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SubjectRe: [PATCH] random: cap jitter samples per bit to factor of HZ
From
On 7/13/22 16:11, Jason A. Donenfeld wrote:
> Currently the jitter mechanism will require two timer ticks per
> iteration, and it requires N iterations per bit. This N is determined
> with a small measurement, and if it's too big, it won't waste time with
> jitter entropy because it'd take too long or not have sufficient entropy
> anyway.
>
> With the current max N of 32, there are large timeouts on systems with a
> small CONFIG_HZ. Rather than set that maximum to 32, instead choose a
> factor of CONFIG_HZ. In this case, 1/30 seems to yield sane values for
> different configurations of CONFIG_HZ.
>
> Reported-by: Vladimir Murzin <vladimir.murzin@arm.com>
> Fixes: 78c768e619fb ("random: vary jitter iterations based on cycle counter speed")
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> Vladimir - Can you let me know if this appears to fix the issue you're
> seeing? -Jason

Works for me, thanks! :)

>
> drivers/char/random.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index e3dd1dd3dd22..a1af90bacc9f 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1174,7 +1174,7 @@ static void __cold entropy_timer(struct timer_list *timer)
> */
> static void __cold try_to_generate_entropy(void)
> {
> - enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = 32 };
> + enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 30 };
> struct entropy_timer_state stack;
> unsigned int i, num_different = 0;
> unsigned long last = random_get_entropy();

FWIW

Tested-by: Vladimir Murzin <vladimir.murzin@arm.com>

Cheers
Vladimir

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