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[83.34.184.239]) by smtp.gmail.com with ESMTPSA id o125sm5847821wma.27.2020.08.05.23.30.41 (version=TLS1_2 cipher=ECDHE-ECDSA-CHACHA20-POLY1305 bits=256/256); Wed, 05 Aug 2020 23:30:42 -0700 (PDT) From: "Jorge Ramirez-Ortiz, Foundries" X-Google-Original-From: "Jorge Ramirez-Ortiz, Foundries" Date: Thu, 6 Aug 2020 08:30:40 +0200 To: Sumit Garg Cc: "Jorge Ramirez-Ortiz, Foundries" , Matt Mackall , Herbert Xu , Jens Wiklander , Arnd Bergmann , ricardo@foundries.io, Michael Scott , Greg Kroah-Hartman , op-tee@lists.trustedfirmware.org, "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" , Linux Kernel Mailing List Subject: Re: [PATCHv2 2/2] hwrng: optee: fix wait use case Message-ID: <20200806063040.GA27943@trex> References: <20200723084622.31134-1-jorge@foundries.io> <20200723084622.31134-2-jorge@foundries.io> <20200724142305.GA24164@trex> <20200805203817.GA12229@trex> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-crypto-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org On 06/08/20, Sumit Garg wrote: > On Thu, 6 Aug 2020 at 02:08, Jorge Ramirez-Ortiz, Foundries > 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 > > > wrote: > > > > > > > > On 24/07/20, Sumit Garg wrote: > > > > > On Thu, 23 Jul 2020 at 14:16, Jorge Ramirez-Ortiz 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 > > > > > > --- > > > > > > 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? > > -Sumit > > > > > > > > > -Sumit > > > > > > > > > > > > > > > > > -Sumit > > > > > > > > > > > return read; > > > > > > msleep((1000 * (max - read)) / pvt_data->data_rate); > > > > > > } else { > > > > > > -- > > > > > > 2.17.1 > > > > > >