[PATCH] usb: ftdi-elan: Fix undefined behaviour

[PATCH] usb: ftdi-elan: Fix undefined behaviour

[cgel.zte]

From: Lv Ruyi <lv.ruyi@zte.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.

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[];
};

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@zte.com.cn>
Signed-off-by: Lv Ruyi <lv.ruyi@zte.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[];
 };
 #define kref_to_usb_ftdi(d) container_of(d, struct usb_ftdi, kref)
 #define platform_device_to_usb_ftdi(d) container_of(d, struct usb_ftdi, \
-- 
2.25.1

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

[Greg KH]

On Fri, Apr 01, 2022 at 07:57:47AM +0000, cgel.zte@gmail.com wrote:
> From: Lv Ruyi <lv.ruyi@zte.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@zte.com.cn>
> Signed-off-by: Lv Ruyi <lv.ruyi@zte.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