On Wed, Apr 13, 2022 at 6:55 AM Jason A. Donenfeld <Jason@xxxxxxxxx> wrote: > > 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@xxxxxxxxxxxxx> > Cc: Arnd Bergmann <arnd@xxxxxxxx> > Cc: Theodore Ts'o <tytso@xxxxxxx> > Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx> > --- > include/linux/timex.h | 8 ++++++++ > kernel/time/timekeeping.c | 10 ++++++++++ > 2 files changed, 18 insertions(+) > > diff --git a/include/linux/timex.h b/include/linux/timex.h > index 5745c90c8800..fbbe34226044 100644 > --- a/include/linux/timex.h > +++ b/include/linux/timex.h > @@ -62,6 +62,8 @@ > #include <linux/types.h> > #include <linux/param.h> > > +extern 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 'does not have a usable get_cycles(), ...' as clearly some arches have get_cycles() and yet still need a fallback. Why not handle the 'if get_cycles() returns 0 do the fallback' within a weak random_get_entropy() function? Then more arches don't need any random_get_entropy() implementation. Rob
