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Subject: Re: [PATCH][next] drm/vboxvideo/vboxvideo.h: Replace zero-length
 array with flexible-array member
To:     "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        David Airlie <airlied@linux.ie>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
References: <20200305105558.GA19124@embeddedor>
 <8e2ab9a2-fb47-1d61-d09c-0510ad5ee5ff@redhat.com>
 <20200306104118.GV2363188@phenom.ffwll.local>
From:   Hans de Goede <hdegoede@redhat.com>
Message-ID: <b6b0d74d-f08a-e5af-b17c-899d6d6da487@redhat.com>
Date:   Fri, 6 Mar 2020 12:04:52 +0100
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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@embeddedor.com>
>>
>> Patch looks good to me:
>>
>> Reviewed-by: Hans de Goede <hdegoede@redhat.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.

Sorry about that, atm I'm on the road which makes pushing things somewhat
inconvenient, so if someone else can push this, that would be great.

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)
>>>
>>
>