From: Mike Frysinger Subject: Re: [PATCH] hwrng: do not warn when there are no devices Date: Mon, 19 Jun 2017 12:03:37 -0700 Message-ID: References: <20170512041734.31945-1-vapier@gentoo.org> <20170619041234.GA12984@gondor.apana.org.au> <20170619062115.GA13498@gondor.apana.org.au> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Cc: Herbert Xu , Matt Mackall , linux-crypto@vger.kernel.org To: PrasannaKumar Muralidharan Return-path: Received: from mail-vk0-f48.google.com ([209.85.213.48]:33797 "EHLO mail-vk0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753150AbdFSTEJ (ORCPT ); Mon, 19 Jun 2017 15:04:09 -0400 Received: by mail-vk0-f48.google.com with SMTP id g66so57526347vki.1 for ; Mon, 19 Jun 2017 12:04:08 -0700 (PDT) In-Reply-To: Sender: linux-crypto-owner@vger.kernel.org List-ID: On Mon, Jun 19, 2017 at 2:43 AM, PrasannaKumar Muralidharan wrote: > On 19 June 2017 at 11:51, Herbert Xu wrote: >> On Sun, Jun 18, 2017 at 10:00:17PM -0700, Mike Frysinger wrote: >>> >>> in order to make tpm-rng react in the way you're implying, the TPM >>> subsystem would need to add a notification chain for transitions from >>> none<->some devices, then tpm-rng could subscribe to that, and during >>> those transition points, it would call hwrng_register/hwrng_unregister >>> to make itself visible accordingly to the hwrng subsystem. maybe >>> someone on the TPM side would be interested in writing all that logic, >>> but it sounds excessive for this minor usage. the current tpm-rng >>> driver is *extremely* simple -- it's 3 funcs, each of which are 1 >>> line. >> >> It's simple and it's broken, as far as the way it hooks into the >> hwrng is concerned. > > ********************************************************************************************* > diff --git a/drivers/char/hw_random/tpm-rng.c b/drivers/char/hw_random/tpm-rng.c > index d6d4482..4861b35 100644 > --- a/drivers/char/hw_random/tpm-rng.c > +++ b/drivers/char/hw_random/tpm-rng.c > @@ -22,6 +22,10 @@ > #include > > #define MODULE_NAME "tpm-rng" > +#define MAX_RETRIES 30 > + > +static struct delayed_work check_tpm_work; > +static int retry_count; > > static int tpm_rng_read(struct hwrng *rng, void *data, size_t max, bool wait) > { > @@ -33,9 +37,27 @@ static struct hwrng tpm_rng = { > .read = tpm_rng_read, > }; > > +static void check_tpm_presence(struct work_struct *work) > +{ > + u8 data = 0; > + if (tpm_get_random(TPM_ANY_NUM, &data, 1) > 0) { > + hwrng_register(&tpm_rng); > + } else { > + if (retry_count < MAX_RETRIES) { > + retry_count++; > + schedule_delayed_work(&check_tpm_work, HZ * 10); > + } else { > + pr_err("Could not find any TPM chip, not > registering rng"); > + } > + } > +} > + > static int __init rng_init(void) > { > - return hwrng_register(&tpm_rng); > + INIT_DELAYED_WORK(&check_tpm_work, check_tpm_presence); > + check_tpm_presence(NULL); > + > + return 0; > } > module_init(rng_init); > ********************************************************************************************* > > Why not something like this? Patch is completely untested. If this > idea seems useful I can clean the code but would require help in > testing. first, that's not how deferred device probing works in the kernel. drivers shouldn't be doing their own sleeping. but we can ignore that because no amount of delay/retries will work -- TPMs can come & go at anytime via hotplugging or module loading/unloading. so the only way to pull it off would be to do something like what i described -- extending the tpm framework so that it can signal children to come up/go down. imo, standing all of that up is over-engineering and not worth the effort, so i'm not going to do it. but maybe you can convince some of the TPM maintainers it's worthwhile. -mike