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

Message ID 20220408182145.142506-6-Jason@zx2c4.com (mailing list archive)
State New, archived
Headers show
Series archs/random: fallback to using sched_clock() if no cycle counter | expand

Commit Message

Jason A. Donenfeld April 8, 2022, 6:21 p.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 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@arndb.de>
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 arch/arm/include/asm/timex.h | 11 +++++++++++
 1 file changed, 11 insertions(+)
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Patch

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