Re: [PATCH] ath10k: work around uninitialized vht_pfr variable

[Brian Norris <briannorris@chromium.org>]

Hi Arnd,

On Mon, Jul 8, 2019 at 5:50 AM Arnd Bergmann <arnd@arndb.de> wrote:
>
> As clang points out, the vht_pfr is assigned to a struct member
> without being initialized in one case:
>
> drivers/net/wireless/ath/ath10k/mac.c:7528:7: error: variable 'vht_pfr' is used uninitialized whenever 'if' condition
>       is false [-Werror,-Wsometimes-uninitialized]
>                 if (!ath10k_mac_can_set_bitrate_mask(ar, band, mask,
>                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/net/wireless/ath/ath10k/mac.c:7551:20: note: uninitialized use occurs here
>                 arvif->vht_pfr = vht_pfr;
>                                  ^~~~~~~
> drivers/net/wireless/ath/ath10k/mac.c:7528:3: note: remove the 'if' if its condition is always true
>                 if (!ath10k_mac_can_set_bitrate_mask(ar, band, mask,
>                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/net/wireless/ath/ath10k/mac.c:7483:12: note: initialize the variable 'vht_pfr' to silence this warning
>         u8 vht_pfr;
>
> Add an explicit but probably incorrect initialization here.
> I suspect we want a better fix here, but chose this approach to
> illustrate the issue.
>
> Fixes: 8b97b055dc9d ("ath10k: fix failure to set multiple fixed rate")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/net/wireless/ath/ath10k/mac.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/net/wireless/ath/ath10k/mac.c b/drivers/net/wireless/ath/ath10k/mac.c
> index e43a566eef77..0606416dc971 100644
> --- a/drivers/net/wireless/ath/ath10k/mac.c
> +++ b/drivers/net/wireless/ath/ath10k/mac.c
> @@ -7541,6 +7541,8 @@ static int ath10k_mac_op_set_bitrate_mask(struct ieee80211_hw *hw,
>                                                                 &vht_nss,
>                                                                 true);

^^ Technically, this call to ath10k_mac_bitrate_mask_get_single_rate()
can fail to assign 'vht_pfr' as well. I can't immediately tell whether
it provably will never hit the -EINVAL case, but if we do, then you'd
have another uninitialized case.

I *believe* it shouldn't fail, since we already pre-checked the VHT
MCS lists for "exactly 1" rate. But it still seems like better code to
pre-initialize and/or add error-handling, so we don't rely on that
implicit proof.

I'm not quite sure yet what the "better" answer should be for
resolving this, but at a minimum, I think the above could be improved.

Brian

>                         update_bitrate_mask = false;
> +               } else {
> +                       vht_pfr = 0;
>                 }
>
>                 mutex_lock(&ar->conf_mutex);
> --
> 2.20.0
>