Messages in this thread | | | Date | Fri, 8 Apr 2022 19:33:53 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH RFC v1 07/10] arm64: use sched_clock() for random_get_entropy() instead of zero |
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On Fri, Apr 08, 2022 at 08:21:42PM +0200, Jason A. Donenfeld wrote: > 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. > > If CONFIG_ARM_ARCH_TIMER=n, then get_cycles() will return 0, so we only > need the fallback code for that case.
In arch/arm64/Kconfig we unconditionally select CONFIG_ARM_ARCH_TIMER, so that configuration shouldn't be possible, and I don't think this patch is necessary.
On arm64 we depend on the architected timer in a bunch of places, so anyone hacking that out has bigger problems.
Thanks, Mark.
> > Cc: Arnd Bergmann <arnd@arndb.de> > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: Will Deacon <will@kernel.org> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> > --- > arch/arm64/include/asm/timex.h | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/arch/arm64/include/asm/timex.h b/arch/arm64/include/asm/timex.h > index cf59ce91b22d..bfebd2e1ce45 100644 > --- a/arch/arm64/include/asm/timex.h > +++ b/arch/arm64/include/asm/timex.h > @@ -13,6 +13,15 @@ > */ > #define get_cycles() arch_timer_read_counter() > > +#ifndef CONFIG_ARM_ARCH_TIMER > +/* > + * The default implementation of random_get_entropy() calls get_cycles(), > + * which will return 0 if CONFIG_ARM_ARCH_TIMER=n, so we fall back to > + * sched_clock() here. Not a great situation, but better than nothing. > + */ > +#define random_get_entropy() ((unsigned long)sched_clock()) > +#endif > + > #include <asm-generic/timex.h> > > #endif > -- > 2.35.1 > > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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