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

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Message-ID: <Y3YNyJ8oeixYuvdI@shell.armlinux.org.uk>
Date:   Thu, 17 Nov 2022 10:32:40 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jakub Kicinski <kuba@...nel.org>,
        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,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mmc@...r.kernel.org, linux-parisc@...r.kernel.org,
        ydroneaud@...eya.com
Subject: Re: [PATCH v2 3/3] treewide: use get_random_u32_between() when
 possible

On Thu, Nov 17, 2022 at 03:05:14AM +0100, Jason A. Donenfeld wrote:
> On Thu, Nov 17, 2022 at 12:55:47AM +0100, Jason A. Donenfeld wrote:
> > 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.
> > I think I can make progress with (1) alone by fiddling around with
> > godbolt enough, like usual.
> 
> The code gen is definitely worse.
> 
> Original half-open interval:
> 
>     return floor + get_random_u32_below(ceil - floor);
> 
> Suggested fully closed interval:
> 	
>     ceil = ceil - floor + 1;
>     return likely(ceil) ? floor + get_random_u32_below(ceil) : get_random_u32();

How many of these uses are going to have ceil and floor as a variable?
If they're constants (e.g. due to being in an inline function with
constant arguments) then the compiler will optimise all of the above
and the assembly code will just be either:

1. a call to get_random_u32()
2. a call to get_random_u32_below() and an addition.

If one passes ceil or floor as a variable, then yes, the code gen is
going to be as complicated as you suggest above.

-- 
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