linux-kernel - Re: [PATCH v2][next] watchdog: cros-ec: Avoid -Wflex-array-member-not-at-end warning

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Message-ID: <202504071212.D6CBE6740C@keescook>
Date: Mon, 7 Apr 2025 12:13:32 -0700
From: Kees Cook <kees@...nel.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Lukasz Majczak <lma@...omium.org>,
	Wim Van Sebroeck <wim@...ux-watchdog.org>,
	Guenter Roeck <linux@...ck-us.net>,
	Benson Leung <bleung@...omium.org>,
	Tzung-Bi Shih <tzungbi@...nel.org>, chrome-platform@...ts.linux.dev,
	linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] watchdog: cros-ec: Avoid
 -Wflex-array-member-not-at-end warning

On Thu, Mar 27, 2025 at 11:12:11AM -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 on-stack definitions 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/watchdog/cros_ec_wdt.c:29:40: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 
> Reviewed-by: Tzung-Bi Shih <tzungbi@...nel.org>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
> Changes v2:
>  - Fix truncated line in changelog text. (Tzung-Bi)
>  - Update variable name: s/buf/msg (Tzung-Bi)
>  - Cast to structs instead of union. (Tzung-Bi)
>  - Add RB tag.
> 
> v1:
>  - Link: https://lore.kernel.org/linux-hardening/Z-SBITmMfwjocYwL@kspp/
> 
>  drivers/watchdog/cros_ec_wdt.c | 30 +++++++++++++-----------------
>  1 file changed, 13 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/watchdog/cros_ec_wdt.c b/drivers/watchdog/cros_ec_wdt.c
> index 716c23f4388c..9ffe7f505645 100644
> --- a/drivers/watchdog/cros_ec_wdt.c
> +++ b/drivers/watchdog/cros_ec_wdt.c
> @@ -25,26 +25,22 @@ static int cros_ec_wdt_send_cmd(struct cros_ec_device *cros_ec,
>  				union cros_ec_wdt_data *arg)
>  {
>  	int ret;
> -	struct {
> -		struct cros_ec_command msg;
> -		union cros_ec_wdt_data data;
> -	} __packed buf = {
> -		.msg = {
> -			.version = 0,
> -			.command = EC_CMD_HANG_DETECT,
> -			.insize  = (arg->req.command == EC_HANG_DETECT_CMD_GET_STATUS) ?
> -				   sizeof(struct ec_response_hang_detect) :
> -				   0,
> -			.outsize = sizeof(struct ec_params_hang_detect),
> -		},
> -		.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_wdt_data));
> +
> +	msg->version = 0;

Technically redundant, but it was redundant before, so keeping style is
okay.

> +	msg->command = EC_CMD_HANG_DETECT;
> +	msg->insize  = (arg->req.command == EC_HANG_DETECT_CMD_GET_STATUS) ?
> +		   sizeof(struct ec_response_hang_detect) :
> +		   0;
> +	msg->outsize = sizeof(struct ec_params_hang_detect);
> +	*(struct ec_params_hang_detect *)msg->data = arg->req;
> +
> +	ret = cros_ec_cmd_xfer_status(cros_ec, msg);
>  	if (ret < 0)
>  		return ret;
>  
> -	arg->resp = buf.data.resp;
> +	arg->resp = *(struct ec_response_hang_detect *)msg->data;

msg->data used twice and a "sizeof()" earlier... might be nicer to have
an explicit pointer?

-Kees

>  
>  	return 0;
>  }
> -- 
> 2.43.0
> 

-- 
Kees Cook