@@ -62,6 +62,8 @@
#include <linux/types.h>
#include <linux/param.h>
+unsigned long random_get_entropy_fallback(void);
+
#include <asm/timex.h>
#ifndef random_get_entropy
@@ -74,8 +76,14 @@
*
* 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 random_get_entropy_fallback().
*/
+#ifdef get_cycles
#define random_get_entropy() ((unsigned long)get_cycles())
+#else
+#define random_get_entropy() random_get_entropy_fallback()
+#endif
#endif
/*
@@ -17,6 +17,7 @@
#include <linux/clocksource.h>
#include <linux/jiffies.h>
#include <linux/time.h>
+#include <linux/timex.h>
#include <linux/tick.h>
#include <linux/stop_machine.h>
#include <linux/pvclock_gtod.h>
@@ -2380,6 +2381,15 @@ static int timekeeping_validate_timex(const struct __kernel_timex *txc)
return 0;
}
+/**
+ * random_get_entropy_fallback - Returns the raw clock source value,
+ * used by random.c for platforms with no valid random_get_entropy().
+ */
+unsigned long random_get_entropy_fallback(void)
+{
+ return tk_clock_read(&tk_core.timekeeper.tkr_mono);
+}
+EXPORT_SYMBOL_GPL(random_get_entropy_fallback);
/**
* do_adjtimex() - Accessor function to NTP __do_adjtimex function
The addition of random_get_entropy_fallback() provides access to whichever time source has the highest frequency, which is useful for gathering entropy on platforms without available cycle counters. It's not necessarily as good as being able to quickly access a cycle counter that the CPU has, but it's still something, even when it falls back to being jiffies-based. 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 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. Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Theodore Ts'o <tytso@mit.edu> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> --- include/linux/timex.h | 8 ++++++++ kernel/time/timekeeping.c | 10 ++++++++++ 2 files changed, 18 insertions(+)