linux-hardening - Re: [PATCH v2][next] wifi: ath6kl: wmi: Avoid -Wflex-array-member-not-at-end warning

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Message-ID: <d2a02b4c-5f69-4f5e-bc0e-428b64e99d36@oss.qualcomm.com>
Date: Wed, 18 Feb 2026 16:18:33 -0800
From: Jeff Johnson <jeff.johnson@....qualcomm.com>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Kalle Valo <quic_kvalo@...cinc.com>
Cc: linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] wifi: ath6kl: wmi: Avoid
 -Wflex-array-member-not-at-end warning

On 2/16/2026 9:10 PM, 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.
> 
> struct bss_bias_info is a flexible structure, this is a structure
> that contains a flexible-array member (struct bss_bias bss_bias[]).
> 
> Since struct roam_ctrl_cmd is defined by hardware, we create the new
> struct bss_bias_info_hdr type, and use it to replace the object type
> causing trouble in struct roam_ctrl_cmd, namely struct bss_bias_info.
> 
> Also, once -fms-extensions is enabled, we can use transparent struct
> members in struct bss_bias_info.
> 
> Notice that the newly created type does not contain the flex-array
> member `bss_bias`, which is the object causing the -Wfamnae warning.
> 
> After these changes, the size of struct roam_ctrl_cmd, along
> with its member's offsets remain the same, hence the memory layout
> doesn't change:
> 
> Before changes:
> struct roam_ctrl_cmd {
> 	union {
> 		u8                 bssid[6];             /*     0     6 */
> 		u8                 roam_mode;            /*     0     1 */
> 		struct bss_bias_info bss;            	 /*     0     1 */
> 		struct low_rssi_scan_params params;      /*     0     8 */
> 	} info;                                          /*     0     8 */
> 	u8                         roam_ctrl;            /*     8     1 */
> 
> 	/* size: 9, cachelines: 1, members: 2 */
> 	/* last cacheline: 9 bytes */
> } __attribute__((__packed__));
> 
> After changes:
> struct roam_ctrl_cmd {
> 	union {
> 		u8                 bssid[6];             /*     0     6 */
> 		u8                 roam_mode;            /*     0     1 */
> 		struct bss_bias_info_hdr bss;            /*     0     1 */
> 		struct low_rssi_scan_params params;      /*     0     8 */
> 	} info;                                          /*     0     8 */
> 	u8                         roam_ctrl;            /*     8     1 */
> 
> 	/* size: 9, cachelines: 1, members: 2 */
> 	/* last cacheline: 9 bytes */
> } __attribute__((__packed__));
> 
> With these changes fix the following warning:
> 
> drivers/net/wireless/ath/ath6kl/wmi.h:1658:20: 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:
>  - Create new separate struct bss_bias_info_hdr, and use
>    transparent struct members (in struct bss_bias_info)
>    instead of rearranging members in struct roam_ctrl_cmd.
>  - Update subject line - Add 'wifi:' prefix.
> 
> v1:
>  - Link: https://lore.kernel.org/linux-hardening/aR153k4ExCD-QTMq@kspp/
> 
>  drivers/net/wireless/ath/ath6kl/wmi.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/wireless/ath/ath6kl/wmi.h b/drivers/net/wireless/ath/ath6kl/wmi.h
> index 3080d82e25cc..d2a5c96fc878 100644
> --- a/drivers/net/wireless/ath/ath6kl/wmi.h
> +++ b/drivers/net/wireless/ath/ath6kl/wmi.h
> @@ -1635,8 +1635,12 @@ struct bss_bias {
>  	s8 bias;
>  } __packed;
>  
> -struct bss_bias_info {
> +struct bss_bias_info_hdr {
>  	u8 num_bss;
> +} __packed;
> +
> +struct bss_bias_info {
> +	struct bss_bias_info_hdr;
>  	struct bss_bias bss_bias[];
>  } __packed;
>  
> @@ -1652,7 +1656,7 @@ struct roam_ctrl_cmd {
>  	union {
>  		u8 bssid[ETH_ALEN]; /* WMI_FORCE_ROAM */
>  		u8 roam_mode; /* WMI_SET_ROAM_MODE */
> -		struct bss_bias_info bss; /* WMI_SET_HOST_BIAS */
> +		struct bss_bias_info_hdr bss; /* WMI_SET_HOST_BIAS */
>  		struct low_rssi_scan_params params; /* WMI_SET_LRSSI_SCAN_PARAMS
>  						     */
>  	} __packed info;

That bss member appears to be completely unused
(bssid, roam_mode, and params are used)

So IMO the better solution is to remove bss from the union.
And I think struct bss_bias and struct bss_bias_info can also be removed.

/jeff