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 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 }