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Message-ID: <Y0I4si9+cMracPAq@zx2c4.com>
Date: Sat, 8 Oct 2022 20:57:54 -0600
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: David Laight <David.Laight@...LAB.COM>
Cc: Kees Cook <keescook@...omium.org>,
Christophe Leroy <christophe.leroy@...roup.eu>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"patches@...ts.linux.dev" <patches@...ts.linux.dev>,
Andreas Noever <andreas.noever@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Borislav Petkov <bp@...en8.de>,
Catalin Marinas <catalin.marinas@....com>,
Christoph Böhmwalder
<christoph.boehmwalder@...bit.com>, Christoph Hellwig <hch@....de>,
Daniel Borkmann <daniel@...earbox.net>,
Dave Airlie <airlied@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"David S . Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Florian Westphal <fw@...len.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"H . Peter Anvin" <hpa@...or.com>,
Heiko Carstens <hca@...ux.ibm.com>,
Helge Deller <deller@....de>,
Herbert Xu <herbert@...dor.apana.org.au>,
Huacai Chen <chenhuacai@...nel.org>,
Hugh Dickins <hughd@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
"James E . J . Bottomley" <jejb@...ux.ibm.com>,
Jan Kara <jack@...e.com>, Jason Gunthorpe <jgg@...pe.ca>,
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Jonathan Corbet <corbet@....net>,
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Toke Høiland-Jørgensen <toke@...e.dk>,
Chuck Lever <chuck.lever@...cle.com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v3 3/5] treewide: use get_random_u32() when possible
On Sat, Oct 08, 2022 at 09:53:33PM +0000, David Laight wrote:
> From: Jason A. Donenfeld
> > Sent: 07 October 2022 18:56
> ...
> > > Given these kinds of less mechanical changes, it may make sense to split
> > > these from the "trivial" conversions in a treewide patch. The chance of
> > > needing a revert from the simple 1:1 conversions is much lower than the
> > > need to revert by-hand changes.
> > >
> > > The Cocci script I suggested in my v1 review gets 80% of the first
> > > patch, for example.
> >
> > I'll split things up into a mechanical step and a non-mechanical step.
> > Good idea.
>
> I'd also do something about the 'get_random_int() & 3' cases.
> (ie remainder by 2^n-1)
> These can be converted to 'get_random_u8() & 3' (etc).
> So they only need one random byte (not 4) and no multiply.
>
> Possibly something based on (the quickly typed, and not C):
> #define get_random_below(val) [
> if (builtin_constant(val))
> BUILD_BUG_ON(!val || val > 0x100000000ull)
> if (!(val & (val - 1)) {
> if (val <= 0x100)
> return get_random_u8() & (val - 1);
> if (val <= 0x10000)
> return get_random_u16() & (val - 1);
> return get_random_u32() & (val - 1);
> }
> }
> BUILD_BUG_ON(sizeof (val) > 4);
> return ((u64)get_random_u32() * val) >> 32;
This is already how the prandom_u32_max() implementation works, as
suggested in the cover letter. The multiplication by constants in it
reduces to bit shifts and you already get all the manual masking
possible.
> get_random_below() is a much better name than prandom_u32_max().
Yes, but that name is reserved for when I succeed at making a function
that bounds with a uniform distribution. prandom_u32_max()'s
distribution is non-uniform since it doesn't do rejection sampling. Work
in progress is on https://git.zx2c4.com/linux-rng/commit/?h=jd/get_random_u32_below .
But out of common respect for this already huge thread with a massive
CC list, if you want to bikeshed my WIP stuff, please start a new thread
for that and not bog this one down. IOW, no need to reply here directly.
That'd annoy me.
Jason
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