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Subject: Re: [PATCH] can: Replace zero-length array with flexible-array
To:     "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Marc Kleine-Budde <mkl@pengutronix.de>
Cc:     linux-can@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20200507185118.GA14022@embeddedor>
From:   Oliver Hartkopp <socketcan@hartkopp.net>
Message-ID: <0420f571-2d6a-c830-2029-8da60e3c2094@hartkopp.net>
Date:   Tue, 12 May 2020 15:30:59 +0200
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On 2020-05-07 20:51, 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]
> 
> 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@kernel.org>
> ---
>   include/linux/can/skb.h |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/can/skb.h b/include/linux/can/skb.h
> index a954def26c0d..900b9f4e0605 100644
> --- a/include/linux/can/skb.h
> +++ b/include/linux/can/skb.h
> @@ -34,7 +34,7 @@
>   struct can_skb_priv {
>   	int ifindex;
>   	int skbcnt;
> -	struct can_frame cf[0];
> +	struct can_frame cf[];
>   };
>   
>   static inline struct can_skb_priv *can_skb_prv(struct sk_buff *skb)
> 

Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>

@Gustavo: Just to be sure:

 From the referenced URLs I got the information that the sizeof() 
operator causes problems when applied to e.g. cf[0].

We don't have this case in our code - but one question remains to me:

We are using the above construct to ensure the padding between the two 
'int' values and the struct can_frame which enforces a 64 bit alignment.

This intention is not affected by the patch, right?

Best,
Oliver