linux-kernel - Re: [PATCH] usb: ftdi-elan: Fix undefined behaviour

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Date:   Fri, 1 Apr 2022 11:10:19 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     cgel.zte@...il.com
Cc:     weiyongjun1@...wei.com, colin.king@...el.com, lv.ruyi@....com.cn,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Zeal Robot <zealci@....com.cn>
Subject: Re: [PATCH] usb: ftdi-elan: Fix undefined behaviour

On Fri, Apr 01, 2022 at 07:57:47AM +0000, cgel.zte@...il.com wrote:
> From: Lv Ruyi <lv.ruyi@....com.cn>
> 
> The use of zero-sized array causes undefined behaviour when it is not
> the last member in a structure. As it happens to be in this case.

What do you mean by "undefined behavior" here?  How is this working now?

> Also, the current code makes use of a language extension to the C90
> standard, but the preferred mechanism to declare variable-length
> types such as this one is a flexible array member, introduced in
> C99:
> 
> struct foo {
>         int stuff;
>         struct boo array[];
> };

gcc and clang both support [0] for a very long time so this isn't an
issue.

> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last. Which is beneficial
> to cultivate a high-quality code.
> 
> Reported-by: Zeal Robot <zealci@....com.cn>
> Signed-off-by: Lv Ruyi <lv.ruyi@....com.cn>
> ---
>  drivers/usb/misc/ftdi-elan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/misc/ftdi-elan.c b/drivers/usb/misc/ftdi-elan.c
> index 6c38c62d29b2..e818d2ed6831 100644
> --- a/drivers/usb/misc/ftdi-elan.c
> +++ b/drivers/usb/misc/ftdi-elan.c
> @@ -171,7 +171,6 @@ struct usb_ftdi {
>  	struct delayed_work command_work;
>  	struct delayed_work respond_work;
>  	struct u132_platform_data platform_data;
> -	struct resource resources[0];
>  	struct platform_device platform_dev;
>  	unsigned char *bulk_in_buffer;
>  	size_t bulk_in_size;
> @@ -185,6 +184,7 @@ struct usb_ftdi {
>  	int expected;
>  	int received;
>  	int ed_found;
> +	struct resource resources[];

How did you test and verify that this change is correct?

thanks,

greg k-h