Re: [PATCH] greybus: Avoid fake flexible array for response data

[Alex Elder <elder@ieee.org>]

On 2/16/24 5:28 PM, Kees Cook wrote:
> 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.

The only down side I see is that it sort of disrupts a pattern
used on Greybus request handlers (and the response structure definitions).

I think a one-line comment in place of each of these two
definitions would be helpful, something like:
/* gb_fw_download_fetch_firmware_response contains no data */

And then add a similar comment above the calls to
gb_operation_response_alloc().

Otherwise this looks good.

Reviewed-by: Alex Elder <elder@linaro.org>

> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> Cc: Viresh Kumar <vireshk@kernel.org>
> Cc: Johan Hovold <johan@kernel.org>
> Cc: Alex Elder <elder@kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
> Cc: greybus-dev@lists.linaro.org
> Cc: linux-staging@lists.linux.dev
> ---
>   drivers/staging/greybus/bootrom.c         | 7 +++----
>   drivers/staging/greybus/fw-download.c     | 7 +++----
>   include/linux/greybus/greybus_protocols.h | 8 --------
>   3 files changed, 6 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/staging/greybus/bootrom.c b/drivers/staging/greybus/bootrom.c
> index 79581457c4af..76ad5f289072 100644
> --- a/drivers/staging/greybus/bootrom.c
> +++ b/drivers/staging/greybus/bootrom.c
> @@ -243,10 +243,10 @@ static int gb_bootrom_get_firmware(struct gb_operation *op)
>   	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 */
> @@ -280,15 +280,14 @@ static int gb_bootrom_get_firmware(struct gb_operation *op)
>   		goto unlock;
>   	}
>   
> -	if (!gb_operation_response_alloc(op, sizeof(*firmware_response) + size,
> -					 GFP_KERNEL)) {
> +	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);
> diff --git a/drivers/staging/greybus/fw-download.c b/drivers/staging/greybus/fw-download.c
> index 543692c567f9..2d73bb729aa2 100644
> --- a/drivers/staging/greybus/fw-download.c
> +++ b/drivers/staging/greybus/fw-download.c
> @@ -271,11 +271,11 @@ static int fw_download_fetch_firmware(struct gb_operation *op)
>   	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)) {
> @@ -325,8 +325,7 @@ static int fw_download_fetch_firmware(struct gb_operation *op)
>   		goto put_fw;
>   	}
>   
> -	if (!gb_operation_response_alloc(op, sizeof(*response) + size,
> -					 GFP_KERNEL)) {
> +	if (!gb_operation_response_alloc(op, size, GFP_KERNEL)) {
>   		dev_err(fw_download->parent,
>   			"error allocating fetch firmware response\n");
>   		ret = -ENOMEM;
> @@ -334,7 +333,7 @@ static int fw_download_fetch_firmware(struct gb_operation *op)
>   	}
>   
>   	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,
> diff --git a/include/linux/greybus/greybus_protocols.h b/include/linux/greybus/greybus_protocols.h
> index aeb8f9243545..603acdcc0c6b 100644
> --- a/include/linux/greybus/greybus_protocols.h
> +++ b/include/linux/greybus/greybus_protocols.h
> @@ -232,10 +232,6 @@ struct gb_fw_download_fetch_firmware_request {
>   	__le32			size;
>   } __packed;
>   
> -struct gb_fw_download_fetch_firmware_response {
> -	__u8			data[0];
> -} __packed;
> -
>   /* firmware download release firmware request */
>   struct gb_fw_download_release_firmware_request {
>   	__u8			firmware_id;
> @@ -414,10 +410,6 @@ struct gb_bootrom_get_firmware_request {
>   	__le32			size;
>   } __packed;
>   
> -struct gb_bootrom_get_firmware_response {
> -	__u8			data[0];
> -} __packed;
> -
>   /* Bootrom protocol Ready to boot request */
>   struct gb_bootrom_ready_to_boot_request {
>   	__u8			status;