fix guarded sample rate of 1 (issue #1085)

Author: daanx

include/mimalloc/types.h

@@ -512,7 +512,6 @@ struct mi_heap_s {
   size_t                guarded_size_min;                    // minimal size for guarded objects
   size_t                guarded_size_max;                    // maximal size for guarded objects
   size_t                guarded_sample_rate;                 // sample rate (set to 0 to disable guarded pages)
-  size_t                guarded_sample_seed;                 // starting sample count
   size_t                guarded_sample_count;                // current sample count (counting down to 0)
   #endif
   mi_page_t*            pages_free_direct[MI_PAGES_DIRECT];  // optimize: array where every entry points a page with possibly free blocks in the corresponding queue for that size.

src/init.c

@@ -110,7 +110,7 @@ mi_decl_cache_align const mi_heap_t _mi_heap_empty = {
   false,            // can reclaim
   0,                // tag
   #if MI_GUARDED
-  0, 0, 0, 0, 1,    // count is 1 so we never write to it (see `internal.h:mi_heap_malloc_use_guarded`)
+  0, 0, 0, 1,       // count is 1 so we never write to it (see `internal.h:mi_heap_malloc_use_guarded`)
   #endif
   MI_SMALL_PAGES_EMPTY,
   MI_PAGE_QUEUES_EMPTY
@@ -153,7 +153,7 @@ mi_decl_cache_align mi_heap_t _mi_heap_main = {
   false,            // can reclaim
   0,                // tag
   #if MI_GUARDED
-  0, 0, 0, 0, 0,
+  0, 0, 0, 0,
   #endif
   MI_SMALL_PAGES_EMPTY,
   MI_PAGE_QUEUES_EMPTY
@@ -165,15 +165,14 @@ mi_stats_t _mi_stats_main = { MI_STAT_VERSION, MI_STATS_NULL };
 
 #if MI_GUARDED
 mi_decl_export void mi_heap_guarded_set_sample_rate(mi_heap_t* heap, size_t sample_rate, size_t seed) {
-  heap->guarded_sample_seed = seed;
-  if (heap->guarded_sample_seed == 0) {
-    heap->guarded_sample_seed = _mi_heap_random_next(heap);
-  }
   heap->guarded_sample_rate  = sample_rate;
-  if (heap->guarded_sample_rate >= 1) {
-    heap->guarded_sample_seed = heap->guarded_sample_seed % heap->guarded_sample_rate;
+  heap->guarded_sample_count = sample_rate;  // count down samples
+  if (heap->guarded_sample_rate > 1) {
+    if (seed == 0) {
+      seed = _mi_heap_random_next(heap);
+    }
+    heap->guarded_sample_count = (seed % heap->guarded_sample_rate) + 1;  // start at random count between 1 and `sample_rate`
   }
-  heap->guarded_sample_count = heap->guarded_sample_seed;  // count down samples
 }
 
 mi_decl_export void mi_heap_guarded_set_size_bound(mi_heap_t* heap, size_t min, size_t max) {

test/main-override-static.c

@@ -43,7 +43,7 @@ int main() {
   // corrupt_free();
   // block_overflow1();
   // block_overflow2();
-  // test_canary_leak();
+  test_canary_leak();
   // test_aslr();
   // invalid_free();
   // test_reserved();