linux-kernel - Re: [PATCH] dmaengine: ti: omap-dma: Replace zero-length array with flexible-array member

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Message-ID: <2affea92-6748-76f1-6fe4-025fa7053a4f@ti.com>
Date:   Thu, 13 Feb 2020 09:06:20 +0200
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Vinod Koul <vkoul@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>
CC:     <dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dmaengine: ti: omap-dma: Replace zero-length array with
 flexible-array member



On 13/02/2020 2.39, 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")

Acked-by: Peter Ujfalusi <peter.ujfalusi@...com>

> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> ---
>  drivers/dma/ti/omap-dma.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/ti/omap-dma.c b/drivers/dma/ti/omap-dma.c
> index a014ab96e673..918301e17552 100644
> --- a/drivers/dma/ti/omap-dma.c
> +++ b/drivers/dma/ti/omap-dma.c
> @@ -124,7 +124,7 @@ struct omap_desc {
>  	uint32_t csdp;		/* CSDP value */
>  
>  	unsigned sglen;
> -	struct omap_sg sg[0];
> +	struct omap_sg sg[];
>  };
>  
>  enum {
> 

- Péter

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