Re: [PATCH wireless-drivers] mt76x0: Remove VLA usage

[Kees Cook]

On Wed, Aug 8, 2018 at 2:46 AM, Kalle Valo <kvalo@xxxxxxxxxxxxxx> wrote:
> Stanislaw Gruszka <sgruszka@xxxxxxxxxx> writes:
>
>> On Tue, Aug 07, 2018 at 03:50:40PM -0700, Kees Cook wrote:
>>> Even with "const" variables, the compiler will generate warnings about
>>> VLA usage. In the quest to remove all VLAs from the kernel[1], this uses
>>> a #define instead of a const to do the array sizing.
>>>
>>> [1]
>>> https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@xxxxxxxxxxxxxx
>>>
>>> Fixes: e87b5039511a ("mt76x0: eeprom files")
>>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>>> ---
>>> Please include this for the v4.19 merge window. The VLA was introduced
>>> with the new source file (which I also note is missing a SPDX line), so
>>
>> I thought SPDX line is needed only if file has no license and eeprom.c
>> file and other mt76x0 files have specified the license. Is SPDX still
>> needed in that case ?

I thought all source files needed SPDX: https://lwn.net/Articles/739183/

>>
>>> +#define MT_MAP_READS        DIV_ROUND_UP(MT_EFUSE_USAGE_MAP_SIZE, 16)
>>>  static int
>>>  mt76x0_efuse_physical_size_check(struct mt76x0_dev *dev)
>>>  {
>>> -    const int map_reads = DIV_ROUND_UP(MT_EFUSE_USAGE_MAP_SIZE, 16);
>>> -    u8 data[map_reads * 16];
>>
>> Why this is variable length array? DIV_ROUND_UP can not be calculated
>> at compile time? But if so, macro do not change the situation either.
>
> The commit log mentioned:
>
>   "Even with "const" variables, the compiler will generate warnings about
>    VLA usage."
>
> So I guess the compiler (gcc?) is just not smart enough in this case?

Correct. This is technically a false positive, but with the goal of
adding -Wvla to the build globally, we have to get rid of these as
well. It's a little frustrating, I agree, but with all others fixed
now, these stand out. :)

-Kees

-- 
Kees Cook
Pixel Security