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Fixed lookahead overflow and removed unbounded lookahead pointers

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As pointed out by davidefer, the lookahead pointer modular arithmetic
does not work around integer overflow when the pointer size is not a
multiple of the block count.

To avoid overflow problems, the easy solution is to stop trying to
work around integer overflows and keep the lookahead offset inside the
block device. To make this work, the ack was modified into a resetable
counter that is decremented every block allocation.

As a plus, quite a bit of the allocation logic ended up simplified.
This commit is contained in:
Christopher Haster
2018-04-10 15:14:27 -05:00
parent 6a89ecba39
commit f935fc0be6
3 changed files with 29 additions and 28 deletions
Download Patch File Download Diff File Expand all files Collapse all files

44
lfs.c
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@@ -270,8 +270,7 @@ int lfs_deorphan(lfs_t *lfs);
static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
lfs_t *lfs = p;
lfs_block_t off = (((lfs_soff_t)(block - lfs->free.begin)
% (lfs_soff_t)(lfs->cfg->block_count))
lfs_block_t off = ((block - lfs->free.off)
+ lfs->cfg->block_count) % lfs->cfg->block_count;
if (off < lfs->free.size) {
@@ -283,20 +282,22 @@ static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
while (true) {
while (lfs->free.off != lfs->free.size) {
lfs_block_t off = lfs->free.off;
lfs->free.off += 1;
while (lfs->free.i != lfs->free.size) {
lfs_block_t off = lfs->free.i;
lfs->free.i += 1;
lfs->free.ack -= 1;
if (!(lfs->free.buffer[off / 32] & (1U << (off % 32)))) {
// found a free block
*block = (lfs->free.begin + off) % lfs->cfg->block_count;
*block = (lfs->free.off + off) % lfs->cfg->block_count;
// eagerly find next off so an alloc ack can
// discredit old lookahead blocks
while (lfs->free.off != lfs->free.size &&
(lfs->free.buffer[lfs->free.off / 32] &
(1U << (lfs->free.off % 32)))) {
lfs->free.off += 1;
while (lfs->free.i != lfs->free.size &&
(lfs->free.buffer[lfs->free.i / 32]
& (1U << (lfs->free.i % 32)))) {
lfs->free.i += 1;
lfs->free.ack -= 1;
}
return 0;
@@ -304,15 +305,16 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
}
// check if we have looked at all blocks since last ack
if (lfs->free.off == lfs->free.ack - lfs->free.begin) {
LFS_WARN("No more free space %d", lfs->free.off + lfs->free.begin);
//if (lfs->free.i == lfs->free.ack - lfs->free.off) {
if (lfs->free.ack == 0) {
LFS_WARN("No more free space %d", lfs->free.i + lfs->free.off);
return LFS_ERR_NOSPC;
}
lfs->free.begin += lfs->free.size;
lfs->free.size = lfs_min(lfs->cfg->lookahead,
lfs->free.ack - lfs->free.begin);
lfs->free.off = 0;
lfs->free.off = (lfs->free.off + lfs->free.size)
% lfs->cfg->block_count;
lfs->free.size = lfs_min(lfs->cfg->lookahead, lfs->free.ack);
lfs->free.i = 0;
// find mask of free blocks from tree
memset(lfs->free.buffer, 0, lfs->cfg->lookahead/8);
@@ -324,7 +326,7 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
}
static void lfs_alloc_ack(lfs_t *lfs) {
lfs->free.ack = lfs->free.begin+lfs->free.off + lfs->cfg->block_count;
lfs->free.ack = lfs->cfg->block_count;
}
@@ -2103,9 +2105,9 @@ int lfs_format(lfs_t *lfs, const struct lfs_config *cfg) {
// create free lookahead
memset(lfs->free.buffer, 0, lfs->cfg->lookahead/8);
lfs->free.begin = 0;
lfs->free.size = lfs_min(lfs->cfg->lookahead, lfs->cfg->block_count);
lfs->free.off = 0;
lfs->free.size = lfs_min(lfs->cfg->lookahead, lfs->cfg->block_count);
lfs->free.i = 0;
lfs_alloc_ack(lfs);
// create superblock dir
@@ -2182,9 +2184,9 @@ int lfs_mount(lfs_t *lfs, const struct lfs_config *cfg) {
}
// setup free lookahead
lfs->free.begin = 0;
lfs->free.size = 0;
lfs->free.off = 0;
lfs->free.size = 0;
lfs->free.i = 0;
lfs_alloc_ack(lfs);
// load superblock