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

[Kees Cook <keescook@chromium.org>]

On Sat, Mar 10, 2018 at 3:06 PM, Arend van Spriel
<arend.vanspriel@broadcom.com> 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@gmail.com>
>> ---
>>   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);
>>
>>   /* 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

-- 
Kees Cook
Pixel Security