linux-kernel - Re: [PATCH] random: use rejection sampling for uniform bounded random integers

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Message-ID: <Y02ehdiCtHyLErB8@sol.localdomain>
Date:   Mon, 17 Oct 2022 11:27:17 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        sneves@....uc.pt
Subject: Re: [PATCH] random: use rejection sampling for uniform bounded
 random integers

On Sun, Oct 16, 2022 at 08:37:53PM -0600, Jason A. Donenfeld wrote:
> In order to be efficient, we implement a kernel-specific variant of
> Daniel Lemire's algorithm from "Fast Random Integer Generation in an
> Interval", linked below. The kernel's variant takes advantage of
> constant folding to avoid divisions entirely in the vast majority of
> cases, works on both 32-bit and 64-bit architectures, and requests a
> minimal amount of bytes from the RNG.
> 
> Link: https://arxiv.org/pdf/1805.10941.pdf

Thanks for doing this!  Your code looks correct, but it was hard for me to
understand until I read the paper that is linked to.  Could you include a brief
comment in the code that explains the algorithm?  Also, though the code looks
correct, I assume that you've also explicitly tested that each of the four code
paths produce uniform random numbers as intended?

- Eric