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

Kalle Valo
2018-08-08T09:47:04Z

Stanislaw Gruszka <sgruszka@redhat.com> 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@mail.gmail.com
>>
>> Fixes: e87b5039511a ("mt76x0: eeprom files")
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>> 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 ?
>
>> +#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?

--
Kalle Valo