linux-kernel - Re: [PATCH] crypto: s5p-sss - Replace zero-length array with flexible-array member

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Date:   Fri, 14 Feb 2020 08:09:36 +0100
From:   Kamil Konieczny <k.konieczny@...sung.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Vladimir Zapolskiy <vz@...ia.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>
Cc:     linux-crypto@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH] crypto: s5p-sss - Replace zero-length array with
 flexible-array member

On 13.02.2020 18:21, 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://protect2.fireeye.com/url?k=7fbec6f4-22720d30-7fbf4dbb-0cc47a314e9a-2a4d03985644c7ed&u=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>
> ---
>  drivers/crypto/s5p-sss.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/crypto/s5p-sss.c b/drivers/crypto/s5p-sss.c
> index d66e20a2f54c..2a16800d2579 100644
> --- a/drivers/crypto/s5p-sss.c
> +++ b/drivers/crypto/s5p-sss.c
> @@ -369,7 +369,7 @@ struct s5p_hash_reqctx {
>  	bool			error;
>  
>  	u32			bufcnt;
> -	u8			buffer[0];
> +	u8			buffer[];
>  };
>  
>  /**
> 

Looks good to me.

Acked-by: Kamil Konieczny <k.konieczny@...sung.com>

-- 
Best regards,
Kamil Konieczny
Samsung R&D Institute Poland