netdev - Re: [DRAFT PATCH] random32: make prandom

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Date:   Mon, 10 Aug 2020 16:48:29 +0200
From:   Willy Tarreau <w@....eu>
To:     David Laight <David.Laight@...lab.com>
Cc:     George Spelvin <lkml@....org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "aksecurity@...il.com" <aksecurity@...il.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "Jason@...c4.com" <Jason@...c4.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tytso@....edu" <tytso@....edu>,
        "lkml.mplumb@...il.com" <lkml.mplumb@...il.com>,
        "stephen@...workplumber.org" <stephen@...workplumber.org>,
        "fw@...len.de" <fw@...len.de>
Subject: Re: [DRAFT PATCH] random32: make prandom_u32() output unpredictable

On Mon, Aug 10, 2020 at 12:01:11PM +0000, David Laight wrote:
> >  /*
> >   * On 32-bit machines, we use HSipHash, a reduced-width version of SipHash.
> >   * This is weaker, but 32-bit machines are not used for high-traffic
> > @@ -375,6 +377,12 @@ static u32 siprand_u32(struct siprand_state *s)
> >  {
> >  	unsigned long v0 = s->v[0], v1 = s->v[1], v2 = s->v[2], v3 = s->v[3];
> > 
> > +	if (++s->count >= 8) {
> > +		v3 ^= s->noise;
> > +		s->noise += random_get_entropy();
> > +		s->count = 0;
> > +	}
> > +
> 
> Using:
> 	if (s->count-- <= 0) {
> 		...
> 		s->count = 8;
> 	}
> probably generates better code.
> Although you may want to use a 'signed int' instead of 'unsigned long'.

Yeah I know, it's just because I only slightly changed the previous code
there. I had an earlier version that kept the rand state fully padded
when storing intermediate values. That's among the final cleanups I'll
bring if we go down that route.

Thanks!

Willy