wifi: rtw89: coex: add annotation __counted_by() for struct rtw89_btc_btf_set_slot_table

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

Use struct_size() and flex_array_size() helpers to calculate proper sizes
for allocation and memcpy().

Don't change logic at all, and result is identical as before.

Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231011063725.25276-1-pkshih@realtek.com

drivers/net/wireless/realtek/rtw89/coex.c

@@ -237,7 +237,7 @@ struct rtw89_btc_btf_set_report {
 struct rtw89_btc_btf_set_slot_table {
 	u8 fver;
 	u8 tbl_num;
-	u8 buf[];
+	struct rtw89_btc_fbtc_slot tbls[] __counted_by(tbl_num);
 } __packed;
 
 struct rtw89_btc_btf_set_mon_reg {
@@ -1821,19 +1821,17 @@ static void rtw89_btc_fw_en_rpt(struct rtw89_dev *rtwdev,
 static void rtw89_btc_fw_set_slots(struct rtw89_dev *rtwdev, u8 num,
 				   struct rtw89_btc_fbtc_slot *s)
 {
-	struct rtw89_btc_btf_set_slot_table *tbl = NULL;
-	u8 *ptr = NULL;
-	u16 n = 0;
+	struct rtw89_btc_btf_set_slot_table *tbl;
+	u16 n;
 
-	n = sizeof(*s) * num + sizeof(*tbl);
+	n = struct_size(tbl, tbls, num);
 	tbl = kmalloc(n, GFP_KERNEL);
 	if (!tbl)
 		return;
 
 	tbl->fver = BTF_SET_SLOT_TABLE_VER;
 	tbl->tbl_num = num;
-	ptr = &tbl->buf[0];
-	memcpy(ptr, s, num * sizeof(*s));
+	memcpy(tbl->tbls, s, flex_array_size(tbl, tbls, num));
 
 	_send_fw_cmd(rtwdev, BTFC_SET, SET_SLOT_TABLE, tbl, n);