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. Finally, since random_get_entropy_fallback() is used during extremely early boot when randomizing freelists in mm_init(), it can be called before timekeeping has been initialized. In that case there really is nothing we can do; jiffies hasn't even started ticking yet. So just give up and return 0.
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| @@ -17,6 +17,7 @@ · 16 unmodified lines | ||
| 17 | 17 | #include <linux/clocksource.h> |
| 18 | 18 | #include <linux/jiffies.h> |
| 19 | 19 | #include <linux/time.h> |
| 20 | +#include <linux/timex.h> | |
| 20 | 21 | #include <linux/tick.h> |
| 21 | 22 | #include <linux/stop_machine.h> |
| 22 | 23 | #include <linux/pvclock_gtod.h> |
| @@ -2380,6 +2381,20 @@ static int timekeeping_validate_timex(const struct __kernel_timex *txc) · 2357 unmodified lines | ||
| 2380 | 2381 | return 0; |
| 2381 | 2382 | } |
| 2382 | 2383 | |
| 2384 | +/** | |
| 2385 | + * random_get_entropy_fallback - Returns the raw clock source value, | |
| 2386 | + * used by random.c for platforms with no valid random_get_entropy(). | |
| 2387 | + */ | |
| 2388 | +unsigned long random_get_entropy_fallback(void) | |
| 2389 | +{ | |
| 2390 | + struct tk_read_base *tkr = &tk_core.timekeeper.tkr_mono; | |
| 2391 | + struct clocksource *clock = READ_ONCE(tkr->clock); | |
| 2392 | + | |
| 2393 | + if (unlikely(timekeeping_suspended || !clock)) | |
| 2394 | + return 0; | |
| 2395 | + return clock->read(clock); | |
| 2396 | +} | |
| 2397 | +EXPORT_SYMBOL_GPL(random_get_entropy_fallback); | |
| 2383 | 2398 | |
| 2384 | 2399 | /** |
| 2385 | 2400 | * do_adjtimex() - Accessor function to NTP __do_adjtimex function |