wifi: rtw89: coex: add annotation __counted_by() to struct rtw89_btc_btf_set_mon_reg

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 <[e-mail]>
Signed-off-by: Ping-Ke Shih <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Kalle Valo <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

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

···
 struct rtw89_btc_btf_set_mon_reg {
 	u8 fver;
 	u8 reg_num;
-	u8 buf[];
+	struct rtw89_btc_fbtc_mreg regs[] __counted_by(reg_num);
 } __packed;
 
 enum btc_btf_set_cx_policy {
···
 	const struct rtw89_chip_info *chip = rtwdev->chip;
 	const struct rtw89_btc_ver *ver = rtwdev->btc.ver;
 	struct rtw89_btc_btf_set_mon_reg *monreg = NULL;
-	u8 n, *ptr = NULL, ulen, cxmreg_max;
+	u8 n, ulen, cxmreg_max;
 	u16 sz = 0;
 
 	n = chip->mon_reg_num;
···
 		return;
 	}
 
-	ulen = sizeof(struct rtw89_btc_fbtc_mreg);
-	sz = (ulen * n) + sizeof(*monreg);
+	ulen = sizeof(monreg->regs[0]);
+	sz = struct_size(monreg, regs, n);
 	monreg = kmalloc(sz, GFP_KERNEL);
 	if (!monreg)
 		return;
 
 	monreg->fver = ver->fcxmreg;
 	monreg->reg_num = n;
-	ptr = &monreg->buf[0];
-	memcpy(ptr, chip->mon_reg, n * ulen);
+	memcpy(monreg->regs, chip->mon_reg, flex_array_size(monreg, regs, n));
 	rtw89_debug(rtwdev, RTW89_DBG_BTC,
 		    "[BTC], %s(): sz=%d ulen=%d n=%d\n",
 		    __func__, sz, ulen, n);