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

[Jason A. Donenfeld]

Hi Eric,

On Sat, Mar 12, 2022 at 2:39 PM Eric Biggers <ebiggers@xxxxxxxxxx> 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