linux-kernel - Re: [PATCH][next] intel-ish-hid: ishtp: hbm.h: Replace zero-length array with flexible-array member

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Date:   Sat, 21 Mar 2020 00:08:19 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
cc:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] intel-ish-hid: ishtp: hbm.h: Replace zero-length
 array with flexible-array member

On Thu, 19 Mar 2020, 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>
> ---
>  drivers/hid/intel-ish-hid/ishtp/hbm.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hid/intel-ish-hid/ishtp/hbm.h b/drivers/hid/intel-ish-hid/ishtp/hbm.h
> index bb85985b1620..7c445b203f2a 100644
> --- a/drivers/hid/intel-ish-hid/ishtp/hbm.h
> +++ b/drivers/hid/intel-ish-hid/ishtp/hbm.h
> @@ -82,7 +82,7 @@ struct ishtp_msg_hdr {
>  
>  struct ishtp_bus_message {
>  	uint8_t hbm_cmd;
> -	uint8_t data[0];
> +	uint8_t data[];
>  } __packed;
>  
>  /**

Applied this one too, thanks Gustavo.

-- 
Jiri Kosina
SUSE Labs