Re: [RFC] rtw88: fix subscript above array bounds compiler warning

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Fri, May 03, 2019 at 12:01:05PM +0000, Tony Chuang wrote:
> > Subject: [RFC] rtw88: fix subscript above array bounds compiler warning
> > 
> > My compiler complain about:
> > 
> > drivers/net/wireless/realtek/rtw88/phy.c: In function
> > ‘rtw_phy_rf_power_2_rssi’:
> > drivers/net/wireless/realtek/rtw88/phy.c:430:26: warning: array subscript is
> > above array bounds [-Warray-bounds]
> >   linear = db_invert_table[i][j];
> > 
> > According to comment power_db should be in range 1 ~ 96 .
> > Correct rtw_phy_power_2_db() to make max power 96 db
> > (still min is 0). This make the warning gone.
> > 
> > However power >= 20 check still looks somewhat suspicious to me.
> > 
> > Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
> > ---
> >  drivers/net/wireless/realtek/rtw88/phy.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/net/wireless/realtek/rtw88/phy.c
> > b/drivers/net/wireless/realtek/rtw88/phy.c
> > index 35a35dbca85f..a716a44d78b0 100644
> > --- a/drivers/net/wireless/realtek/rtw88/phy.c
> > +++ b/drivers/net/wireless/realtek/rtw88/phy.c
> > @@ -410,12 +410,12 @@ void rtw_phy_dynamic_mechanism(struct rtw_dev
> > *rtwdev)
> > 
> >  static u8 rtw_phy_power_2_db(s8 power)
> >  {
> > -	if (power <= -100 || power >= 20)
> > +	if (power <= -96 || power >= 20)
> >  		return 0;
> >  	else if (power >= 0)
> > -		return 100;
> > +		return 96;
> >  	else
> > -		return 100 + power;
> > +		return 96 + power;
> >  }
> > 
> >  static u64 rtw_phy_db_2_linear(u8 power_db)
> > --
> 
> I think I should check with the radio team, that if the power from the
> rx descriptor generated by hardware can possibly get >= 20
> 
> And also check what the actual logic they expected to deal with the power.
> Thanks for reporting it.

Yeah, this could be just teoretical issue as we can not get power
values >= 0 from HW. However I think compiler correctly complains, as
for power_db=100 we get i = ((100 - 1) >> 3) = 12 , what exceed by one
max first index of db_invert_table[][], which should be in range
0 - 11.

Stanislaw