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random: Fix fast_mix() function

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There was a bad typo in commit 43759d4f42 ("random: use an improved
fast_mix() function") and I didn't notice because it "looked right", so
I saw what I expected to see when I reviewed it.

Only months later did I look and notice it's not the Threefish-inspired
mix function that I had designed and optimized.

Mea Culpa.  Each input bit still has a chance to affect each output bit,
and the fast pool is spilled *long* before it fills, so it's not a total
disaster, but it's definitely not the intended great improvement.

I'm still working on finding better rotation constants.  These are good
enough, but since it's unrolled twice, it's possible to get better
mixing for free by using eight different constants rather than repeating
the same four.

Signed-off-by: George Spelvin <linux@horizon.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org  # v3.16+
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
George Spelvin 2015-02-07 00:32:06 -05:00 committed by Linus Torvalds
parent bfa76d4957
commit 19acc77a36
1 changed files with 4 additions and 4 deletions
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8
drivers/char/random.c
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@ -569,19 +569,19 @@ static void fast_mix(struct fast_pool *f)
__u32 c = f->pool[2], d = f->pool[3];
a += b; c += d;
b = rol32(a, 6); d = rol32(c, 27);
b = rol32(b, 6); d = rol32(d, 27);
d ^= a; b ^= c;
a += b; c += d;
b = rol32(a, 16); d = rol32(c, 14);
b = rol32(b, 16); d = rol32(d, 14);
d ^= a; b ^= c;
a += b; c += d;
b = rol32(a, 6); d = rol32(c, 27);
b = rol32(b, 6); d = rol32(d, 27);
d ^= a; b ^= c;
a += b; c += d;
b = rol32(a, 16); d = rol32(c, 14);
b = rol32(b, 16); d = rol32(d, 14);
d ^= a; b ^= c;
f->pool[0] = a; f->pool[1] = b;