linux-kernel - Re: [PATCH] dmaengine: uniphier-mdmac: replace zero-length array with flexible-array member

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Date:   Tue, 18 Feb 2020 10:25:36 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        "open list:DMA GENERIC OFFLOAD ENGINE SUBSYSTEM" 
        <dmaengine@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dmaengine: uniphier-mdmac: replace zero-length array with
 flexible-array member

On Thu, Feb 13, 2020 at 9:35 AM Gustavo A. R. Silva
<gustavo@...eddedor.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@...eddedor.com>

Acked-by: Masahiro Yamada <yamada.masahiro@...ionext.com>


> ---
>  drivers/dma/uniphier-mdmac.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/dma/uniphier-mdmac.c b/drivers/dma/uniphier-mdmac.c
> index 21b8f1131d55..618839df0748 100644
> --- a/drivers/dma/uniphier-mdmac.c
> +++ b/drivers/dma/uniphier-mdmac.c
> @@ -68,7 +68,7 @@ struct uniphier_mdmac_device {
>         struct dma_device ddev;
>         struct clk *clk;
>         void __iomem *reg_base;
> -       struct uniphier_mdmac_chan channels[0];
> +       struct uniphier_mdmac_chan channels[];
>  };
>
>  static struct uniphier_mdmac_chan *
> --
> 2.23.0
>


-- 
Best Regards
Masahiro Yamada