Re: [PATCH] video: fbdev: via_aux_vt1632: remove VLA usage

[Gustavo A. R. Silva]

Hi Emil,

On 03/08/2018 05:58 AM, Emil Velikov wrote:
> Hi Gustavo,
>
> On 7 March 2018 at 19:54, Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx> wrote:
> > In preparation to enabling -Wvla, remove VLA and replace it
> > with a fixed-length array instead. Also, remove variable 'len'.
> What is the reason behind adding -Wvla? Is there a thread some that I
> can read up on?

Sure. This is the thread: https://lkml.org/lkml/2018/3/7/621

> > Notice that no new IDs have been added in seven years.
> >
> > Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> > ---
> >   drivers/video/fbdev/via/via_aux_vt1632.c | 6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/video/fbdev/via/via_aux_vt1632.c b/drivers/video/fbdev/via/via_aux_vt1632.c
> > index d24f4cd..0cd0d2a 100644
> > --- a/drivers/video/fbdev/via/via_aux_vt1632.c
> > +++ b/drivers/video/fbdev/via/via_aux_vt1632.c
> > @@ -35,10 +35,10 @@ static void probe(struct via_aux_bus *bus, u8 addr)
> >                  .addr   =       addr,
> >                  .name   =       name};
> >          /* check vendor id and device id */
> > -       const u8 id[] = {0x06, 0x11, 0x92, 0x31}, len = ARRAY_SIZE(id);
> > -       u8 tmp[len];
> > +       const u8 id[4] = {0x06, 0x11, 0x92, 0x31};
> > +       u8 tmp[4];
> >
> > -       if (!via_aux_read(&drv, 0x00, tmp, len) || memcmp(id, tmp, len))
> > +       if (!via_aux_read(&drv, 0x00, tmp, 4) || memcmp(id, tmp, 4))
>
> Generally, hard coding a bunch of numbers like that makes for
> confusing and fragile code.

You are correct.

> A lot simpler and more obvious solution is like the following.
> It silences the compiler warning (-Wvla - pedantic) you while keeping
> the original clarity.
>
>          const u8 id[] = {0x06, 0x11, 0x92, 0x31}, len = ARRAY_SIZE(id);
> -       u8 tmp[len];
> +       u8 tmp[ARRAY_SIZE(id)]; // Using len causes a Wvla warning

Yeah, this works just fine.

There are a total of four instances of this same issue in other files 
for the VIA component. I'll fix them and send a single patch shortly.

Thanks for the feedback.
I appreciate it.
--
Gustavo