[PATCH RFC v1 05/10] arm: use sched_clock() for random_get_entropy() instead of zero

[Jason A. Donenfeld]

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 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: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
 arch/arm/include/asm/timex.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/arm/include/asm/timex.h b/arch/arm/include/asm/timex.h
index 7c3b3671d6c2..1c51580ee55d 100644
--- a/arch/arm/include/asm/timex.h
+++ b/arch/arm/include/asm/timex.h
@@ -9,7 +9,18 @@
 #ifndef _ASMARM_TIMEX_H
 #define _ASMARM_TIMEX_H
 
+#include <linux/sched/clock.h>
+
 typedef unsigned long cycles_t;
 #define get_cycles()	({ cycles_t c; read_current_timer(&c) ? 0 : c; })
 
+static inline unsigned long random_get_entropy(void)
+{
+	unsigned long ret = get_cycles();
+	if (ret)
+		return ret;
+	return sched_clock();
+}
+#define random_get_entropy random_get_entropy
+
 #endif
-- 
2.35.1