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

[Amit Shah <amit.shah@redhat.com>]

On (Mon) 14 Jul 2014 [08:37:00], Jason Cooper wrote:
> 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@chromium.org>
> > CC: Jason Cooper <jason@lakedaemon.net>
> > CC: Herbert Xu <herbert@gondor.apana.org.au>
> > Signed-off-by: Amit Shah <amit.shah@redhat.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.

Yea; I felt that too.  I thought of a do_add_early_randomness()
instead, but that seemed awkward too.  I forgot to mention I was
planning on revisiting this naming for v1.

> > +{
> > +	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.

Sounds like a good idea to me.  Though, changes in core.c that
increase the time in hwrng_register() or hwrng_init() may not get
noticed by rng drivers and they may suddenly start failing for no
apparent reason.  Seems like a far stretch, though.  Does anyone else
have an opinion on this?

Thanks,

		Amit