Re: [PATCH RFC v1 07/10] arm64: use sched_clock() for random_get_entropy() instead of zero

[Mark Rutland]

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@xxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
> ---
>  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
> 
> 
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