diff mbox series

[v5,08/11] um: use fallback for random_get_entropy() instead of zero

Message ID 20220419111650.1582274-9-Jason@zx2c4.com (mailing list archive)
State New, archived
Headers show
Series archs/random: fallback to best raw ktime when no cycle counter | expand

Commit Message

Jason A. Donenfeld April 19, 2022, 11:16 a.m. UTC
In the event that random_get_entropy() can't access a cycle counter or
similar, falling back to returning 0 is really not the best we can do.
Instead, at least calling random_get_entropy_fallback() would be
preferable, because that always needs to return _something_, even
falling back to jiffies eventually. It's not as though
random_get_entropy_fallback() is super high precision or guaranteed to
be entropic, but basically anything that's not zero all the time is
better than returning zero all the time.

This is accomplished by just including the asm-generic code like on
other architectures, which means we can get rid of the empty stub
function here.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 arch/um/include/asm/timex.h | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

Comments

Johannes Berg April 19, 2022, 11:33 a.m. UTC | #1
On Tue, 2022-04-19 at 13:16 +0200, Jason A. Donenfeld wrote:
> In the event that random_get_entropy() can't access a cycle counter or
> similar, falling back to returning 0 is really not the best we can do.
> Instead, at least calling random_get_entropy_fallback() would be
> preferable, because that always needs to return _something_, even
> falling back to jiffies eventually. It's not as though
> random_get_entropy_fallback() is super high precision or guaranteed to
> be entropic, but basically anything that's not zero all the time is
> better than returning zero all the time.
> 
> This is accomplished by just including the asm-generic code like on
> other architectures, which means we can get rid of the empty stub
> function here.


LGTM, actually better than before, though not even sure any drivers in
ARCH=um have interrupts that say they can be used for this.

Acked-by: Johannes Berg <johannes@sipsolutions.net>

I assume you're going to take this through the random tree?

johannes
Jason A. Donenfeld April 19, 2022, 11:37 a.m. UTC | #2
Hi Johannes,

On Tue, Apr 19, 2022 at 01:33:21PM +0200, Johannes Berg wrote:
> On Tue, 2022-04-19 at 13:16 +0200, Jason A. Donenfeld wrote:
> > In the event that random_get_entropy() can't access a cycle counter or
> > similar, falling back to returning 0 is really not the best we can do.
> > Instead, at least calling random_get_entropy_fallback() would be
> > preferable, because that always needs to return _something_, even
> > falling back to jiffies eventually. It's not as though
> > random_get_entropy_fallback() is super high precision or guaranteed to
> > be entropic, but basically anything that's not zero all the time is
> > better than returning zero all the time.
> > 
> > This is accomplished by just including the asm-generic code like on
> > other architectures, which means we can get rid of the empty stub
> > function here.
> 
> 
> LGTM, actually better than before, though not even sure any drivers in
> ARCH=um have interrupts that say they can be used for this.
> 
> Acked-by: Johannes Berg <johannes@sipsolutions.net>
> 
> I assume you're going to take this through the random tree?

Right, that's the plan (as mentioned in the cover letter). Changes to
random.c will depend on these patches being there.

Jason
diff mbox series

Patch

diff --git a/arch/um/include/asm/timex.h b/arch/um/include/asm/timex.h
index e392a9a5bc9b..9f27176adb26 100644
--- a/arch/um/include/asm/timex.h
+++ b/arch/um/include/asm/timex.h
@@ -2,13 +2,8 @@ 
 #ifndef __UM_TIMEX_H
 #define __UM_TIMEX_H
 
-typedef unsigned long cycles_t;
-
-static inline cycles_t get_cycles (void)
-{
-	return 0;
-}
-
 #define CLOCK_TICK_RATE (HZ)
 
+#include <asm-generic/timex.h>
+
 #endif