RE: [PATCH] rtw89: avoid stringop-overflow warning

[Ping-Ke Shih <pkshih@realtek.com>]

> -----Original Message-----
> From: Arnd Bergmann <arnd@kernel.org>
> Sent: Monday, December 4, 2023 3:30 PM
> To: Ping-Ke Shih <pkshih@realtek.com>; Kalle Valo <kvalo@kernel.org>; Gustavo A. R. Silva
> <gustavoars@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>; DeanKu <ku920601@realtek.com>; linux-wireless@vger.kernel.org;
> linux-kernel@vger.kernel.org
> Subject: [PATCH] rtw89: avoid stringop-overflow warning

Subject prefix should be "wifi: rtw89: ..."

> 
> From: Arnd Bergmann <arnd@arndb.de>
> 
> After -Wstringop-overflow got enabled, the rtw89 driver produced
> two odd warnings with gcc-13:
> 
> drivers/net/wireless/realtek/rtw89/coex.c: In function 'rtw89_btc_ntfy_scan_start':
> drivers/net/wireless/realtek/rtw89/coex.c:5362:50: error: writing 1 byte into a region of size 0
> [-Werror=stringop-overflow=]
>  5362 |                 wl->dbcc_info.scan_band[phy_idx] = band;
>       |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~
> In file included from drivers/net/wireless/realtek/rtw89/coex.h:8,
>                  from drivers/net/wireless/realtek/rtw89/coex.c:5:
> drivers/net/wireless/realtek/rtw89/core.h:1441:12: note: at offset [64, 255] into destination object
> 'scan_band' of size 2
>  1441 |         u8 scan_band[RTW89_PHY_MAX]; /* scan band in  each phy */
>       |            ^~~~~~~~~
> drivers/net/wireless/realtek/rtw89/coex.c: In function 'rtw89_btc_ntfy_switch_band':
> drivers/net/wireless/realtek/rtw89/coex.c:5406:50: error: writing 1 byte into a region of size 0
> [-Werror=stringop-overflow=]
>  5406 |                 wl->dbcc_info.scan_band[phy_idx] = band;
>       |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~
> drivers/net/wireless/realtek/rtw89/core.h:1441:12: note: at offset [64, 255] into destination object
> 'scan_band' of size 2
>  1441 |         u8 scan_band[RTW89_PHY_MAX]; /* scan band in  each phy */
>       |            ^~~~~~~~~
> 
> I don't know what happened here, but adding an explicit range check
> shuts up the output.

The callers of these two cases will pass RTW89_PHY_0 (0) as argument of phy_idx,
and will extend to pass RTW89_PHY_1 (1) in the future, but should not be larger
than 1.

I don't mind to add this checking, but I really don't know what happened neither.
A statement 'wl->scan_info.band[phy_idx] = band;' did similar thing in the same
function, but why doesn't gcc complain this?