MINOR: debug/pools: make DEBUG_UAF also detect underflows

Since we use padding before the allocated page, it's trivial to place
the allocated address there and see if it gets mangled once we release
it.

This may be backported to stable releases already using DEBUG_UAF.

include/common/memory.h

@@ -298,7 +298,9 @@ static inline void pool_free_area(void *area, size_t __maybe_unused size)
  * to those of malloc(). However the allocation is rounded up to 4kB so that a
  * full page is allocated. This ensures the object can be freed alone so that
  * future dereferences are easily detected. The returned object is always
- * 16-bytes aligned to avoid issues with unaligned structure objects.
+ * 16-bytes aligned to avoid issues with unaligned structure objects. In case
+ * some padding is added, the area's start address is copied at the end of the
+ * padding to help detect underflows.
  */
 static inline void *pool_alloc_area(size_t size)
 {
@@ -306,17 +308,26 @@ static inline void *pool_alloc_area(size_t size)
 	void *ret;
 
 	ret = mmap(NULL, (size + 4095) & -4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, 0, 0);
-	return ret == MAP_FAILED ? NULL : ret + pad;
+	if (ret == MAP_FAILED)
+		return NULL;
+	if (pad >= sizeof(void *))
+		*(void **)(ret + pad - sizeof(void *)) = ret + pad;
+	return ret + pad;
 }
 
 /* frees an area <area> of size <size> allocated by pool_alloc_area(). The
  * semantics are identical to free() except that the size must absolutely match
- * the one passed to pool_alloc_area().
+ * the one passed to pool_alloc_area(). In case some padding is added, the
+ * area's start address is compared to the one at the end of the padding, and
+ * a segfault is triggered if they don't match, indicating an underflow.
  */
 static inline void pool_free_area(void *area, size_t size)
 {
 	size_t pad = (4096 - size) & 0xFF0;
 
+	if (pad >= sizeof(void *) && *(void **)(area - sizeof(void *)) != area)
+		*(volatile int *)0 = 0;
+
 	munmap(area - pad, (size + 4095) & -4096);
 }