random: allow architectures to optionally define random_get_entropy()

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>
Cc: stable@vger.kernel.org

1 files changed, 4 insertions, 4 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 92e6c67e1ae6..2d5daf9b58e9 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -643,7 +643,7 @@ 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;
 
         mix_pool_bytes(&input_pool, buf, size, NULL);
         mix_pool_bytes(&input_pool, &time, sizeof(time), NULL);
@@ -680,7 +680,7 @@ 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);
 
@@ -747,7 +747,7 @@ 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;
@@ -1485,7 +1485,7 @@ 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);