dma-buf/fence-array: Add flex array to struct dma_fence_array

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1][2].

The "struct dma_fence_array" can be refactored to add a flex array in order
to have the "callback structures allocated behind the array" be more
explicit.

Do so:
   - makes the code more readable and safer.
   - allows using __counted_by() for additional checks
   - avoids some pointer arithmetic in dma_fence_array_enable_signaling()

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: https://github.com/KSPP/linux/issues/160 [2]
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8b4e556e07b5dd78bb8a39b67ea0a43b199083c8.1716652811.git.christophe.jaillet@wanadoo.frSigned-off-by: Christian König <christian.koenig@amd.com>

drivers/dma-buf/dma-fence-array.c

@@ -70,7 +70,7 @@ static void dma_fence_array_cb_func(struct dma_fence *f,
 static bool dma_fence_array_enable_signaling(struct dma_fence *fence)
 {
 	struct dma_fence_array *array = to_dma_fence_array(fence);
-	struct dma_fence_array_cb *cb = (void *)(&array[1]);
+	struct dma_fence_array_cb *cb = array->callbacks;
 	unsigned i;
 
 	for (i = 0; i < array->num_fences; ++i) {
@@ -168,22 +168,20 @@ struct dma_fence_array *dma_fence_array_create(int num_fences,
 					       bool signal_on_any)
 {
 	struct dma_fence_array *array;
-	size_t size = sizeof(*array);
 
 	WARN_ON(!num_fences || !fences);
 
-	/* Allocate the callback structures behind the array. */
-	size += num_fences * sizeof(struct dma_fence_array_cb);
-	array = kzalloc(size, GFP_KERNEL);
+	array = kzalloc(struct_size(array, callbacks, num_fences), GFP_KERNEL);
 	if (!array)
 		return NULL;
 
+	array->num_fences = num_fences;
+
 	spin_lock_init(&array->lock);
 	dma_fence_init(&array->base, &dma_fence_array_ops, &array->lock,
 		       context, seqno);
 	init_irq_work(&array->work, irq_dma_fence_array_work);
 
-	array->num_fences = num_fences;
 	atomic_set(&array->num_pending, signal_on_any ? 1 : num_fences);
 	array->fences = fences;
 

include/linux/dma-fence-array.h

@@ -33,6 +33,7 @@ struct dma_fence_array_cb {
  * @num_pending: fences in the array still pending
  * @fences: array of the fences
  * @work: internal irq_work function
+ * @callbacks: array of callback helpers
  */
 struct dma_fence_array {
 	struct dma_fence base;
@@ -43,6 +44,8 @@ struct dma_fence_array {
 	struct dma_fence **fences;
 
 	struct irq_work work;
+
+	struct dma_fence_array_cb callbacks[] __counted_by(num_fences);
 };
 
 /**