linux-kernel - Re: [PATCH v6 11/17] openrisc: use fallback for random_get

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Date:   Thu, 28 Apr 2022 06:42:30 +0900
From:   Stafford Horne <shorne@...il.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        tglx@...utronix.de, arnd@...db.de, Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>
Subject: Re: [PATCH v6 11/17] openrisc: use fallback for random_get_entropy()
 instead of zero

On Sat, Apr 23, 2022 at 11:26:17PM +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 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@...utronix.de>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Jonas Bonn <jonas@...thpole.se>
> Cc: Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>
> Cc: Stafford Horne <shorne@...il.com>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  arch/openrisc/include/asm/timex.h | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/openrisc/include/asm/timex.h b/arch/openrisc/include/asm/timex.h
> index d52b4e536e3f..115e89b336cd 100644
> --- a/arch/openrisc/include/asm/timex.h
> +++ b/arch/openrisc/include/asm/timex.h
> @@ -23,6 +23,9 @@ static inline cycles_t get_cycles(void)
>  {
>  	return mfspr(SPR_TTCR);
>  }
> +#define get_cycles get_cycles
> +
> +#define random_get_entropy() (((unsigned long)get_cycles()) ?: random_get_entropy_fallback())
>  
>  /* This isn't really used any more */
>  #define CLOCK_TICK_RATE 1000
> -- 
> 2.35.1

Hi Jason,

This looks OK, but I am wondering why we cannot add this to
"include/linux/timex.h" as the default implementation of random_get_entropy
if get_cycles is defined.  I see there are 2 cases:

   1. architectures that define get_cycles, and random_get_entropy is defined in
      include/linux/timex.h.
      (openrisc, sparc*, xtensa*, nios2, um*, arm)

   2. architectures that define random_get_entropy explicitly
      (mips, m68k, riscv, x86)

* Those marked with * just define get_cycles as 0.

I would think in "include/linux/timex.h" we could define.

    #ifndef random_get_entropy
    #ifdef get_cycles
    #define random_get_entropy() (((unsigned long)get_cycles()) ?: random_get_entropy_fallback())
    #else
    #define random_get_entropy()   random_get_entropy_fallback()
    #endif
    #endif

For architectures that define get_cycles as 0, they can be cleaned up.

-Stafford