linux-kernel - Re: [PATCH][next] cfg802154: Replace zero-length array with flexible-array member

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Message-ID: <2711894b-b78d-aebe-79fd-aa274d4ff977@datenfreihafen.org>
Date:   Fri, 28 Feb 2020 16:22:04 +0100
From:   Stefan Schmidt <stefan@...enfreihafen.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Alexander Aring <alex.aring@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     linux-wpan@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] cfg802154: Replace zero-length array with
 flexible-array member

Hello.

On 28.02.20 14:59, 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")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> ---
>   include/net/cfg802154.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/net/cfg802154.h b/include/net/cfg802154.h
> index 6f86073a5d7d..6ed07844eb24 100644
> --- a/include/net/cfg802154.h
> +++ b/include/net/cfg802154.h
> @@ -214,7 +214,7 @@ struct wpan_phy {
>   	/* the network namespace this phy lives in currently */
>   	possible_net_t _net;
>   
> -	char priv[0] __aligned(NETDEV_ALIGN);
> +	char priv[] __aligned(NETDEV_ALIGN);
>   };
>   
>   static inline struct net *wpan_phy_net(struct wpan_phy *wpan_phy)
> 

This patch has been applied to the wpan-next tree and will be
part of the next pull request to net-next. Thanks!

regards
Stefan Schmidt