linux-kernel - Re: [greybus-dev] [PATCH] staging: greybus: Replace zero-length array with flexible-array member

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Date:   Tue, 11 Feb 2020 16:15:35 -0600
From:   Alex Elder <elder@...aro.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Johan Hovold <johan@...nel.org>, Alex Elder <elder@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     devel@...verdev.osuosl.org, greybus-dev@...ts.linaro.org,
        linux-kernel@...r.kernel.org
Subject: Re: [greybus-dev] [PATCH] staging: greybus: Replace zero-length array
 with flexible-array member

On 2/11/20 3:12 PM, 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
> inadvertenly introduced[3] to the codebase from now on.
> 
> 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>
> ---
>  drivers/staging/greybus/raw.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/staging/greybus/raw.c b/drivers/staging/greybus/raw.c
> index 838acbe84ca0..2b301b2aa107 100644
> --- a/drivers/staging/greybus/raw.c
> +++ b/drivers/staging/greybus/raw.c
> @@ -30,7 +30,7 @@ struct gb_raw {
>  struct raw_data {
>  	struct list_head entry;
>  	u32 len;
> -	u8 data[0];
> +	u8 data[];
>  };
>  
>  static struct class *raw_class;
> 

Does the kamlloc() call in receive_data() have any problems
with the sizeof(*raw_data) passed as its argument?

I'm not entirely sure what sizeof(struct-with-flexible-array-member)
produces.

					-Alex