wifi: ath9k: work around memset overflow warning

gcc-9 and some other older versions produce a false-positive warning
for zeroing two fields

In file included from include/linux/string.h:369,
                 from drivers/net/wireless/ath/ath9k/main.c:18:
In function 'fortify_memset_chk',
    inlined from 'ath9k_ps_wakeup' at drivers/net/wireless/ath/ath9k/main.c:140:3:
include/linux/fortify-string.h:462:25: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
  462 |                         __write_overflow_field(p_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Using a struct_group seems to reliably avoid the warning and
not make the code much uglier. The combined memset() should even
save a couple of cpu cycles.

Signed-off-by: Arnd Bergmann <[email protected]>
Acked-by: Toke Høiland-Jørgensen <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Signed-off-by: Kalle Valo <[email protected]>
Link: https://msgid.link/[email protected]

Author: arndb

drivers/net/wireless/ath/ath.h

@@ -171,8 +171,10 @@ struct ath_common {
 	unsigned int clockrate;
 
 	spinlock_t cc_lock;
-	struct ath_cycle_counters cc_ani;
-	struct ath_cycle_counters cc_survey;
+	struct_group(cc,
+		struct ath_cycle_counters cc_ani;
+		struct ath_cycle_counters cc_survey;
+	);
 
 	struct ath_regulatory regulatory;
 	struct ath_regulatory reg_world_copy;

drivers/net/wireless/ath/ath9k/main.c

@@ -135,8 +135,7 @@ void ath9k_ps_wakeup(struct ath_softc *sc)
 	if (power_mode != ATH9K_PM_AWAKE) {
 		spin_lock(&common->cc_lock);
 		ath_hw_cycle_counters_update(common);
-		memset(&common->cc_survey, 0, sizeof(common->cc_survey));
-		memset(&common->cc_ani, 0, sizeof(common->cc_ani));
+		memset(&common->cc, 0, sizeof(common->cc));
 		spin_unlock(&common->cc_lock);
 	}