Bluetooth: hci_conn: Use __counted_by() to avoid -Wfamnae warning

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).

Also, -Wflex-array-member-not-at-end is coming in GCC-14, and we are
getting ready to enable it globally.

So, use the `DEFINE_FLEX()` helper for an on-stack definition of
a flexible structure where the size of the flexible-array member
is known at compile-time, and refactor the rest of the code,
accordingly.

With these changes, fix the following warning:
net/bluetooth/hci_conn.c:2116:50: warning: structure containing a flexible
array member is not at the end of another structure
[-Wflex-array-member-not-at-end]

Link: KSPP#202
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

Author: GustavoARSilva

include/net/bluetooth/hci.h

@@ -2216,7 +2216,7 @@ struct hci_cp_le_big_create_sync {
 	__u8    mse;
 	__le16  timeout;
 	__u8    num_bis;
-	__u8    bis[];
+	__u8    bis[] __counted_by(num_bis);
 } __packed;
 
 #define HCI_OP_LE_BIG_TERM_SYNC			0x206c

net/bluetooth/hci_conn.c

@@ -2109,13 +2109,10 @@ int hci_le_big_create_sync(struct hci_dev *hdev, struct hci_conn *hcon,
 			   struct bt_iso_qos *qos,
 			   __u16 sync_handle, __u8 num_bis, __u8 bis[])
 {
-	struct _packed {
-		struct hci_cp_le_big_create_sync cp;
-		__u8  bis[0x11];
-	} pdu;
+	DEFINE_FLEX(struct hci_cp_le_big_create_sync, pdu, bis, num_bis, 0x11);
 	int err;
 
-	if (num_bis < 0x01 || num_bis > sizeof(pdu.bis))
+	if (num_bis < 0x01 || num_bis > pdu->num_bis)
 		return -EINVAL;
 
 	err = qos_set_big(hdev, qos);
@@ -2125,18 +2122,17 @@ int hci_le_big_create_sync(struct hci_dev *hdev, struct hci_conn *hcon,
 	if (hcon)
 		hcon->iso_qos.bcast.big = qos->bcast.big;
 
-	memset(&pdu, 0, sizeof(pdu));
-	pdu.cp.handle = qos->bcast.big;
-	pdu.cp.sync_handle = cpu_to_le16(sync_handle);
-	pdu.cp.encryption = qos->bcast.encryption;
-	memcpy(pdu.cp.bcode, qos->bcast.bcode, sizeof(pdu.cp.bcode));
-	pdu.cp.mse = qos->bcast.mse;
-	pdu.cp.timeout = cpu_to_le16(qos->bcast.timeout);
-	pdu.cp.num_bis = num_bis;
-	memcpy(pdu.bis, bis, num_bis);
+	pdu->handle = qos->bcast.big;
+	pdu->sync_handle = cpu_to_le16(sync_handle);
+	pdu->encryption = qos->bcast.encryption;
+	memcpy(pdu->bcode, qos->bcast.bcode, sizeof(pdu->bcode));
+	pdu->mse = qos->bcast.mse;
+	pdu->timeout = cpu_to_le16(qos->bcast.timeout);
+	pdu->num_bis = num_bis;
+	memcpy(pdu->bis, bis, num_bis);
 
 	return hci_send_cmd(hdev, HCI_OP_LE_BIG_CREATE_SYNC,
-			    sizeof(pdu.cp) + num_bis, &pdu);
+			    sizeof(*pdu) + num_bis, pdu);
 }
 
 static void create_big_complete(struct hci_dev *hdev, void *data, int err)