linux-kernel - Re: [PATCH][next] leds: Avoid -Wflex-array-member-not-at-end warning

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Message-ID: <c051fd45-7eb0-465d-9e97-af294c453755@t-8ch.de>
Date: Fri, 28 Mar 2025 19:31:31 +0100
From: Thomas Weißschuh <thomas@...ssschuh.net>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Lee Jones <lee@...nel.org>, Pavel Machek <pavel@...nel.org>, 
	Benson Leung <bleung@...omium.org>, Guenter Roeck <groeck@...omium.org>, linux-leds@...r.kernel.org, 
	chrome-platform@...ts.linux.dev, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] leds: Avoid -Wflex-array-member-not-at-end warning

Hi Gustavo,

On 2025-03-28 08:33:22-0600, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> getting ready to enable it, globally.
> 
> Use the `DEFINE_RAW_FLEX()` helper for an on-stack definition of
> a flexible structure where the size of the flexible-array member
> is known at compile-time, and refactor the rest of the code,
> accordingly.
> 
> So, with these changes, fix the following warning:
> 
> drivers/leds/leds-cros_ec.c:70:40: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  drivers/leds/leds-cros_ec.c | 26 +++++++++++---------------
>  1 file changed, 11 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/leds/leds-cros_ec.c b/drivers/leds/leds-cros_ec.c
> index 275522b81ea5..6eab0474f52d 100644
> --- a/drivers/leds/leds-cros_ec.c
> +++ b/drivers/leds/leds-cros_ec.c
> @@ -66,24 +66,20 @@ static int cros_ec_led_send_cmd(struct cros_ec_device *cros_ec,
>  				union cros_ec_led_cmd_data *arg)
>  {
>  	int ret;
> -	struct {
> -		struct cros_ec_command msg;
> -		union cros_ec_led_cmd_data data;
> -	} __packed buf = {
> -		.msg = {
> -			.version = 1,
> -			.command = EC_CMD_LED_CONTROL,
> -			.insize  = sizeof(arg->resp),
> -			.outsize = sizeof(arg->req),
> -		},
> -		.data.req = arg->req
> -	};
> -
> -	ret = cros_ec_cmd_xfer_status(cros_ec, &buf.msg);
> +	DEFINE_RAW_FLEX(struct cros_ec_command, msg, data,
> +			sizeof(union cros_ec_led_cmd_data));
> +
> +	msg->version = 1;
> +	msg->command = EC_CMD_LED_CONTROL;
> +	msg->insize  = sizeof(arg->resp);
> +	msg->outsize = sizeof(arg->req);
> +	*(struct ec_params_led_control *)msg->data = arg->req;

To be honest this looks really ugly and it's not at all obvious what is
going on. We have the utility function cros_ec_cmd() which would be the
nicer alternative. (Without having verified that it avoids the warning).
While it is slightly more expensive, I don't think it matters.
And if it does, the helper can be optimized.

(The same goes for my other cros_ec drivers)

> +
> +	ret = cros_ec_cmd_xfer_status(cros_ec, msg);
>  	if (ret < 0)
>  		return ret;
>  
> -	arg->resp = buf.data.resp;
> +	arg->resp = *(struct ec_response_led_control *)msg->data;
>  
>  	return 0;
>  }


Thomas