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Date:   Mon, 11 May 2020 17:26:27 +0200
From:   Daniel Borkmann <>
To:     Yonghong Song <>,
        "Gustavo A. R. Silva" <>,
        Alexei Starovoitov <>
Cc:     Martin KaFai Lau <>, Song Liu <>,
        Andrii Nakryiko <>,
        John Fastabend <>,
        KP Singh <>,,,
Subject: Re: [PATCH] libbpf: Replace zero-length array with flexible-array

On 5/9/20 9:16 PM, Yonghong Song wrote:
> On 5/7/20 11:50 AM, 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]
>> sizeof(flexible-array-member) triggers a warning because flexible array
>> members have incomplete type[1]. There are some instances of code in
>> which the sizeof operator is being incorrectly/erroneously applied to
>> zero-length arrays and the result is zero. Such instances may be hiding
>> some bugs. So, this work (flexible-array member conversions) will also
>> help to get completely rid of those sorts of issues.
>> This issue was found with the help of Coccinelle.
>> [1]
>> [2]
>> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>> Signed-off-by: Gustavo A. R. Silva <>
> I think this is probably for bpf-next.
> Acked-by: Yonghong Song <>

Applied, thanks!

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