On Tue, Aug 11, 2020 at 05:26:49AM +0000, George Spelvin wrote:
> On Mon, Aug 10, 2020 at 11:04:55PM +0200, Willy Tarreau wrote:
> > What could be improved is the way the input values are mixed (just
> > added hence commutative for now). I didn't want to call a siphash
> > round on the hot paths, but just shifting the previous noise value
> > before adding would work, such as the following for example:
> >
> > void prandom_u32_add_noise(a, b, c, d)
> > {
> > unsigned long *noise = get_cpu_ptr(&net_rand_noise);
> >
> > #if BITS_PER_LONG == 64
> > *noise = rol64(*noise, 7) + a + b + c + d;
> > #else
> > *noise = rol32(*noise, 7) + a + b + c + d;
> > #endif
> > put_cpu_ptr(&net_rand_noise);
> >
> > }
>
> If you think this is enough seed material, I'm fine with it.
>
> I don't hugely like the fact that you sum all the inputs, since
> entropy tends to be concentrated in the low-order words, and summing
> risks cancellation.
Yes I've figured this. But I thought it was still better than
a pure xor which would cancell the high bits from pointers.
> You can't afford even one SIPROUND as a non-cryptographic hash? E.g.
That's what I mentioned above, I'm still hesitating. I need to test.
>
> DEFINE_PER_CPU(unsigned long[4], net_rand_noise);
> EXPORT_SYMBOL(net_rand_noise);
>
> void prandom_u32_add_noise(a, b, c, d)
> {
> unsigned long *noise = get_cpu_ptr(&net_rand_noise);
>
> a ^= noise[0];
> b ^= noise[1];
> c ^= noise[2];
> d ^= noise[3];
> /*
> * This is not used cryptographically; it's just
> * a convenient 4-word hash function.
> */
> SIPROUND(a, b, c, d);
> noise[0] = a;
> noise[1] = b;
> noise[2] = c;
> put_cpu_ptr(&net_rand_noise);
> }
>
> (And then you mix in net_rand_noise[0].)
>
> Other options are HASH_MIX() from fs/namei.c, but that's
> more sequential.
>
> There's also a simple Xorshift generator.
I think a xorshift on each value will have roughly the same cost
as a single SIPROUND. But I've not yet completely eliminated these
options until I've tested. If we lose a few cycles per packet, that
might be OK.
Willy
