random: Fix fast_mix() function

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>
Cc: stable@vger.kernel.org  # v3.16+
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 04645c09fe5e5eee7f6699c451e9e4f6f8368958..9cd6968e2f924bf7eb5c545c4298445651d5665d 100644 (file)
--- 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;