Re: [PATCH 03/10] b43: Remove uninitialized_var() usage

[Nick Desaulniers <ndesaulniers@google.com>]

On Wed, Jun 3, 2020 at 4:32 PM Kees Cook <keescook@chromium.org> wrote:
>
> Using uninitialized_var() is dangerous as it papers over real bugs[1]
> (or can in the future), and suppresses unrelated compiler warnings (e.g.
> "unused variable"). If the compiler thinks it is uninitialized, either
> simply initialize the variable or make compiler changes. As a precursor
> to removing[2] this[3] macro[4], just initialize this variable to NULL,
> and make the (unreachable!) code do a conditional test.
>
> [1] https://lore.kernel.org/lkml/20200603174714.192027-1-glider@google.com/
> [2] https://lore.kernel.org/lkml/CA+55aFw+Vbj0i=1TGqCR5vQkCzWJ0QxK6CernOU6eedsudAixw@mail.gmail.com/
> [3] https://lore.kernel.org/lkml/CA+55aFwgbgqhbp1fkxvRKEpzyR5J8n1vKT1VZdz9knmPuXhOeg@mail.gmail.com/
> [4] https://lore.kernel.org/lkml/CA+55aFz2500WfbKXAx8s67wrm9=yVJu65TpLgN_ybYNv0VEOKA@mail.gmail.com/
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  drivers/net/wireless/broadcom/b43/phy_n.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/wireless/broadcom/b43/phy_n.c b/drivers/net/wireless/broadcom/b43/phy_n.c
> index d3c001fa8eb4..88cdcea10d61 100644
> --- a/drivers/net/wireless/broadcom/b43/phy_n.c
> +++ b/drivers/net/wireless/broadcom/b43/phy_n.c
> @@ -4222,7 +4222,7 @@ static void b43_nphy_tx_gain_table_upload(struct b43_wldev *dev)

The TODOs and `#if 0` in this function are concerning.  It looks like
`rf_pwr_offset_table` is only used when `phy->rev` is >=7 && < 19.

Further, the loop has a case for `phy->rev >= 19` but we would have
returned earlier if that was the case.

>         u32 rfpwr_offset;
>         u8 pga_gain, pad_gain;
>         int i;
> -       const s16 *uninitialized_var(rf_pwr_offset_table);
> +       const s16 *rf_pwr_offset_table = NULL;
>
>         table = b43_nphy_get_tx_gain_table(dev);
>         if (!table)
> @@ -4256,9 +4256,13 @@ static void b43_nphy_tx_gain_table_upload(struct b43_wldev *dev)
>                         pga_gain = (table[i] >> 24) & 0xf;
>                         pad_gain = (table[i] >> 19) & 0x1f;
>                         if (b43_current_band(dev->wl) == NL80211_BAND_2GHZ)
> -                               rfpwr_offset = rf_pwr_offset_table[pad_gain];
> +                               rfpwr_offset = rf_pwr_offset_table
> +                                               ? rf_pwr_offset_table[pad_gain]
> +                                               : 0;
>                         else
> -                               rfpwr_offset = rf_pwr_offset_table[pga_gain];
> +                               rfpwr_offset = rf_pwr_offset_table
> +                                               ? rf_pwr_offset_table[pga_gain]
> +                                               : 0;


The code is trying to check `phy->rev >= 7 && phy->rev < 19` once
before the loop, then set `rf_pwr_offset_table`, so having another
conditional on `rf_pwr_offset_table` in the loop is unnecessary. I'm
ok with initializing it to `NULL`, but I'm not sure the conditional
check is necessary.  Do you get a compiler warning otherwise?

>                 } else {
>                         pga_gain = (table[i] >> 24) & 0xF;
>                         if (b43_current_band(dev->wl) == NL80211_BAND_2GHZ)
> --
> 2.25.1
>
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-- 
Thanks,
~Nick Desaulniers