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random: drop trickle mode

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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>
This commit is contained in:
Theodore Ts'o 2013-10-02 21:10:35 -04:00
parent 6e9fa2c8a6
commit 95b709b6be
1 changed files with 2 additions and 17 deletions
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19
drivers/char/random.c
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@ -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.
@ -313,14 +311,6 @@ 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
@ -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();
}