linux-kernel - Re: [PATCH v2] random: use SipHash as interrupt entropy accumulator

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Date:   Sat, 12 Mar 2022 17:07:05 -0700
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>
Subject: Re: [PATCH v2] random: use SipHash as interrupt entropy accumulator

Hi Eric,

On Sat, Mar 12, 2022 at 2:39 PM Eric Biggers <ebiggers@...nel.org> wrote:
> On Sun, Mar 06, 2022 at 09:51:23AM -0700, Jason A. Donenfeld wrote:
> > For this, we make use of SipHash-1-x on 64-bit and HalfSipHash-1-x on
> > 32-bit, which are already in use in the kernel and achieve the same
> > performance as the function they replace. It would be nice to do two
> > rounds, but we don't exactly have the CPU budget handy for that, and one
> > round alone is already sufficient.
> >
>
> I'm a bit confused by the argument here.  It's not SipHash-1-x that's used
> elsewhere in the kernel, but rather SipHash-2-4.  HalfSipHash-1-3 is used too

Actually the hsiphash family of functions are aliased to SipHash-1-3 on 64-bit:

/* Note that on 64-bit, we make HalfSipHash1-3 actually be SipHash1-3, for
 * performance reasons. On 32-bit, below, we actually implement HalfSipHash1-3.
 */

> So on 64-bit platforms it now throws away half of the pool.
>
> It should use 'u8 pool[sizeof(fast_pool->pool)]' to avoid hardcoding a size.

Actually the commit message notes that we intentionally dump half of
it on 64bit. This is intentional.

Jason