linux-kernel - Re: [PATCH] ethernet: i825xx: sun3_82586: Replace zero-length array with flexible-array

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Date:   Thu, 21 May 2020 14:44:45 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Sam Creasey <sammy@...my.net>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ethernet: i825xx: sun3_82586: Replace zero-length array
 with flexible-array

Hi Andrew,

I wonder if you could take this in your tree.

Apparently, Sam hasn't been active since 2007.

Thanks
--
Gustavo

On Thu, May 07, 2020 at 02:24:56PM -0500, 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@...nel.org>
> ---
>  drivers/net/ethernet/i825xx/sun3_82586.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/i825xx/sun3_82586.h b/drivers/net/ethernet/i825xx/sun3_82586.h
> index 79aef681ac85..451cb3d26cb5 100644
> --- a/drivers/net/ethernet/i825xx/sun3_82586.h
> +++ b/drivers/net/ethernet/i825xx/sun3_82586.h
> @@ -250,7 +250,7 @@ struct mcsetup_cmd_struct
>    unsigned short cmd_cmd;
>    unsigned short cmd_link;
>    unsigned short mc_cnt;		/* number of bytes in the MC-List */
> -  unsigned char  mc_list[0][6];  	/* pointer to 6 bytes entries */
> +  unsigned char  mc_list[][6];  	/* pointer to 6 bytes entries */
>  };
>  
>  /*
>