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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
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