linux-kernel - Re: [PATCH] rtc: sun6i: Use struct

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Message-ID: <CAGXu5jKPOVRk5DbFMKu0BV_K-ES+j+6_aApjyo0X8zXBfTYDnw@mail.gmail.com>
Date:   Thu, 23 Aug 2018 13:56:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>, linux-rtc@...r.kernel.org,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rtc: sun6i: Use struct_size() in kzalloc()

On Thu, Aug 23, 2018 at 11:51 AM, Gustavo A. R. Silva
<gustavo@...eddedor.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@...eddedor.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