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

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Message-ID: <Y05P+KTzFHGaK4C3@sol.localdomain>
Date:   Tue, 18 Oct 2022 00:04:24 -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 Mon, Oct 17, 2022 at 11:31:03PM -0600, Jason A. Donenfeld wrote:
> On Mon, Oct 17, 2022 at 11:27:17AM -0700, Eric Biggers wrote:
> > 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?
> 
> Yes, I've tested those, and they work. (Threw a lot of cores and ram at
> it.)
> 
> I could include a comment, sure. What do you have in mind? A
> line-by-line thing, or just a short blurb at the top of the function?
> 
> Jason

A comment at the top of the function would be good.

- Eric