linux-hardening - Re: [PATCH v2][next] iio: cros_ec: Avoid -Wflex-array-member-not-at-end warning

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Message-ID: <202503180840.77F2F47DE8@keescook>
Date: Tue, 18 Mar 2025 08:42:41 -0700
From: Kees Cook <kees@...nel.org>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Jonathan Cameron <jic23@...nel.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Benson Leung <bleung@...omium.org>,
	Guenter Roeck <groeck@...omium.org>,
	Tzung-Bi Shih <tzungbi@...nel.org>, linux-iio@...r.kernel.org,
	chrome-platform@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] iio: cros_ec: Avoid
 -Wflex-array-member-not-at-end warning

On Tue, Mar 18, 2025 at 08:33:20PM +1030, 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/iio/common/cros_ec_sensors/cros_ec_sensors_core.c:39:40: 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>
> ---
> Changes in v2:
>  - Use MAX() to calculate the number of bytes for the flex array. (Kees)
> 
> v1:
>  - Link: https://lore.kernel.org/linux-hardening/Z9dy43vUUh4goi-Q@kspp/
> 
>  .../cros_ec_sensors/cros_ec_sensors_core.c    | 28 ++++++++-----------
>  1 file changed, 11 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/iio/common/cros_ec_sensors/cros_ec_sensors_core.c b/drivers/iio/common/cros_ec_sensors/cros_ec_sensors_core.c
> index 7751d6f69b12..40d5b10c74e0 100644
> --- a/drivers/iio/common/cros_ec_sensors/cros_ec_sensors_core.c
> +++ b/drivers/iio/common/cros_ec_sensors/cros_ec_sensors_core.c
> @@ -34,25 +34,19 @@
>  static int cros_ec_get_host_cmd_version_mask(struct cros_ec_device *ec_dev,
>  					     u16 cmd_offset, u16 cmd, u32 *mask)
>  {
> +	DEFINE_RAW_FLEX(struct cros_ec_command, buf, data,
> +			MAX(sizeof(struct ec_response_get_cmd_versions),
> +			    sizeof(struct ec_params_get_cmd_versions)));
>  	int ret;
> -	struct {
> -		struct cros_ec_command msg;
> -		union {
> -			struct ec_params_get_cmd_versions params;
> -			struct ec_response_get_cmd_versions resp;
> -		};
> -	} __packed buf = {
> -		.msg = {
> -			.command = EC_CMD_GET_CMD_VERSIONS + cmd_offset,
> -			.insize = sizeof(struct ec_response_get_cmd_versions),
> -			.outsize = sizeof(struct ec_params_get_cmd_versions)
> -			},
> -		.params = {.cmd = cmd}
> -	};
> -
> -	ret = cros_ec_cmd_xfer_status(ec_dev, &buf.msg);
> +
> +	buf->command = EC_CMD_GET_CMD_VERSIONS + cmd_offset;
> +	buf->insize = sizeof(struct ec_response_get_cmd_versions);
> +	buf->outsize = sizeof(struct ec_params_get_cmd_versions);
> +	((struct ec_params_get_cmd_versions *)buf->data)->cmd = cmd;
> +
> +	ret = cros_ec_cmd_xfer_status(ec_dev, buf);
>  	if (ret >= 0)
> -		*mask = buf.resp.version_mask;
> +		*mask = ((struct ec_response_get_cmd_versions *)buf->data)->version_mask;
>  	return ret;
>  }

Since "params" is used twice, I'd say do like the other patch and assign
it to a new variable:

	struct ec_response_get_cmd_versions *params = buf->data;
	...
	params->cmd = cmd;
	...
		*mask = params->version_mask;


>  
> -- 
> 2.43.0
> 

-- 
Kees Cook