[PATCH RFC v1 01/10] random: use sched_clock() for random_get_entropy() if no get_cycles()

From: Jason A. Donenfeld
Date: Fri Apr 08 2022 - 14:22:39 EST


In the event that a given arch does not define get_cycles(), falling
back to the get_cycles() default implementation that returns 0 is really
not the best we can do. Instead, at least calling sched_clock() would be
preferable, because that always needs to return _something_, even
falling back to jiffies eventually. It's not as though sched_clock() 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.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
include/linux/timex.h | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/include/linux/timex.h b/include/linux/timex.h
index 5745c90c8800..bd78f784762e 100644
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -61,6 +61,7 @@
#include <linux/compiler.h>
#include <linux/types.h>
#include <linux/param.h>
+#include <linux/sched/clock.h>

#include <asm/timex.h>

@@ -74,8 +75,13 @@
*
* By default we use get_cycles() for this purpose, but individual
* architectures may override this in their asm/timex.h header file.
+ * If a given arch does not have get_cycles(), then we fallback to
+ * using sched_clock().
*/
+#ifdef get_cycles
#define random_get_entropy() ((unsigned long)get_cycles())
+#else
+#define random_get_entropy() ((unsigned long)sched_clock())
#endif

/*
--
2.35.1