| Message ID | 20220408182145.142506-8-Jason@zx2c4.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | archs/random: fallback to using sched_clock() if no cycle counter | expand |
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
Hi Mark, On Fri, Apr 08, 2022 at 07:33:53PM +0100, Mark Rutland wrote: > 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 for the tip. I'll drop this patch from the series. Jason
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
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. 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(+)