Re: [PATCH] rtc: sun6i: Use struct_size() in kzalloc()

[Kees Cook <keescook@chromium.org>]

On Thu, Aug 23, 2018 at 11:51 AM, Gustavo A. R. Silva
<gustavo@embeddedor.com> wrote:
> One of the more common cases of allocation size calculations is finding
> the size of a structure that has a zero-sized array at the end, along
> with memory for some number of elements for that array. For example:
>
> struct foo {
>         int stuff;
>         void *entry[];
> };
>
> instance = kzalloc(sizeof(struct foo) + sizeof(void *) * count, GFP_KERNEL);
>
> Instead of leaving these open-coded and prone to type mistakes, we can
> now use the new struct_size() helper:
>
> instance = kzalloc(struct_size(instance, entry, count), GFP_KERNEL);
>
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> ---
>  drivers/rtc/rtc-sun6i.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/rtc/rtc-sun6i.c b/drivers/rtc/rtc-sun6i.c
> index 2cd5a7b..fe07310 100644
> --- a/drivers/rtc/rtc-sun6i.c
> +++ b/drivers/rtc/rtc-sun6i.c
> @@ -199,8 +199,7 @@ static void __init sun6i_rtc_clk_init(struct device_node *node)
>         if (!rtc)
>                 return;
>
> -       clk_data = kzalloc(sizeof(*clk_data) + (sizeof(*clk_data->hws) * 2),
> -                          GFP_KERNEL);
> +       clk_data = kzalloc(struct_size(clk_data, hws, 2), GFP_KERNEL);
>         if (!clk_data) {
>                 kfree(rtc);
>                 return;

This looks like entirely correct to me, but I'm surprised the
Coccinelle script didn't discover this. I guess the isomorphisms don't
cover the parenthesis?

-Kees

-- 
Kees Cook
Pixel Security