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Date: Sat, 8 Oct 2022 21:53:33 +0000
From: David Laight <David.Laight@...LAB.COM>
To: "'Jason A. Donenfeld'" <Jason@...c4.com>,
Kees Cook <keescook@...omium.org>
CC: 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>,
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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
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;
}
get_random_below() is a much better name than prandom_u32_max().
David
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