linux-kernel - Re: [PATCH][next] mlxsw: Replace zero-length array with flexible-array member

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Message-ID: <1f264459-4627-d889-5623-1f18b55019f9@embeddedor.com>
Date:   Wed, 19 Feb 2020 13:58:53 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Ido Schimmel <idosch@...lanox.com>
Cc:     Jiri Pirko <jiri@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] mlxsw: Replace zero-length array with
 flexible-array member



On 2/19/20 10:43, Ido Schimmel wrote:
> On Tue, Feb 18, 2020 at 02:57:05PM -0600, 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>
> 
> Thanks, Gustavo. Looks good to me. Ran a few tests with a debug config
> and nothing exploded.
> 

haha great!

> I was just about to submit some patches myself, but they will conflict
> with this patch, so I will wait :)
> 

:)

> Reviewed-by: Ido Schimmel <idosch@...lanox.com>
> Tested-by: Ido Schimmel <idosch@...lanox.com>
> 

Thanks!
--
Gustavo