linux-kernel - Re: [PATCH] platform/x86: i2c-multi-instantiate: Replace zero-length array with flexible-array member

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Date:   Thu, 27 Feb 2020 13:40:17 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Hans de Goede <hdegoede@...hat.com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>
Cc:     platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] platform/x86: i2c-multi-instantiate: Replace zero-length
 array with flexible-array member



On 2/27/20 13:01, Hans de Goede wrote:
> Hi,
> 
> 
> On 2/27/20 8:03 PM, Gustavo A. R. Silva wrote:
>> Hi all,
>>
>> Friendly ping: who can take this?
> 
> FWIW, the patch looks good to me:
> 
> Reviewed-by: Hans de Goede <hdegoede@...hat.com>
> 

Thanks, Hans.
--
Gustavo

> Regards,
> 
> Hans
> 
> 
> 
>> On 2/12/20 18:55, Gustavo A. R. Silva wrote:
>>> The current codebase makes use of the zero-length array language
>>> extension to the C90 standard, but the preferred mechanism to declare
>>> variable-length types such as these ones is a flexible array member[1][2],
>>> introduced in C99:
>>>
>>> struct foo {
>>>          int stuff;
>>>          struct boo array[];
>>> };
>>>
>>> By making use of the mechanism above, we will get a compiler warning
>>> in case the flexible array does not occur last in the structure, which
>>> will help us prevent some kind of undefined behavior bugs from being
>>> inadvertently introduced[3] to the codebase from now on.
>>>
>>> Also, notice that, dynamic memory allocations won't be affected by
>>> this change:
>>>
>>> "Flexible array members have incomplete type, and so the sizeof operator
>>> may not be applied. As a quirk of the original implementation of
>>> zero-length arrays, sizeof evaluates to zero."[1]
>>>
>>> This issue was found with the help of Coccinelle.
>>>
>>> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
>>> [2] https://github.com/KSPP/linux/issues/21
>>> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>>>
>>> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
>>> ---
>>>   drivers/platform/x86/i2c-multi-instantiate.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/platform/x86/i2c-multi-instantiate.c b/drivers/platform/x86/i2c-multi-instantiate.c
>>> index ffb8d5d1eb5f..6acc8457866e 100644
>>> --- a/drivers/platform/x86/i2c-multi-instantiate.c
>>> +++ b/drivers/platform/x86/i2c-multi-instantiate.c
>>> @@ -28,7 +28,7 @@ struct i2c_inst_data {
>>>     struct i2c_multi_inst_data {
>>>       int num_clients;
>>> -    struct i2c_client *clients[0];
>>> +    struct i2c_client *clients[];
>>>   };
>>>     static int i2c_multi_inst_count(struct acpi_resource *ares, void *data)
>>>
>>
>