Re: [PATCH 0/7] Add arc4random support

[Yann Droneaud <ydroneaud@opteya.com>]

Hi,

Le 13/04/2022 à 22:23, Adhemerval Zanella via Libc-alpha a écrit :

> This patch adds the arc4random, arc4random_buf, and arc4random_uniform
> along with optimized versions for x86_64, aarch64, and powerpc64.
>
> The generic implementation is based on scalar Chacha20, with a global
> cache and locking.  It uses getrandom or /dev/urandom as fallback to
> get the initial entropy, and reseeds the internal state on every 16MB
> of consumed entropy.
>
> It maintains an internal buffer which consumes at maximum one page on
> most systems (assuming 4k pages).  The internal buffer optimizes the
> cipher encrypt calls, by amortize arc4random calls (where both
> function call and locks cost are the dominating factor).
>
> Fork detection is done by checking if MADV_WIPEONFORK supported.  If not
> the fork callback will reset the state on the fork call.  It does not
> handle direct clone calls, nor vfork or _Fork (arc4random is not
> async-signal-safe due the internal lock usage, althought the
> implementation does try to handle fork cases).
>
> The generic ChaCha20 implementation is based on the RFC8439 [1], which
> a simple memcpy with xor implementation.

The xor (with 0) is a waste of CPU cycles as the ChaCha20 keystream is 
the PRNG output.

Regards.

-- 

Yann Droneaud

OPTEYA