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

[keescook@chromium.org (Kees Cook)]

On Thu, Aug 23, 2018 at 2:48 PM, Joe Perches <joe@perches.com> wrote:
> Forwarding a question about coccinelle and isomorphisms from Kees Cook
>
> ---------- Forwarded message ----------
> From: Kees Cook <keescook@chromium.org>
> To: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
> Cc: Alessandro Zummo <a.zummo@towertech.it>, Alexandre Belloni <alexandre.belloni@bootlin.com>, Maxime Ripard <maxime.ripard@bootlin.com>, Chen-Yu Tsai <wens@csie.org>, linux-rtc at vger.kernel.org, linux-arm-kernel <linux-arm-kernel@lists.infradead.org>, LKML <linux-kernel@vger.kernel.org>
> Bcc:
> Date: Thu, 23 Aug 2018 13:56:35 -0700
> Subject: Re: [PATCH] rtc: sun6i: Use struct_size() in kzalloc()
> 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?

I had this:

@@
identifier alloc =~
"kmalloc|kzalloc|kmalloc_node|kzalloc_node|vmalloc|vzalloc|kvmalloc|kvzalloc";
identifier VAR, ELEMENT;
expression COUNT;
@@

- alloc(sizeof(*VAR) + COUNT * sizeof(*VAR->ELEMENT)
+ alloc(struct_size(VAR, ELEMENT, COUNT)
  , ...)

But I needed to explicitly change the rule to:

(
- alloc(sizeof(*VAR) + COUNT * sizeof(*VAR->ELEMENT)
+ alloc(struct_size(VAR, ELEMENT, COUNT)
  , ...)
|
- alloc(sizeof(*VAR) + (COUNT * sizeof(*VAR->ELEMENT))
+ alloc(struct_size(VAR, ELEMENT, COUNT)
  , ...)
)

to add the ()s. I would expect arithmetic commutative expressions to
match? But I had to add parens?

-Kees

-- 
Kees Cook
Pixel Security