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Message-ID: <CAGXu5jKDA=L5E6Cu_oRe-M=ouuhrccCZHyM+z6_UMAUqe9xnRQ@mail.gmail.com>
Date: Sat, 10 Mar 2018 15:12:40 -0800
From: Kees Cook <keescook@...omium.org>
To: Arend van Spriel <arend.vanspriel@...adcom.com>
Cc: Andreas Christoforou <andreaschristofo@...il.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
QCA ath9k Development <ath9k-devel@....qualcomm.com>, Kalle Valo <kvalo@...eaurora.org>,
linux-wireless <linux-wireless@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drivers: net: wireless: ath: ath9: dfs: remove VLA usage
On Sat, Mar 10, 2018 at 3:06 PM, Arend van Spriel
<arend.vanspriel@...adcom.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@...il.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
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