linux-kernel - Re: [PATCH] auxdisplay: charlcd: replace zero-length array with flexible-array member

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Message-ID: <0dd75019-4d02-24ee-fdf1-f713a99d4141@embeddedor.com>
Date:   Thu, 13 Feb 2020 00:13:41 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] auxdisplay: charlcd: replace zero-length array with
 flexible-array member

Hi Miguel,

On 2/12/20 14:49, Miguel Ojeda wrote:
> Hi Gustavo,
> 
> On Wed, Feb 12, 2020 at 8:49 PM Gustavo A. R. Silva
> <gustavo@...eddedor.com> 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
>> inadvertenly introduced[3] to the codebase from now on.
> 
> s:inadvertenly:inadvertently:
> 

Thanks for this.

>> 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>
> 
> I saw the discussion regarding this -- thanks! Do you want me to
> handle this or will you push everything centrally? If the latter, have
> my
> 

Please, go ahead and handle it.

> Acked-by: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
> 

Thanks
--
Gustavo