add initial slab allocation quarantine

README.md

@@ -107,6 +107,8 @@ and will be migrated to the main configuration when proper sanity checks and
 documentation are written. The following advanced options are available:
 
 ```
+#define SLAB_QUARANTINE_RANDOM_SIZE 0
+#define SLAB_QUARANTINE_QUEUE_SIZE 0
 #define GUARD_SLABS_INTERVAL 1
 #define GUARD_SIZE_DIVISOR 2
 #define REGION_QUARANTINE_RANDOM_SIZE 128
@@ -189,7 +191,7 @@ was a bit less important and if a core goal was finding latent bugs.
 * Fine-grained randomization within memory regions
     * Randomly sized guard regions for large allocations
     * Random slot selection within slabs
-    * [in-progress] Randomized delayed free for slab allocations
+    * Randomized delayed free for slab allocations
     * [in-progress] Randomized allocation of slabs
     * [more randomization coming as the implementation is matured]
 * Slab allocations are zeroed on free
@@ -204,8 +206,7 @@ was a bit less important and if a core goal was finding latent bugs.
 * Detection of write-after-free by verifying zero filling is intact
 * Memory in fresh allocations is consistently zeroed due to it either being
   fresh pages or zeroed on free after previous usage
-* [in-progress] Delayed free via a combination of FIFO and randomization for
-  slab allocations
+* Delayed free via a combination of FIFO and randomization for slab allocations
 * Random canaries placed after each slab allocation to *absorb*
   and then later detect overflows/underflows
     * High entropy per-slab random values

config.h

@@ -3,6 +3,8 @@
 
 #include <stdbool.h>
 
+#define SLAB_QUARANTINE_RANDOM_SIZE 0
+#define SLAB_QUARANTINE_QUEUE_SIZE 0
 #define GUARD_SLABS_INTERVAL 1
 #define GUARD_SIZE_DIVISOR 2
 #define REGION_QUARANTINE_RANDOM_SIZE 128

malloc.c

@@ -28,6 +28,8 @@ extern int __register_atfork(void (*)(void), void (*)(void), void (*)(void), voi
 #define atfork pthread_atfork
 #endif
 
+#define SLAB_QUARANTINE (SLAB_QUARANTINE_RANDOM_SIZE > 0 || SLAB_QUARANTINE_QUEUE_SIZE > 0)
+
 static_assert(sizeof(void *) == 8, "64-bit only");
 
 static_assert(!WRITE_AFTER_FREE_CHECK || ZERO_ON_FREE, "WRITE_AFTER_FREE_CHECK depends on ZERO_ON_FREE");
@@ -65,6 +67,9 @@ struct slab_metadata {
 #ifdef SLAB_METADATA_COUNT
     u16 count;
 #endif
+#if SLAB_QUARANTINE
+    u64 quarantine[4];
+#endif
 };
 
 static const size_t min_align = 16;
@@ -172,6 +177,15 @@ struct __attribute__((aligned(CACHELINE_SIZE))) size_class {
     struct libdivide_u32_t size_divisor;
     struct libdivide_u64_t slab_size_divisor;
 
+#if SLAB_QUARANTINE_RANDOM_SIZE > 0
+    void *quarantine_random[SLAB_QUARANTINE_RANDOM_SIZE];
+#endif
+
+#if SLAB_QUARANTINE_QUEUE_SIZE > 0
+    void *quarantine_queue[SLAB_QUARANTINE_QUEUE_SIZE];
+    size_t quarantine_queue_index;
+#endif
+
     // slabs with at least one allocated slot and at least one free slot
     //
     // LIFO doubly-linked list
@@ -264,6 +278,23 @@ static bool get_slot(struct slab_metadata *metadata, size_t index) {
     return (metadata->bitmap[bucket] >> (index - bucket * 64)) & 1UL;
 }
 
+#if SLAB_QUARANTINE
+static void set_quarantine(struct slab_metadata *metadata, size_t index) {
+    size_t bucket = index / 64;
+    metadata->quarantine[bucket] |= 1UL << (index - bucket * 64);
+}
+
+static void clear_quarantine(struct slab_metadata *metadata, size_t index) {
+    size_t bucket = index / 64;
+    metadata->quarantine[bucket] &= ~(1UL << (index - bucket * 64));
+}
+
+static bool get_quarantine(struct slab_metadata *metadata, size_t index) {
+    size_t bucket = index / 64;
+    return (metadata->quarantine[bucket] >> (index - bucket * 64)) & 1UL;
+}
+#endif
+
 static u64 get_mask(size_t slots) {
     return slots < 64 ? ~0UL << slots : 0;
 }
@@ -548,6 +579,46 @@ static inline void deallocate_small(void *p, const size_t *expected_size) {
         }
     }
 
+#if SLAB_QUARANTINE
+    if (get_quarantine(metadata, slot)) {
+        fatal_error("double free (quarantine)");
+    }
+
+    set_quarantine(metadata, slot);
+
+#if SLAB_QUARANTINE_RANDOM_SIZE > 0
+    size_t random_index = get_random_u16_uniform(&c->rng, 16);
+    void *substitute = c->quarantine_random[random_index];
+    c->quarantine_random[random_index] = p;
+
+    if (substitute == NULL) {
+        mutex_unlock(&c->lock);
+        return;
+    }
+
+    p = substitute;
+#endif
+
+#if SLAB_QUARANTINE_QUEUE_SIZE > 0
+    void *substitute = c->quarantine_queue[c->quarantine_queue_index];
+    c->quarantine_queue[c->quarantine_queue_index] = p;
+    c->quarantine_queue_index = (c->quarantine_queue_index + 1) % SLAB_QUARANTINE_QUEUE_SIZE;
+
+    if (substitute == NULL) {
+        mutex_unlock(&c->lock);
+        return;
+    }
+
+    p = substitute;
+#endif
+
+    metadata = get_metadata(c, p);
+    slab = get_slab(c, slab_size, metadata);
+    slot = libdivide_u32_do((char *)p - (char *)slab, &c->size_divisor);
+
+    clear_quarantine(metadata, slot);
+#endif
+
     if (!has_free_slots(slots, metadata)) {
         metadata->next = c->partial_slabs;
         metadata->prev = NULL;