linux-kernel - Re: [PATCH] dmaengine: at_xdmac: Replace zero-length array with flexible-array

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Message-ID: <20200511054803.yrz4hc4y4z5vscpl@sekiro>
Date:   Mon, 11 May 2020 07:48:03 +0200
From:   Ludovic Desroches <ludovic.desroches@...rochip.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dmaengine: at_xdmac: Replace zero-length array with
 flexible-array

On Thu, May 07, 2020 at 02:00:46PM -0500, Gustavo A. R. Silva wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> 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] 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 <gustavoars@...nel.org>
Acked-by: Ludovic Desroches<ludovic.desroches@...rochip.com>

Ludovic Desroches
> ---
>  drivers/dma/at_xdmac.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/at_xdmac.c b/drivers/dma/at_xdmac.c
> index bb0eaf38b594..fd92f048c491 100644
> --- a/drivers/dma/at_xdmac.c
> +++ b/drivers/dma/at_xdmac.c
> @@ -212,7 +212,7 @@ struct at_xdmac {
>         struct clk              *clk;
>         u32                     save_gim;
>         struct dma_pool         *at_xdmac_desc_pool;
> -       struct at_xdmac_chan    chan[0];
> +       struct at_xdmac_chan    chan[];
>  };
> 
> 
>