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Adopt Brent's algorithm for cycle detection #809

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Apr 26, 2023
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Adopt Brent's algorithm for cycle detection #809

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74 changes: 54 additions & 20 deletions lfs.c
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Original file line number Diff line number Diff line change
Expand Up @@ -279,14 +279,12 @@ static inline int lfs_pair_cmp(
paira[0] == pairb[1] || paira[1] == pairb[0]);
}

#ifndef LFS_READONLY
static inline bool lfs_pair_sync(
static inline bool lfs_pair_issync(
const lfs_block_t paira[2],
const lfs_block_t pairb[2]) {
return (paira[0] == pairb[0] && paira[1] == pairb[1]) ||
(paira[0] == pairb[1] && paira[1] == pairb[0]);
}
#endif

static inline void lfs_pair_fromle32(lfs_block_t pair[2]) {
pair[0] = lfs_fromle32(pair[0]);
Expand Down Expand Up @@ -4110,14 +4108,23 @@ static int lfs_rawmount(lfs_t *lfs, const struct lfs_config *cfg) {

// scan directory blocks for superblock and any global updates
lfs_mdir_t dir = {.tail = {0, 1}};
lfs_block_t cycle = 0;
lfs_block_t tortoise[2] = {LFS_BLOCK_NULL, LFS_BLOCK_NULL};
lfs_size_t tortoise_i = 1;
lfs_size_t tortoise_period = 1;
while (!lfs_pair_isnull(dir.tail)) {
if (cycle >= lfs->cfg->block_count/2) {
// loop detected
// detect cycles with Brent's algorithm
if (lfs_pair_issync(dir.tail, tortoise)) {
LFS_WARN("Cycle detected in tail list");
err = LFS_ERR_CORRUPT;
goto cleanup;
}
cycle += 1;
if (tortoise_i == tortoise_period) {
tortoise[0] = dir.tail[0];
tortoise[1] = dir.tail[1];
tortoise_i = 0;
tortoise_period *= 2;
}
tortoise_i += 1;

// fetch next block in tail list
lfs_stag_t tag = lfs_dir_fetchmatch(lfs, &dir, dir.tail,
Expand Down Expand Up @@ -4268,13 +4275,22 @@ int lfs_fs_rawtraverse(lfs_t *lfs,
}
#endif

lfs_block_t cycle = 0;
lfs_block_t tortoise[2] = {LFS_BLOCK_NULL, LFS_BLOCK_NULL};
lfs_size_t tortoise_i = 1;
lfs_size_t tortoise_period = 1;
while (!lfs_pair_isnull(dir.tail)) {
if (cycle >= lfs->cfg->block_count/2) {
// loop detected
// detect cycles with Brent's algorithm
if (lfs_pair_issync(dir.tail, tortoise)) {
LFS_WARN("Cycle detected in tail list");
return LFS_ERR_CORRUPT;
}
cycle += 1;
if (tortoise_i == tortoise_period) {
tortoise[0] = dir.tail[0];
tortoise[1] = dir.tail[1];
tortoise_i = 0;
tortoise_period *= 2;
}
tortoise_i += 1;

for (int i = 0; i < 2; i++) {
int err = cb(data, dir.tail[i]);
Expand Down Expand Up @@ -4353,13 +4369,22 @@ static int lfs_fs_pred(lfs_t *lfs,
// iterate over all directory directory entries
pdir->tail[0] = 0;
pdir->tail[1] = 1;
lfs_block_t cycle = 0;
lfs_block_t tortoise[2] = {LFS_BLOCK_NULL, LFS_BLOCK_NULL};
lfs_size_t tortoise_i = 1;
lfs_size_t tortoise_period = 1;
while (!lfs_pair_isnull(pdir->tail)) {
if (cycle >= lfs->cfg->block_count/2) {
// loop detected
// detect cycles with Brent's algorithm
if (lfs_pair_issync(pdir->tail, tortoise)) {
LFS_WARN("Cycle detected in tail list");
return LFS_ERR_CORRUPT;
}
cycle += 1;
if (tortoise_i == tortoise_period) {
tortoise[0] = pdir->tail[0];
tortoise[1] = pdir->tail[1];
tortoise_i = 0;
tortoise_period *= 2;
}
tortoise_i += 1;

if (lfs_pair_cmp(pdir->tail, pair) == 0) {
return 0;
Expand Down Expand Up @@ -4409,13 +4434,22 @@ static lfs_stag_t lfs_fs_parent(lfs_t *lfs, const lfs_block_t pair[2],
// use fetchmatch with callback to find pairs
parent->tail[0] = 0;
parent->tail[1] = 1;
lfs_block_t cycle = 0;
lfs_block_t tortoise[2] = {LFS_BLOCK_NULL, LFS_BLOCK_NULL};
lfs_size_t tortoise_i = 1;
lfs_size_t tortoise_period = 1;
while (!lfs_pair_isnull(parent->tail)) {
if (cycle >= lfs->cfg->block_count/2) {
// loop detected
// detect cycles with Brent's algorithm
if (lfs_pair_issync(parent->tail, tortoise)) {
LFS_WARN("Cycle detected in tail list");
return LFS_ERR_CORRUPT;
}
cycle += 1;
if (tortoise_i == tortoise_period) {
tortoise[0] = parent->tail[0];
tortoise[1] = parent->tail[1];
tortoise_i = 0;
tortoise_period *= 2;
}
tortoise_i += 1;

lfs_stag_t tag = lfs_dir_fetchmatch(lfs, parent, parent->tail,
LFS_MKTAG(0x7ff, 0, 0x3ff),
Expand Down Expand Up @@ -4532,7 +4566,7 @@ static int lfs_fs_deorphan(lfs_t *lfs, bool powerloss) {
}
lfs_pair_fromle32(pair);

if (!lfs_pair_sync(pair, pdir.tail)) {
if (!lfs_pair_issync(pair, pdir.tail)) {
// we have desynced
LFS_DEBUG("Fixing half-orphan "
"{0x%"PRIx32", 0x%"PRIx32"} "
Expand Down