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Subject[PATCH, RFC 07/12] random: allow architectures to optionally define random_get_entropy()
Date
Allow architectures which have a disabled get_cycles() function to
provide a random_get_entropy() function which provides a fine-grained,
rapidly changing counter that can be used by the /dev/random driver.

For example, an architecture might have a rapidly changing register
used to control random TLB cache eviction, or DRAM refresh that
doesn't meet the requirements of get_cycles(), but which is good
enough for the needs of the random driver.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
---
drivers/char/random.c | 8 ++++----
include/linux/timex.h | 17 +++++++++++++++++
2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 9564144..ba23d5c 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -707,7 +707,7 @@ struct timer_rand_state {
*/
void add_device_randomness(const void *buf, unsigned int size)
{
- unsigned long time = get_cycles() ^ jiffies;
+ unsigned long time = random_get_entropy() ^ jiffies;
unsigned long flags;

trace_add_device_randomness(size, _RET_IP_);
@@ -751,7 +751,7 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
goto out;

sample.jiffies = jiffies;
- sample.cycles = get_cycles();
+ sample.cycles = random_get_entropy();
sample.num = num;
mix_pool_bytes(&input_pool, &sample, sizeof(sample), NULL);

@@ -818,7 +818,7 @@ void add_interrupt_randomness(int irq, int irq_flags)
struct fast_pool *fast_pool = &__get_cpu_var(irq_randomness);
struct pt_regs *regs = get_irq_regs();
unsigned long now = jiffies;
- __u32 input[4], cycles = get_cycles();
+ __u32 input[4], cycles = random_get_entropy();

input[0] = cycles ^ jiffies;
input[1] = irq;
@@ -1580,7 +1580,7 @@ unsigned int get_random_int(void)

hash = get_cpu_var(get_random_int_hash);

- hash[0] += current->pid + jiffies + get_cycles();
+ hash[0] += current->pid + jiffies + random_get_entropy();
md5_transform(hash, random_int_secret);
ret = hash[0];
put_cpu_var(get_random_int_hash);
diff --git a/include/linux/timex.h b/include/linux/timex.h
index b3726e6..f9780cc 100644
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -64,6 +64,23 @@

#include <asm/timex.h>

+#ifndef random_get_entropy
+/*
+ * The random_get_entropy() function is used by the /dev/random driver
+ * in order to extract entropy via the relative unpredictability of
+ * when an interrupt takes places versus a high speed, fine-grained
+ * timing source or cycle counter. Since it will be occurred on every
+ * single interrupt, it must have a very low cost/overhead.
+ *
+ * By default we use get_cycles() for this purpose, but individual
+ * architectures may override this in their asm/timex.h header file.
+ */
+static inline cycles_t random_get_entropy(void)
+{
+ return get_cycles();
+}
+#endif
+
/*
* SHIFT_PLL is used as a dampening factor to define how much we
* adjust the frequency correction for a given offset in PLL mode.
--
1.7.12.rc0.22.gcdd159b


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