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

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 92e6c67..2d5daf9 100644 (file)
--- 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);

diff --git a/include/linux/timex.h b/include/linux/timex.h
index b3726e6..da4c32d 100644 (file)
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -64,6 +64,20 @@
 
 #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.
+ */
+#define random_get_entropy()   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.