random: drop trickle mode

The add_timer_randomness() used to drop into trickle mode when entropy
pool was estimated to be 87.5% full.  This was important when
add_timer_randomness() was used to sample interrupts.  It's not used
for this any more --- add_interrupt_randomness() now uses fast_mix()
instead.  By elimitating trickle mode, it allows us to fully utilize
entropy provided by add_input_randomness() and add_disk_randomness()
even when the input pool is above the old trickle threshold of 87.5%.

This helps to answer the criticism in [1] in their hypothetical
scenario where our entropy estimator was inaccurate, even though the
measurements in [2] seem to indicate that our entropy estimator given
real-life entropy collection is actually pretty good, albeit on the
conservative side (which was as it was designed).

[1] http://eprint.iacr.org/2013/338.pdf
[2] http://eprint.iacr.org/2012/251.pdf

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 2 insertions, 17 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 7ae7ea6..6da3f25 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -282,10 +282,8 @@
 #define LONGS(x) (((x) + sizeof(unsigned long) - 1)/sizeof(unsigned long))
 
 /*
- * To allow fractional bits to be tracked, the following fields contain
- * this many fractional bits:
- *
- * entropy_count, trickle_thresh
+ * To allow fractional bits to be tracked, the entropy_count field is
+ * denominated in units of 1/8th bits.
  *
  * 2*(ENTROPY_SHIFT + log2(poolbits)) must <= 31, or the multiply in
  * credit_entropy_bits() needs to be 64 bits wide.
@@ -314,14 +312,6 @@ static int random_write_wakeup_thresh = 128;
 static int random_min_urandom_seed = 60;
 
 /*
- * When the input pool goes over trickle_thresh, start dropping most
- * samples to avoid wasting CPU time and reduce lock contention.
- */
-static const int trickle_thresh = (INPUT_POOL_WORDS * 28) << ENTROPY_SHIFT;
-
-static DEFINE_PER_CPU(int, trickle_count);
-
-/*
  * Originally, we used a primitive polynomial of degree .poolwords
  * over GF(2).  The taps for various sizes are defined below.  They
  * were chosen to be evenly spaced except for the last tap, which is 1
@@ -755,10 +745,6 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 	long delta, delta2, delta3;
 
 	preempt_disable();
-	/* if over the trickle threshold, use only 1 in 4096 samples */
-	if (ENTROPY_BITS(&input_pool) > trickle_thresh &&
-	    ((__this_cpu_inc_return(trickle_count) - 1) & 0xfff))
-		goto out;
 
 	sample.jiffies = jiffies;
 	sample.cycles = random_get_entropy();
@@ -800,7 +786,6 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 		credit_entropy_bits(&input_pool,
 				    min_t(int, fls(delta>>1), 11));
 	}
-out:
 	preempt_enable();
 }