Re: [PATCH] dma: map_benchmark: turn dma_sg_map_param buf into a flexible array

[Kees Cook <kees@kernel.org>]

On Mon, May 25, 2026 at 03:06:28PM -0700, Rosen Penev wrote:
> The buf pointer was kmalloc_array()'d immediately after the parent
> struct allocation, with the count (granule, validated to 1..1024 by
> the ioctl) trivially available beforehand.  Move buf to the struct
> tail as a flexible array member and fold the two allocations into a
> single kzalloc_flex(), dropping the kfree(params->buf) in both the
> prepare error path and unprepare.
> 
> Add __counted_by for extra runtime analysis.
> 
> Assisted-by: Claude:Opus-4.7
> Signed-off-by: Rosen Penev <rosenp@gmail.com>
> ---
>  kernel/dma/map_benchmark.c | 29 +++++++++++++----------------
>  1 file changed, 13 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/dma/map_benchmark.c b/kernel/dma/map_benchmark.c
> index 29eeb5fdf199..a65da5c7710c 100644
> --- a/kernel/dma/map_benchmark.c
> +++ b/kernel/dma/map_benchmark.c
> @@ -121,35 +121,35 @@ static struct map_benchmark_ops dma_single_map_benchmark_ops = {
>  struct dma_sg_map_param {
>  	struct sg_table sgt;
>  	struct device *dev;
> -	void **buf;
>  	u32 npages;
>  	u32 dma_dir;
> +	void *buf[] __counted_by(npages);
>  };
>  
>  static void *dma_sg_map_benchmark_prepare(struct map_benchmark_data *map)
>  {
> +	struct dma_sg_map_param *params;
>  	struct scatterlist *sg;
> +	u32 npages;
>  	int i;
>  
> -	struct dma_sg_map_param *params = kzalloc(sizeof(*params), GFP_KERNEL);
> -
> -	if (!params)
> -		return NULL;
>  	/*
>  	 * Set the number of scatterlist entries based on the granule.
>  	 * In SG mode, 'granule' represents the number of scatterlist entries.
>  	 * Each scatterlist entry corresponds to a single page.
>  	 */
> -	params->npages = map->bparam.granule;
> +	npages = map->bparam.granule;
> +
> +	params = kzalloc_flex(*params, buf, npages);
> +	if (!params)
> +		return NULL;
> +
> +	params->npages = npages;
>  	params->dma_dir = map->bparam.dma_dir;
>  	params->dev = map->dev;
> -	params->buf = kmalloc_array(params->npages, sizeof(*params->buf),
> -				    GFP_KERNEL);
> -	if (!params->buf)
> -		goto out;
>  
> -	if (sg_alloc_table(&params->sgt, params->npages, GFP_KERNEL))
> -		goto free_buf;
> +	if (sg_alloc_table(&params->sgt, npages, GFP_KERNEL))

nit: I think it's better to use the params->npages here just because it
is obviously tied to params->sgt, and reduces code churn for this patch
(i.e. that line would be left alone).

Otherwise looks good.

-Kees

> +		goto free_params;
>  
>  	for_each_sgtable_sg(&params->sgt, sg, i) {
>  		params->buf[i] = (void *)__get_free_page(GFP_KERNEL);
> @@ -166,9 +166,7 @@ static void *dma_sg_map_benchmark_prepare(struct map_benchmark_data *map)
>  		free_page((unsigned long)params->buf[i]);
>  
>  	sg_free_table(&params->sgt);
> -free_buf:
> -	kfree(params->buf);
> -out:
> +free_params:
>  	kfree(params);
>  	return NULL;
>  }
> @@ -183,7 +181,6 @@ static void dma_sg_map_benchmark_unprepare(void *mparam)
>  
>  	sg_free_table(&params->sgt);
>  
> -	kfree(params->buf);
>  	kfree(params);
>  }
>  
> -- 
> 2.54.0
> 

-- 
Kees Cook