAME_INODE (*sb, state->dev_ino))
77 return true;
78
79 /* If the number of `descending' chdir calls is a power of two,
80 record the dev/ino of the current directory. */
81 if (is_zero_or_power_of_two (++(state->chdir_counter)))
82 {
83 /* On all architectures that we know about, if the counter
84 overflows then there is a directory cycle here somewhere,
85 even if we haven't detected it yet. Typically this happens
86 only after the counter is incremented 2**64 times, so it's a
87 fairly theoretical point. */
88 if (state->chdir_counter == 0)
89 return true;
90
91 state->dev_ino.st_dev = sb->st_dev;
92 state->dev_ino.st_ino = sb->st_ino;
93 }
94
95 return false;
96 }
97