linux-kernel - Re: [PATCH][next] drm/vboxvideo/vboxvideo.h: Replace zero-length array with flexible-array member

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Date:   Sat, 7 Mar 2020 13:03:33 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] drm/vboxvideo/vboxvideo.h: Replace zero-length
 array with flexible-array member

Hi,

On 3/6/20 11:41 AM, Daniel Vetter wrote:
> On Thu, Mar 05, 2020 at 03:22:38PM +0100, Hans de Goede wrote:
>> Hi,
>>
>> On 3/5/20 11:55 AM, Gustavo A. R. Silva wrote:
>>> The current codebase makes use of the zero-length array language
>>> extension to the C90 standard, but the preferred mechanism to declare
>>> variable-length types such as these ones is a flexible array member[1][2],
>>> 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 in the structure, which
>>> will help us prevent some kind of undefined behavior bugs from being
>>> inadvertently introduced[3] to the codebase from now on.
>>>
>>> Also, notice that, dynamic memory allocations won't be affected by
>>> this change:
>>>
>>> "Flexible array members have incomplete type, and so the sizeof operator
>>> may not be applied. As a quirk of the original implementation of
>>> zero-length arrays, sizeof evaluates to zero."[1]
>>>
>>> This issue was found with the help of Coccinelle.
>>>
>>> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
>>> [2] https://github.com/KSPP/linux/issues/21
>>> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>>>
>>> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
>>
>> Patch looks good to me:
>>
>> Reviewed-by: Hans de Goede <hdegoede@...hat.com>
> 
> You're also going to push this? r-b by maintainers without any hint to
> what's going to happen is always rather confusing.

I've pushed this now, sorry for the confusion I will be more
clear about my intentions next time.

Regards,

Hans



>>> ---
>>>    drivers/gpu/drm/vboxvideo/vboxvideo.h | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/vboxvideo/vboxvideo.h b/drivers/gpu/drm/vboxvideo/vboxvideo.h
>>> index 0592004f71aa..a5de40fe1a76 100644
>>> --- a/drivers/gpu/drm/vboxvideo/vboxvideo.h
>>> +++ b/drivers/gpu/drm/vboxvideo/vboxvideo.h
>>> @@ -138,7 +138,7 @@ struct vbva_buffer {
>>>    	u32 data_len;
>>>    	/* variable size for the rest of the vbva_buffer area in VRAM. */
>>> -	u8 data[0];
>>> +	u8 data[];
>>>    } __packed;
>>>    #define VBVA_MAX_RECORD_SIZE (128 * 1024 * 1024)
>>>
>>
>