linux-kernel - Re: [RFC PATCH 1/3] hw_random: allow RNG devices to give early randomness after a delay

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Message-ID: <20140714123700.GP13108@titan.lakedaemon.net>
Date:	Mon, 14 Jul 2014 08:37:00 -0400
From:	Jason Cooper <jason@...edaemon.net>
To:	Amit Shah <amit.shah@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Virtualization List <virtualization@...ts.linux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	herbert@...dor.apana.org.au, keescook@...omium.org
Subject: Re: [RFC PATCH 1/3] hw_random: allow RNG devices to give early
 randomness after a delay

On Mon, Jul 14, 2014 at 10:05:19AM +0530, Amit Shah wrote:
> Some RNG devices may not be ready to give early randomness at probe()
> time, and hence lose out on the opportunity to contribute to system
> randomness at boot- or device hotplug- time.
> 
> This commit schedules a delayed work item for such devices, and fetches
> early randomness after a delay.  Currently the delay is 500ms, which is
> enough for the lone device that needs such treatment: virtio-rng.
> 
> CC: Kees Cook <keescook@...omium.org>
> CC: Jason Cooper <jason@...edaemon.net>
> CC: Herbert Xu <herbert@...dor.apana.org.au>
> Signed-off-by: Amit Shah <amit.shah@...hat.com>
> ---
>  drivers/char/hw_random/core.c | 20 +++++++++++++++++++-
>  include/linux/hw_random.h     |  8 ++++++++
>  2 files changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index c4419ea..2a765fd 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -63,7 +63,7 @@ static size_t rng_buffer_size(void)
>  	return SMP_CACHE_BYTES < 32 ? 32 : SMP_CACHE_BYTES;
>  }
>  
> -static void add_early_randomness(struct hwrng *rng)
> +static void get_early_randomness(struct hwrng *rng)
>  {
>  	unsigned char bytes[16];
>  	int bytes_read;
> @@ -79,6 +79,21 @@ static void add_early_randomness(struct hwrng *rng)
>  		add_device_randomness(bytes, bytes_read);
>  }
>  
> +static void sched_init_random(struct work_struct *work)
> +{
> +	struct hwrng *rng = container_of(work, struct hwrng, dwork.work);
> +
> +	get_early_randomness(rng);
> +}
> +
> +static void add_early_randomness(struct hwrng *rng)

The add/get naming seems awkward in the above hunks.

> +{
> +	if (!(rng->flags & HWRNG_DELAY_READ_AT_INIT))
> +		return get_early_randomness(rng);
> +
> +	schedule_delayed_work(&rng->dwork, msecs_to_jiffies(500));
> +}
> +

Perhaps instead of rng->flags and a hardcoded delay, we could have
rng->seed_delay = msecs_to_jiffies(500) in virtio-rng?  Then you can
just call unconditionally:

	schedule_delayed_work(&rng->dwork, rng->seed_delay);

I think that would be a more extensible solution should other drivers
show up with the same issue.

thx,

Jason.
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