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>
---
Compile tested only.

Also, I don't think that 'cb' is a great name and the associated kernel-doc
description could certainly be improved.
Any proposal welcomed :)
---
 drivers/dma-buf/dma-fence-array.c | 10 ++++------
 include/linux/dma-fence-array.h   |  3 +++
 2 files changed, 7 insertions(+), 6 deletions(-)

Am 18.05.24 um 19:47 schrieb Christophe JAILLET:
> 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>
> ---
> Compile tested only.
>
> Also, I don't think that 'cb' is a great name and the associated kernel-doc
> description could certainly be improved.
> Any proposal welcomed :)

Ah, yes. That was also on my TODO list for a very long time.

> ---
>   drivers/dma-buf/dma-fence-array.c | 10 ++++------
>   include/linux/dma-fence-array.h   |  3 +++
>   2 files changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-fence-array.c b/drivers/dma-buf/dma-fence-array.c
> index 9b3ce8948351..9c55afaca607 100644
> --- a/drivers/dma-buf/dma-fence-array.c
> +++ b/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->cb;
>   	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, cb, 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;
>   
> diff --git a/include/linux/dma-fence-array.h b/include/linux/dma-fence-array.h
> index ec7f25def392..a793f9d5c73b 100644
> --- a/include/linux/dma-fence-array.h
> +++ b/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
> + * @cb: 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 cb[] __counted_by(num_fences);

Please name that callbacks, apart from that looks good to me.

Regards,
Christian.


>   };
>   
>   /**