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

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Message-ID: <20180723215620.GH532@ziepe.ca>
Date:   Mon, 23 Jul 2018 15:56:20 -0600
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Tadeusz Struk <tadeusz.struk@...el.com>
Cc:     James Bottomley <James.Bottomley@...senPartnership.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        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 Mon, Jul 23, 2018 at 02:38:08PM -0700, Tadeusz Struk wrote:
> On 07/23/2018 02:13 PM, James Bottomley wrote:
> > The current patch does, you even provided a use case in your last email
> >  (it's do command to get sizing followed by do command with correctly
> > sized buffer). 
> 
> The example I provided was: #1 send a command, #2 read the response header
> (10 bytes), get the actual response size from the header and then #3 read
> the full response (response size - size of the header bytes).

The proposed patch doesn't clear the data_pending if the entire buffer
is not consumed, so of course it is ABI breaking, that really isn't OK.

> > However, if you tie it to O_NONBLOCK, it won't because no-one currently
> > opens the TPM device non blocking so it's an ABI conformant
> > discriminator of the uses.  Tying to O_NONBLOCK should be simple
> > because it's in file->f_flags.
> 
> I think that it might be an option. Especially that I have this on top of
> the async patch. Let's discuss this when Jarkko is back.

Maybe you could do this by requiring the userspace to call pread()
with a non-zero offset to get the trailing segment of the last
executed command and leave normal read/pread(off=0) with the semantics
as they have today.

Jason