dma-buf: Proactively round up to kmalloc bucket size

Instead of discovering the kmalloc bucket size _after_ allocation, round
up proactively so the allocation is explicitly made for the full size,
allowing the compiler to correctly reason about the resulting size of
the buffer through the existing __alloc_size() hint.

Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221018090858.never.941-kees@kernel.org

drivers/dma-buf/dma-resv.c

@@ -98,12 +98,17 @@ static void dma_resv_list_set(struct dma_resv_list *list,
 static struct dma_resv_list *dma_resv_list_alloc(unsigned int max_fences)
 {
 	struct dma_resv_list *list;
+	size_t size;
 
-	list = kmalloc(struct_size(list, table, max_fences), GFP_KERNEL);
+	/* Round up to the next kmalloc bucket size. */
+	size = kmalloc_size_roundup(struct_size(list, table, max_fences));
+
+	list = kmalloc(size, GFP_KERNEL);
 	if (!list)
 		return NULL;
 
-	list->max_fences = (ksize(list) - offsetof(typeof(*list), table)) /
+	/* Given the resulting bucket size, recalculated max_fences. */
+	list->max_fences = (size - offsetof(typeof(*list), table)) /
 		sizeof(*list->table);
 
 	return list;