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

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 03/10/2018 05:12 PM, Kees Cook wrote:
> 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);
>>>

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