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

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Date:   Fri, 6 Mar 2020 11:41:18 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        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

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.
-Daniel

> 
> 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)
> > 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch