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

[Hans de Goede]

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@xxxxxxxxxxxxxx>
> >
> > Patch looks good to me:
> >
> > Reviewed-by: Hans de Goede <hdegoede@xxxxxxxxxx>
>
> 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)