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

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Message-ID: <4f7c8a2f-e49e-beb5-9188-1bebac804d8d@intel.com>
Date:   Fri, 28 Feb 2020 19:26:09 +0100
From:   Björn Töpel <bjorn.topel@...el.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Magnus Karlsson <magnus.karlsson@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [next] xdp: Replace zero-length array with flexible-array
 member

On 2020-02-28 18:14, Jonathan Lemon wrote:
> 
> 
> On 28 Feb 2020, at 5:19, 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>
> 
> Acked-by: Jonathan Lemon <jonathan.lemon@...il.com>
> 

Acked-by: Björn Töpel <bjorn.topel@...el.com>