Re: [PATCHv2 2/2] hwrng: optee: fix wait use case

[Jorge Ramirez-Ortiz, Foundries]

On 06/08/20, Sumit Garg wrote:
> On Thu, 6 Aug 2020 at 12:00, Jorge Ramirez-Ortiz, Foundries
> <jorge@xxxxxxxxxxxx> wrote:
> >
> > On 06/08/20, Sumit Garg wrote:
> > > On Thu, 6 Aug 2020 at 02:08, Jorge Ramirez-Ortiz, Foundries
> > > <jorge@xxxxxxxxxxxx> wrote:
> > > >
> > > > On 05/08/20, Sumit Garg wrote:
> > > > > Apologies for my delayed response as I was busy with some other tasks
> > > > > along with holidays.
> > > >
> > > > no pb! was just making sure this wasnt falling through some cracks.
> > > >
> > > > >
> > > > > On Fri, 24 Jul 2020 at 19:53, Jorge Ramirez-Ortiz, Foundries
> > > > > <jorge@xxxxxxxxxxxx> wrote:
> > > > > >
> > > > > > On 24/07/20, Sumit Garg wrote:
> > > > > > > On Thu, 23 Jul 2020 at 14:16, Jorge Ramirez-Ortiz <jorge@xxxxxxxxxxxx> wrote:
> > > > > > > >
> > > > > > > > The current code waits for data to be available before attempting a
> > > > > > > > second read. However the second read would not be executed as the
> > > > > > > > while loop exits.
> > > > > > > >
> > > > > > > > This fix does not wait if all data has been read and reads a second
> > > > > > > > time if only partial data was retrieved on the first read.
> > > > > > > >
> > > > > > > > This fix also does not attempt to read if not data is requested.
> > > > > > >
> > > > > > > I am not sure how this is possible, can you elaborate?
> > > > > >
> > > > > > currently, if the user sets max 0, get_optee_rng_data will regardless
> > > > > > issuese a call to the secure world requesting 0 bytes from the RNG
> > > > > >
> > > > >
> > > > > This case is already handled by core API: rng_dev_read().
> > > >
> > > > ah ok good point, you are right
> > > > but yeah, there is no consequence to the actual patch.
> > > >
> > >
> > > So, at least you could get rid of the corresponding text from commit message.
> > >
> > > > >
> > > > > > with this patch, this request is avoided.
> > > > > >
> > > > > > >
> > > > > > > >
> > > > > > > > Signed-off-by: Jorge Ramirez-Ortiz <jorge@xxxxxxxxxxxx>
> > > > > > > > ---
> > > > > > > >  v2: tidy up the while loop to avoid reading when no data is requested
> > > > > > > >
> > > > > > > >  drivers/char/hw_random/optee-rng.c | 4 ++--
> > > > > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/drivers/char/hw_random/optee-rng.c b/drivers/char/hw_random/optee-rng.c
> > > > > > > > index 5bc4700c4dae..a99d82949981 100644
> > > > > > > > --- a/drivers/char/hw_random/optee-rng.c
> > > > > > > > +++ b/drivers/char/hw_random/optee-rng.c
> > > > > > > > @@ -122,14 +122,14 @@ static int optee_rng_read(struct hwrng *rng, void *buf, size_t max, bool wait)
> > > > > > > >         if (max > MAX_ENTROPY_REQ_SZ)
> > > > > > > >                 max = MAX_ENTROPY_REQ_SZ;
> > > > > > > >
> > > > > > > > -       while (read == 0) {
> > > > > > > > +       while (read < max) {
> > > > > > > >                 rng_size = get_optee_rng_data(pvt_data, data, (max - read));
> > > > > > > >
> > > > > > > >                 data += rng_size;
> > > > > > > >                 read += rng_size;
> > > > > > > >
> > > > > > > >                 if (wait && pvt_data->data_rate) {
> > > > > > > > -                       if (timeout-- == 0)
> > > > > > > > +                       if ((timeout-- == 0) || (read == max))
> > > > > > >
> > > > > > > If read == max, would there be any sleep?
> > > > > >
> > > > > > no but I see no reason why there should be a wait since we already have
> > > > > > all the data that we need; the msleep is only required when we need to
> > > > > > wait for the RNG to generate entropy for the number of bytes we are
> > > > > > requesting. if we are requesting 0 bytes, the entropy is already
> > > > > > available. at leat this is what makes sense to me.
> > > > > >
> > > > >
> > > > > Wouldn't it lead to a call as msleep(0); that means no wait as well?
> > > >
> > > > I dont understand: there is no reason to wait if read == max and this
> > > > patch will not wait: if read == max it calls 'return read'
> > > >
> > > > am I misunderstanding your point?
> > >
> > > What I mean is that we shouldn't require this extra check here as
> > > there wasn't any wait if read == max with existing implementation too.
> >
> > um, I am getting confused Sumit
> >
> > with the exisiting implementation (the one we aim to replace), if get_optee_rng_data reads all the values requested on the first call (ie, read = 0) with wait set to true, the call will wait with msleep(0). Which is unnecessary and waits for a jiffy (ie, the call to msleep 0 will schedule a one jiffy timeout interrruptible)
> >
> > with this alternative implementation, msleep(0) does not get called.
> >
> > are we in synch?
> 
> Ah, I see msleep(0) also by default schedules timeout for 1 jiffy. So
> we are in sync now. Probably you can clarify this in commit message as
> well to avoid confusion.

ok will do.
shall I add your reviewed-by line or just resend?

> 
> -Sumit
> 
> >
> > >
> > > -Sumit
> > >
> > > >
> > > > >
> > > > > -Sumit
> > > > >
> > > > > >
> > > > > > >
> > > > > > > -Sumit
> > > > > > >
> > > > > > > >                                 return read;
> > > > > > > >                         msleep((1000 * (max - read)) / pvt_data->data_rate);
> > > > > > > >                 } else {
> > > > > > > > --
> > > > > > > > 2.17.1
> > > > > > > >