Re: [PATCH] drivers: net: wireless: ath: ath9: dfs: remove VLA usage

[Gustavo A. R. Silva]

On 03/10/2018 05:12 PM, Kees Cook wrote:
> On Sat, Mar 10, 2018 at 3:06 PM, Arend van Spriel
> <arend.vanspriel@xxxxxxxxxxxx> wrote:
> > On 3/9/2018 1:30 PM, Andreas Christoforou wrote:
> > > The kernel would like to have all stack VLA usage removed.
> >
> >
> > I think there was a remark made earlier to give more explanation here. It
> > should explain why we want "VLA on stack" removed.
> >
> > > Signed-off-by: Andreas Christoforou <andreaschristofo@xxxxxxxxx>
> > > ---
> > >    drivers/net/wireless/ath/ath9k/dfs.c | 3 +--
> > >    1 file changed, 1 insertion(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/net/wireless/ath/ath9k/dfs.c
> > > b/drivers/net/wireless/ath/ath9k/dfs.c
> > > index 6fee9a4..cfb0f84 100644
> > > --- a/drivers/net/wireless/ath/ath9k/dfs.c
> > > +++ b/drivers/net/wireless/ath/ath9k/dfs.c
> > > @@ -41,7 +41,6 @@ static const int BIN_DELTA_MAX                = 10;
> > >
> > >    /* we need at least 3 deltas / 4 samples for a reliable chirp detection
> > > */
> > >    #define NUM_DIFFS 3
> > > -static const int FFT_NUM_SAMPLES       = (NUM_DIFFS + 1);

What about this instead?

#define FFT_NUM_SAMPLES	(NUM_DIFFS + 1)

> > >    /* Threshold for difference of delta peaks */
> > >    static const int MAX_DIFF             = 2;
> > > @@ -101,7 +100,7 @@ static bool ath9k_check_chirping(struct ath_softc *sc,
> > > u8 *data,
> > >                                   int datalen, bool is_ctl, bool is_ext)
> > >    {
> > >          int i;
> > > -       int max_bin[FFT_NUM_SAMPLES];
> > > +       int max_bin[NUM_DIFFS + 1];
> >
> >
> > Just wondering. Is this actually a VLA. FFT_NUM_SAMPLES was static const so
> > not really going to show a lot of variation. This array will always have the
> > same size on the stack.
>
> The problem is that it's not a "constant expression", so the compiler
> frontend still yells about it under -Wvla. I would characterize this
> mainly as a fix for "accidental VLA" or "misdetected VLA" or something
> like that. AIUI, there really isn't a functional change here.
>
> -Kees

--
Gustavo