[v6,08/17] riscv: use fallback for random_get_entropy() instead of zero

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@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 arch/riscv/include/asm/timex.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Sat, 23 Apr 2022 14:26:14 PDT (-0700), Jason@zx2c4.com 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.

Makes sense: we had an architecturally-mandated timer at the time, but 
we don't any more.  Every real implementation has a timer right now, but 
that may change in the future so it doesn't hurt to fix it before it's 
broken.

> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Paul Walmsley <paul.walmsley@sifive.com>
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  arch/riscv/include/asm/timex.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/timex.h b/arch/riscv/include/asm/timex.h
> index 507cae273bc6..d6a7428f6248 100644
> --- a/arch/riscv/include/asm/timex.h
> +++ b/arch/riscv/include/asm/timex.h
> @@ -41,7 +41,7 @@ static inline u32 get_cycles_hi(void)
>  static inline unsigned long random_get_entropy(void)
>  {
>  	if (unlikely(clint_time_val == NULL))
> -		return 0;
> +		return random_get_entropy_fallback();
>  	return get_cycles();
>  }
>  #define random_get_entropy()	random_get_entropy()

Fine for me if this goes in via some other tree, but also happy to take 
it via the RISC-V tree if you'd like.  IMO we could just call this a 
fix, maybe

Fixes: aa9887608e77 ("RISC-V: Check clint_time_val before use")

(but that just brought this back, so there's likely older kernels broken 
too).  Shouldn't be breaking any real hardware, though, so no rush on my 
end.

Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>

Thanks!

Hi Palmer,

On Mon, Apr 25, 2022 at 4:55 PM Palmer Dabbelt <palmer@dabbelt.com> wrote:
> Fine for me if this goes in via some other tree, but also happy to take
> it via the RISC-V tree if you'd like.

I'm going to take this series through the random.git tree, as I've got
things that build on top of it for random.c slated for 5.19.

> IMO we could just call this a
> fix, maybe
>
> Fixes: aa9887608e77 ("RISC-V: Check clint_time_val before use")
>
> (but that just brought this back, so there's likely older kernels broken
> too).  Shouldn't be breaking any real hardware, though, so no rush on my
> end.

That'd be fine with me, but it'd involve also backporting the
timekeeping patch, which adds a new API, so maybe we better err on the
side of caution with that new code.

> Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
> Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>

Thanks for the review.

> Makes sense: we had an architecturally-mandated timer at the time, but
> we don't any more.

That's too bad. Out of curiosity, what happened? Was that deemed too
expensive for certain types of chips that western digital wanted to
produce for their hard drives, or some really constrained use case
like that?

Jason

On Mon, 25 Apr 2022 08:02:49 PDT (-0700), Jason@zx2c4.com wrote:
> Hi Palmer,
>
> On Mon, Apr 25, 2022 at 4:55 PM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>> Fine for me if this goes in via some other tree, but also happy to take
>> it via the RISC-V tree if you'd like.
>
> I'm going to take this series through the random.git tree, as I've got
> things that build on top of it for random.c slated for 5.19.
>
>> IMO we could just call this a
>> fix, maybe
>>
>> Fixes: aa9887608e77 ("RISC-V: Check clint_time_val before use")
>>
>> (but that just brought this back, so there's likely older kernels broken
>> too).  Shouldn't be breaking any real hardware, though, so no rush on my
>> end.
>
> That'd be fine with me, but it'd involve also backporting the
> timekeeping patch, which adds a new API, so maybe we better err on the
> side of caution with that new code.

wFM.  Like I said this isn't going to break any existing hardware, and 
anyone trying to ship something without the timers is likely going to be 
in for way more trouble than this so will probably be stuck with newer 
kernels anyway.

>> Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
>> Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
>
> Thanks for the review.
>
>> Makes sense: we had an architecturally-mandated timer at the time, but
>> we don't any more.
>
> That's too bad. Out of curiosity, what happened? Was that deemed too
> expensive for certain types of chips that western digital wanted to
> produce for their hard drives, or some really constrained use case
> like that?

No idea, but it was at the beginning of the "everything is 
optional"-ification of the ISA so I'm guessing it's just part of that.