Re: [PATCH 2/3] fpga: dfl-afu: fix integer truncation of npages in afu_dma_pin_pages()

On Thu, Apr 02, 2026 at 06:54:45AM -0600, Sebastian Alba Vives wrote:
> From: Sebastian Josue Alba Vives <[email protected]>
> 
> In afu_dma_pin_pages(), npages is declared as int but is assigned from
> region->length >> PAGE_SHIFT where region->length is u64. This causes
> implicit truncation on 64-bit systems when length is large.

How can length be that large?  You are shifting down, not up.

> 
> The truncated value is then passed to account_locked_vm() (which takes
> unsigned long) with implicit sign extension, and to pin_user_pages_fast()
> which takes int nr_pages, potentially causing incorrect VM accounting.
> 
> Change npages to unsigned long and add a cap to prevent values exceeding
> INT_MAX from reaching pin_user_pages_fast().
> 
> Signed-off-by: Sebastian Alba Vives <[email protected]>
> ---
>  drivers/fpga/dfl-afu-dma-region.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/fpga/dfl-afu-dma-region.c b/drivers/fpga/dfl-afu-dma-region.c
> index 87652d5..0d1f973 100644
> --- a/drivers/fpga/dfl-afu-dma-region.c
> +++ b/drivers/fpga/dfl-afu-dma-region.c
> @@ -34,10 +34,13 @@ void afu_dma_region_init(struct dfl_feature_dev_data *fdata)
>  static int afu_dma_pin_pages(struct dfl_feature_dev_data *fdata,
>  			     struct dfl_afu_dma_region *region)
>  {
> -	int npages = region->length >> PAGE_SHIFT;
> +	unsigned long npages = region->length >> PAGE_SHIFT;
>  	struct device *dev = &fdata->dev->dev;
>  	int ret, pinned;
>  
> +	if (npages > INT_MAX)
> +		return -EINVAL;

Why INT_MAX?  SHouldn't this be much smaller?

thanks,

greg k-h