Re: [PATCH v2] Bluetooth: Remove VLA usage in aes_cmac

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

On 04/05/2018 03:46 AM, Marcel Holtmann wrote:

>> By the way, what is you opinion on replacing crypto_shash_descsize(ctx) with PAGE_SIZE / 8 in SHASH_DESC_ON_STACK?
>>
>> Does it work for you?
> 
> isn’t that just waste?
> 

Agree.

> The macro itself is this.
> 
> #define SHASH_DESC_ON_STACK(shash, ctx)                           \
>          char __##shash##_desc[sizeof(struct shash_desc) +         \
>                  crypto_shash_descsize(ctx)] CRYPTO_MINALIGN_ATTR; \
>          struct shash_desc *shash = (struct shash_desc *)__##shash##_desc
> 
> For AES-CMAC, we could always do this with a manual macro that gives us the right size. However that is error prone if any internals change. I think what has to happened that crypto_shash_decsize becomes something the compiler can evaluate at compile time.
> 

Yeah. That would imply an analysis of the algorithm each of the callers 
use. In the case of AES-CMAC, what is the maximum digest size?

I tried to find a fixed-length value for AES-CMAC but I didn't get any 
output with git grep -n _DIGEST_SIZE | grep AES

Thanks
--
Gustavo