linux-kernel - Re: [PATCH 02/12] random: make 'add_interrupt

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Date:	Sun, 08 Jul 2012 03:01:09 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	ewust@...ch.edu, zakir@...ch.edu, nadiah@...ucsd.edu,
	jhalderm@...ch.edu, Linus Torvalds <torvalds@...ux-foundation.org>,
	stable@...r.kernel.org
Subject: Re: [PATCH 02/12] random: make 'add_interrupt_randomness()' do
 something sane

On Fri, 2012-07-06 at 18:44 -0400, Theodore Ts'o wrote:
> We've been moving away from add_interrupt_randomness() for various
> reasons: it's too expensive to do on every interrupt, and flooding the
> CPU with interrupts could theoretically cause bogus floods of entropy
> from a somewhat externally controllable source.
> 
> This solves both problems by limiting the actual randomness addition
> to just once a second or after 128 interrupts, whicever comes first.

Seems to be once per 64 interrupts.  add_interrupt_randomness() calls
fast_mix() with nbytes = 20, so it increments fast_pool->count by 20.
add_interrupt_randomness() returns early if (fast_pool->count & 255),
and 64 * 20 == 0x500.

[...]
> +static void fast_mix(struct fast_pool *f, const void *in, int nbytes)
> +{
> +       const char      *bytes = in;
> +       __u32           w;
> +       unsigned        i = f->count;
> +       unsigned        input_rotate = f->rotate;
> +
> +       while (nbytes--) {
> +               w = rol32(*bytes++, input_rotate & 31) ^ f->pool[i & 3] ^
> +                       f->pool[(i + 1) & 3];
> +               f->pool[i & 3] = (w >> 3) ^ twist_table[w & 7];
> +               input_rotate += (i++ & 3) ? 7 : 14;
> +       }
> +       f->count = i;
> +       f->rotate = input_rotate;
> +}
[...]

This seems like a comparatively expensive operation to add to every
interrupt. :-/

Ben.

-- 
Ben Hutchings
Life would be so much easier if we could look at the source code.

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