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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Thu, 13 Feb 2020 09:47:25 -0800
Message-ID: <CAEf4Bzae=Afp_4FGgeFy+=kk64nm1vhRso2zF3j7Qdst66RFZw@mail.gmail.com>
Subject: Re: [PATCH] bpf: queue_stack_maps: Replace zero-length array with
 flexible-array member
To:     "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
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On Thu, Feb 13, 2020 at 7:22 AM Gustavo A. R. Silva
<gustavo@embeddedor.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
> 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@embeddedor.com>
> ---

Sure, why not, though I don't think that's the only one (e.g.,
bpf_storage_buffer's data is zero-length as well).

Acked-by: Andrii Nakryiko <andriin@fb.com>

>  kernel/bpf/queue_stack_maps.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/queue_stack_maps.c b/kernel/bpf/queue_stack_maps.c
> index f697647ceb54..30e1373fd437 100644
> --- a/kernel/bpf/queue_stack_maps.c
> +++ b/kernel/bpf/queue_stack_maps.c
> @@ -19,7 +19,7 @@ struct bpf_queue_stack {
>         u32 head, tail;
>         u32 size; /* max_entries + 1 */
>
> -       char elements[0] __aligned(8);
> +       char elements[] __aligned(8);
>  };
>
>  static struct bpf_queue_stack *bpf_queue_stack(struct bpf_map *map)
> --
> 2.25.0
>