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

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Message-ID: <6FEAF24E-27CF-4840-8134-595D27275976@gmail.com>
Date:   Fri, 28 Feb 2020 09:14:12 -0800
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     "Björn Töpel" <bjorn.topel@...el.com>,
        "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 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>