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

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Message-ID: <0b9f0bb5-6620-0499-6680-5ec595167f23@intel.com>
Date:   Thu, 19 Jul 2018 13:12:19 -0700
From:   Tadeusz Struk <tadeusz.struk@...el.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>,
        jarkko.sakkinen@...ux.intel.com
Cc:     jgg@...pe.ca, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tpm: add support for partial reads

On 07/19/2018 12:52 PM, James Bottomley wrote:
> The ABI break is the error case as I outlined above.  We can't assume
> everyone uses the current interface without getting an error and one
> error and your hosed is a nasty failure case to change the interface
> to. 

Well, if there is a broken application out there that doesn't work today
it will not work after this change neither.

> Plus, if you assume everyone is passing 4k buffers, why would you
> even need the fragmentation case?

So that people don't need to do this anymore and we can run a
spec compliant TCTI on top of /dev/tpm<N>.

> 
>>> It might be possible to layer the behaviour you want compatibly
>>> into the current device structure (say an ioctl to switch to the
>>> fragment behaviour) but I've got to ask why we'd go to the
>>> complexity without a use case?
>> New IOCTL would add extra complexity, which isn't necessary.
> So what's wrong with fragmenting in the layer above the device driver
> (in userspace) and not actually changing the kernel?

Because it is much easier to implement in the driver, and
we can run a spec compliant TCTI on top of /dev/tpm<N>.

Thanks,
-- 
Tadeusz