linux-kernel - Re: [PATCH, RFC 07/12] random: allow architectures to optionally define random_get

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Date:	Mon, 23 Sep 2013 12:38:26 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	hpa@...or.com, joern@...fs.org, macro@...ux-mips.org,
	ralf@...ux-mips.org, dave.taht@...il.com, blogic@...nwrt.org,
	andrewmcgr@...il.com, geert@...ux-m68k.org, tg@...bsd.de
Subject: Re: [PATCH, RFC 07/12] random: allow architectures to optionally define random_get_entropy()

Am Sonntag, 22. September 2013, 16:38:53 schrieb Theodore Ts'o:

Hi Theodore,

>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.

Are there any plans to get such function implemented on large list of 
architectures without a get_cycles function?

Thanks
>
>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@....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.


Ciao
Stephan
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