From: David Laight <David.Laight@ACULAB.COM>
To: "'Jason A. Donenfeld'" <Jason@zx2c4.com>,
Kees Cook <keescook@chromium.org>
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Subject: RE: [PATCH v3 3/5] treewide: use get_random_u32() when possible
Date: Sat, 8 Oct 2022 21:53:33 +0000 [thread overview]
Message-ID: <69080fb8cace486db4e28e2e90f1d550@AcuMS.aculab.com> (raw)
In-Reply-To: <Y0BoQmVauPLC2uW5@zx2c4.com>
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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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2022-10-08 21:54 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-06 16:53 [PATCH v3 0/5] treewide cleanup of random integer usage Jason A. Donenfeld
2022-10-06 16:53 ` [PATCH v3 1/5] treewide: use prandom_u32_max() when possible Jason A. Donenfeld
2022-10-06 16:53 ` [PATCH v3 2/5] treewide: use get_random_{u8,u16}() " Jason A. Donenfeld
2022-10-06 16:53 ` [PATCH v3 3/5] treewide: use get_random_u32() " Jason A. Donenfeld
2022-10-06 17:21 ` Christophe Leroy
2022-10-06 17:24 ` Jason A. Donenfeld
2022-10-06 17:31 ` Christophe Leroy
2022-10-06 17:42 ` Christophe Leroy
2022-10-06 23:36 ` Jason A. Donenfeld
2022-10-07 4:57 ` Christophe Leroy
2022-10-07 14:07 ` Jason A. Donenfeld
2022-10-07 17:12 ` Kees Cook
2022-10-07 17:56 ` Jason A. Donenfeld
2022-10-08 21:53 ` David Laight [this message]
2022-10-09 2:57 ` Jason A. Donenfeld
2022-10-07 9:14 ` David Laight
2022-10-07 9:26 ` Mika Westerberg
2022-10-06 16:53 ` [PATCH v3 4/5] treewide: use get_random_bytes " Jason A. Donenfeld
2022-10-06 17:22 ` Christophe Leroy
2022-10-07 3:09 ` Bagas Sanjaya
2022-10-07 3:23 ` Jason A. Donenfeld
2022-10-06 16:53 ` [PATCH v3 5/5] prandom: remove unused functions Jason A. Donenfeld
2022-10-07 9:11 ` [PATCH v3 0/5] treewide cleanup of random integer usage Ulf Hansson
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