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

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