[PATCH 3.19 009/175] random: Fix fast_mix() function

Greg Kroah-Hartman
2015-03-04T07:08:05Z

From: George Spelvin <linux@horizon.com>

commit 19acc77a36970958a4a0e4daeb2c8cb2aab0ffd4 upstream.

There was a bad typo in commit 43759d4f429c ("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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/char/random.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -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;