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Date:   Tue, 12 Apr 2022 06:56:12 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Jaehee Park <jhpark1013@gmail.com>
Cc:     Johan Hovold <johan@kernel.org>, Alex Elder <elder@kernel.org>,
        greybus-dev@lists.linaro.org, linux-staging@lists.linux.dev,
        linux-kernel@vger.kernel.org, outreachy@lists.linux.dev
Subject: Re: [PATCH] staging: greybus: replace zero-element array with
 flexible-array
Message-ID: <YlUGbFs8oNikJCcv@kroah.com>
References: <20220411211411.GA2796005@jaehee-ThinkPad-X1-Extreme>
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In-Reply-To: <20220411211411.GA2796005@jaehee-ThinkPad-X1-Extreme>
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On Mon, Apr 11, 2022 at 05:14:11PM -0400, Jaehee Park wrote:
> Zero-length and one-element arrays are deprecated. Flexible-array
> members should be used instead. Flexible-array members are
> recommended because this is the way the kernel expects dynamically
> sized trailing elements to be declared.
> Refer to Documentation/process/deprecated.rst.
> 
> Change the zero-length array, buf, in the struct 
> gb_usb_hub_control_response to a flexible array. And add wLength as a 
> member of the struct so that the struct is not a zero-sized struct.
> 
> Issue found by flexible_array coccinelle script.
> 
> Signed-off-by: Jaehee Park <jhpark1013@gmail.com>
> ---
> 
> I have a question for the authors: 
> I saw a fixme comment in the hub_control function in usb.c:
> / FIXME: handle unspecified lengths /
> 
> I was wondering why this comment was left there?
> 
> In this patch, I'm using this struct:
> 
> struct gb_usb_hub_control_response {
>     __le16 wLength;
>     u8 buf[];
> };
> 
> And instead of using response_size, I'm doing this:
> 
> struct gb_usb_hub_control_response *response;
> And using sizeof(*response) as the input to gb_operation_create.
> 
> Would the flexible array address the handling of unspecified lengths 
> issue (in the fixme comment)?

No, you can not change the format of the data on the bus without also
changing the firmware in the device and usually the specification as
well.

>  drivers/staging/greybus/usb.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/staging/greybus/usb.c b/drivers/staging/greybus/usb.c
> index 8e9d9d59a357..d0b2422401df 100644
> --- a/drivers/staging/greybus/usb.c
> +++ b/drivers/staging/greybus/usb.c
> @@ -27,7 +27,8 @@ struct gb_usb_hub_control_request {
>  };
>  
>  struct gb_usb_hub_control_response {
> -	u8 buf[0];
> +	__le16 wLength;
> +	u8 buf[];

What is wrong with buf[0] here?

You can fix this in other ways if you really understand the difference
between [0] and [] in C.  Please look at many of the other conversions
if you wish to do this.

thanks,

greg k-h