Re: [PATCH 2/2] prandom/random32: switch to Xoshiro256++

[Simon Horman <horms@kernel.org>]

On Fri, Feb 14, 2025 at 09:18:40AM +0100, Markus Theil wrote:
> The current Linux PRNG is based on LFSR113, which means:
> - needs some warmup rounds to yield better statistical properties
> - seeds/initial states must be of certain structure
> - does not pass L’Ecuyer's BigCrush in TestU01
> 
> While of course, there is no clear "best" PRNG, replace with
> Xoshiro256++, which seams to be a sensible replacement, from
> todays point of view:
> - only needs one bit set to 1 in the seed, needs no warmup, when
>   seeded with splitmix64.
> - Also has statistical evaluation, like LFSR113.
> - Passes BigCrush in TestU01.
> 
> The code got smaller, because some edge cases are ruled out now.
> I kept the test vectors and adapted them to this RNG.
> 
> Signed-off-by: Markus Theil <theil.markus@gmail.com>

...

> diff --git a/lib/random32.c b/lib/random32.c

...

> +/**
> + * prandom_seed_state - set seed for prandom_u32_state().
> + * @state: pointer to state structure to receive the seed.
> + * @seed: arbitrary 64-bit value to use as a seed.
> + *
> + * splitmix64 init as suggested for xoshiro256++
> + * See: https://prng.di.unimi.it/splitmix64.c
> + */
> +void prandom_seed_state(struct rnd_state *state, u64 seed)
>  {
> -	/* Calling RNG ten times to satisfy recurrence condition */
> -	prandom_u32_state(state);
> -	prandom_u32_state(state);
> -	prandom_u32_state(state);
> -	prandom_u32_state(state);
> -	prandom_u32_state(state);
> -	prandom_u32_state(state);
> -	prandom_u32_state(state);
> -	prandom_u32_state(state);
> -	prandom_u32_state(state);
> -	prandom_u32_state(state);
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(state->s); ++i) {
> +		seed += 0x9e3779b97f4a7c15;
> +		u64 z = seed;
> +		z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9;
> +		z = (z ^ (z >> 27)) * 0x94d049bb133111eb;
> +        state->s[i] = z ^ (z >> 31);

nit: The indentation seems off here.

> +	}
>  }
> +EXPORT_SYMBOL(prandom_seed_state);

...