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uid-range: fix out-of-bounds write in uid_range_partition()

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uid_range_partition() filled the grown entries[] buffer backwards in
place. The backward-fill invariant (the write cursor stays above the
read index) only holds when every source entry contributes at least
one partition; an entry with nr < size contributes zero, so the cursor
stalls while the read index keeps descending. A later multi-part
entry's writes then overwrite the still-live zero-part slot, the
corrupted slot is re-read as a one-part entry, and the next
range->entries[--t] underflows.

Add a forward compaction first pass that drops the zero-part entries
before the backward fill.

Follow-up for 025439faaa

Co-Authored-by: Paul Meyer <katexochen0@gmail.com>
This commit is contained in:
Luca Boccassi
2026-06-24 13:56:37 +01:00
parent f66144cf2a
commit d30e87ea49
2 changed files with 73 additions and 2 deletions
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13
src/basic/uid-range.c
View File

@@ -395,9 +395,18 @@ int uid_range_partition(UIDRange *range, uid_t size) {
if (n_new_entries > range->n_entries && !GREEDY_REALLOC(range->entries, n_new_entries))
return -ENOMEM;
/* Work backwards to avoid overwriting entries we still need to read */
/* Compact in place: drop entries that contribute zero partitions (nr < size). This forward pass
* reads each entry once and only writes to lower-or-equal indices, so it cannot alias an unread
* source entry. */
size_t n_src = 0;
for (size_t i = 0; i < range->n_entries; i++)
if (range->entries[i].nr >= size)
range->entries[n_src++] = range->entries[i];
/* Pre-compaction guarantees every surviving entry contributes at least one partition slot, so the
* write cursor t stays ahead of the read index. */
size_t t = n_new_entries;
for (size_t i = range->n_entries; i > 0; i--) {
for (size_t i = n_src; i > 0; i--) {
UIDRangeEntry *e = range->entries + i - 1;
unsigned n_parts = e->nr / size;

62
src/test/test-uid-range.c
View File

@@ -318,6 +318,68 @@ TEST(uid_range_partition) {
p = uid_range_free(p);
/* Small entry preceding a large entry: the small entry must be dropped and the large entry
* partitioned without the in-place backward-fill write cursor aliasing the still-live small entry
* slot. */
ASSERT_OK(uid_range_add_str(&p, "0-4"));
ASSERT_OK(uid_range_add_str(&p, "100-129"));
ASSERT_EQ(uid_range_entries(p), 2U);
ASSERT_OK(uid_range_partition(p, 10));
ASSERT_EQ(uid_range_entries(p), 3U);
ASSERT_EQ(p->entries[0].start, 100U);
ASSERT_EQ(p->entries[0].nr, 10U);
ASSERT_EQ(p->entries[1].start, 110U);
ASSERT_EQ(p->entries[1].nr, 10U);
ASSERT_EQ(p->entries[2].start, 120U);
ASSERT_EQ(p->entries[2].nr, 10U);
p = uid_range_free(p);
/* A too-small entry between two partitionable entries is dropped; the others still partition. */
ASSERT_OK(uid_range_add_str(&p, "0-4")); /* nr=5 < size -> dropped */
ASSERT_OK(uid_range_add_str(&p, "50-69")); /* nr=20 -> 2 parts */
ASSERT_OK(uid_range_add_str(&p, "200-204")); /* nr=5 < size -> dropped */
ASSERT_OK(uid_range_add_str(&p, "1000-1029")); /* nr=30 -> 3 parts */
ASSERT_EQ(uid_range_entries(p), 4U);
ASSERT_OK(uid_range_partition(p, 10));
ASSERT_EQ(uid_range_entries(p), 5U);
ASSERT_EQ(p->entries[0].start, 50U);
ASSERT_EQ(p->entries[0].nr, 10U);
ASSERT_EQ(p->entries[1].start, 60U);
ASSERT_EQ(p->entries[1].nr, 10U);
ASSERT_EQ(p->entries[2].start, 1000U);
ASSERT_EQ(p->entries[2].nr, 10U);
ASSERT_EQ(p->entries[3].start, 1010U);
ASSERT_EQ(p->entries[3].nr, 10U);
ASSERT_EQ(p->entries[4].start, 1020U);
ASSERT_EQ(p->entries[4].nr, 10U);
p = uid_range_free(p);
/* A too-small entry before a partitionable entry is dropped. */
ASSERT_OK(uid_range_add_str(&p, "0-4")); /* nr=5 < size -> dropped */
ASSERT_OK(uid_range_add_str(&p, "100-119")); /* nr=20 -> 2 parts */
ASSERT_OK(uid_range_partition(p, 10));
ASSERT_EQ(uid_range_entries(p), 2U);
ASSERT_EQ(p->entries[0].start, 100U);
ASSERT_EQ(p->entries[0].nr, 10U);
ASSERT_EQ(p->entries[1].start, 110U);
ASSERT_EQ(p->entries[1].nr, 10U);
p = uid_range_free(p);
/* A too-small entry after a partitionable entry is dropped. */
ASSERT_OK(uid_range_add_str(&p, "0-199")); /* nr=200 -> 2 parts */
ASSERT_OK(uid_range_add_str(&p, "1000-1004")); /* nr=5 < size -> dropped */
ASSERT_OK(uid_range_partition(p, 100));
ASSERT_EQ(uid_range_entries(p), 2U);
ASSERT_EQ(p->entries[0].start, 0U);
ASSERT_EQ(p->entries[0].nr, 100U);
ASSERT_EQ(p->entries[1].start, 100U);
ASSERT_EQ(p->entries[1].nr, 100U);
p = uid_range_free(p);
/* Partition size of 1 */
ASSERT_OK(uid_range_add_str(&p, "100-102"));
ASSERT_OK(uid_range_partition(p, 1));