greybus: Avoid fake flexible array for response data

FORTIFY_SOURCE has been ignoring 0-sized destinations while the kernel
code base has been converted to flexible arrays. In order to enforce
the 0-sized destinations (e.g. with __counted_by), the remaining 0-sized
destinations need to be handled. Instead of converting an empty struct
into using a flexible array, just directly use a pointer without any
additional indirection. Remove struct gb_bootrom_get_firmware_response
and struct gb_fw_download_fetch_firmware_response.

Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: Alex Elder <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/greybus/bootrom.c

···
 	struct gb_bootrom *bootrom = gb_connection_get_data(op->connection);
 	const struct firmware *fw;
 	struct gb_bootrom_get_firmware_request *firmware_request;
-	struct gb_bootrom_get_firmware_response *firmware_response;
 	struct device *dev = &op->connection->bundle->dev;
 	unsigned int offset, size;
 	enum next_request_type next_request;
+	u8 *firmware_response;
 	int ret = 0;
 
 	/* Disable timeouts */
···
 		goto unlock;
 	}
 
-	if (!gb_operation_response_alloc(op, sizeof(*firmware_response) + size,
-					 GFP_KERNEL)) {
+	/* gb_bootrom_get_firmware_response contains only a byte array */
+	if (!gb_operation_response_alloc(op, size, GFP_KERNEL)) {
 		dev_err(dev, "%s: error allocating response\n", __func__);
 		ret = -ENOMEM;
 		goto unlock;
 	}
 
 	firmware_response = op->response->payload;
-	memcpy(firmware_response->data, fw->data + offset, size);
+	memcpy(firmware_response, fw->data + offset, size);
 
 	dev_dbg(dev, "responding with firmware (offs = %u, size = %u)\n",
 		offset, size);

drivers/staging/greybus/fw-download.c

···
 	struct gb_connection *connection = op->connection;
 	struct fw_download *fw_download = gb_connection_get_data(connection);
 	struct gb_fw_download_fetch_firmware_request *request;
-	struct gb_fw_download_fetch_firmware_response *response;
 	struct fw_request *fw_req;
 	const struct firmware *fw;
 	unsigned int offset, size;
 	u8 firmware_id;
+	u8 *response;
 	int ret = 0;
 
 	if (op->request->payload_size != sizeof(*request)) {
···
 		goto put_fw;
 	}
 
-	if (!gb_operation_response_alloc(op, sizeof(*response) + size,
-					 GFP_KERNEL)) {
+	/* gb_fw_download_fetch_firmware_response contains only a byte array */
+	if (!gb_operation_response_alloc(op, size, GFP_KERNEL)) {
 		dev_err(fw_download->parent,
 			"error allocating fetch firmware response\n");
 		ret = -ENOMEM;
···
 	}
 
 	response = op->response->payload;
-	memcpy(response->data, fw->data + offset, size);
+	memcpy(response, fw->data + offset, size);
 
 	dev_dbg(fw_download->parent,
 		"responding with firmware (offs = %u, size = %u)\n", offset,

include/linux/greybus/greybus_protocols.h

···
 	__le32			size;
 } __packed;
 
-struct gb_fw_download_fetch_firmware_response {
-	__u8			data[0];
-} __packed;
+/* gb_fw_download_fetch_firmware_response contains no other data */
 
 /* firmware download release firmware request */
 struct gb_fw_download_release_firmware_request {
···
 	__le32			size;
 } __packed;
 
-struct gb_bootrom_get_firmware_response {
-	__u8			data[0];
-} __packed;
+/* gb_bootrom_get_firmware_response contains no other data */
 
 /* Bootrom protocol Ready to boot request */
 struct gb_bootrom_ready_to_boot_request {