linux-kernel - Re: [PATCH v2 3/3] treewide: use get_random_u32

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Message-ID: <Y3V4g8eorwiU++Y3@zx2c4.com>
Date:   Thu, 17 Nov 2022 00:55:47 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Christoph Böhmwalder 
        <christoph.boehmwalder@...bit.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Richard Weinberger <richard@....at>,
        "Darrick J . Wong" <djwong@...nel.org>,
        SeongJae Park <sj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Helge Deller <deller@....de>, netdev@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-media@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
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        ydroneaud@...eya.com
Subject: Re: [PATCH v2 3/3] treewide: use get_random_u32_between() when
 possible

On Wed, Nov 16, 2022 at 02:43:13PM -0800, Kees Cook wrote:
> On Mon, Nov 14, 2022 at 05:45:58PM +0100, Jason A. Donenfeld wrote:
> > -				(get_random_u32_below(1024) + 1) * PAGE_SIZE;
> > +				get_random_u32_between(1, 1024 + 1) * PAGE_SIZE;
> 
> I really don't like "between". Can't this be named "inclusive" (and
> avoid adding 1 everywhere, which seems ugly), or at least named
> something less ambiguous?
> 
> > -		n = get_random_u32_below(100) + 1;
> > +		n = get_random_u32_between(1, 101);
> 
> Because I find this much less readable. "Below 100" is clear: 0-99
> inclusive, plus 1, so 1-100 inclusive. "Between 1 and 101" is not obvious
> to me to mean: 1-100 inclusive.
> 
> These seem so much nicer:
> 	get_random_u32_inclusive(1, 1024)
> 	get_random_u32_inclusive(1, 100)

Yann pointed out something similar -- the half-closed interval being
confusing -- and while I was initially dismissive, I've warmed up to
doing this fully closed after sending a diff of that:

https://lore.kernel.org/lkml/Y3Qt8HiXj8giOnZy@zx2c4.com/

So okay, let's say that I'll implement the inclusive version instead. We
now have two problems to solve:

1) How/whether to make f(0, UR2_MAX) safe,
   - without additional 64-bit arithmetic,
   - minimizing the number of branches.
   I have a few ideas I'll code golf for a bit.

2) What to call it:
   - between I still like, because it mirrors "I'm thinking of a number
     between 1 and 10 and..." that everybody knows,
   - inclusive I guess works, but it's not a preposition,
   - bikeshed color #3?

I think I can make progress with (1) alone by fiddling around with
godbolt enough, like usual. I could use some more ideas for (2) though.

Jason