From 95b709b6be49e4ff3933ef6a5b5e623de2713a71 Mon Sep 17 00:00:00 2001 From: Theodore Ts'o Date: Wed, 2 Oct 2013 21:10:35 -0400 Subject: [PATCH] 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" --- drivers/char/random.c | 19 ++----------------- 1 file changed, 2 insertions(+), 17 deletions(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 7ae7ea65da68..6da3f250804c 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. @@ -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(); } -- 2.39.5