linux-kernel - Re: [PATCH v4 2/2] tpm: add support for partial reads

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Date:   Mon, 19 Nov 2018 19:28:24 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Tadeusz Struk <tadeusz.struk@...el.com>
Cc:     jgg@...pe.ca, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] tpm: add support for partial reads

On Mon, Nov 19, 2018 at 08:44:49AM -0800, Tadeusz Struk wrote:
> On 11/19/18 5:58 AM, Jarkko Sakkinen wrote:
> > Please explain a scenario where "!ret_size" would no work given that
> > both size and partial_data have always positive value?
> 
> Right, I only looked at the one line above before responding.
> I'll change it to !ret_size
> 
> > 
> > I don't understand. In order to maintain backwards compatibility you can
> > send a new command at any time.
> 
> No, currently it is not possible to send a new command until the previous
> response is consumed. -EBUSY is returned if one sends a new command before
> reading the previous response (or at least part of it). See:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/char/tpm/tpm-dev-common.c#n128

Ah, you are correct.

You should add a boolean flag instead of introducing a new variable for
holding amount that has been read because obviously one read operation
is enough for backwards compatibility.

The code could read the code to data_pending and then set

   priv->data_read = false;

We do not need the original amount for anything.

/Jarkko