linux-kernel - Re: [PATCH v2] tpm: add support for partial reads

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Message-ID: <20181105134436.GB21757@linux.intel.com>
Date:   Mon, 5 Nov 2018 15:44:36 +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  v2] tpm: add support for partial reads

On Mon, Oct 29, 2018 at 12:52:17PM -0700, Tadeusz Struk wrote:
> Currently to read a response from the TPM device an application needs
> provide big enough buffer for the whole response and read it in one go.
> The application doesn't know how big the response it beforehand so it
> always needs to maintain a 4K buffer and read the max (4K).
> In case if the user of the TSS library doesn't provide big enough
> buffer the TCTI spec says that the library should set the required
> size and return TSS2_TCTI_RC_INSUFFICIENT_BUFFER error code so that the
> application could allocate a bigger buffer and call receive again.
> To make it possible in the TSS library, this requires being able to do
> partial reads from the driver.
> The library would read the 10 bytes header first to get the actual size
> of the response from the header, and then read the rest of the response.
> This patch adds support for partial reads, i.e. the user can read the
> response in one or multiple reads, until the whole response is consumed.
> The user can also read only part of the response and ignore
> the rest by issuing a new write to send a new command.

You don't explain what the commit does at all.

> The usecase is implemented in this TSS commit:
> https://github.com/tpm2-software/tpm2-tss/commit/ce982f67a67dc08e24683d30b05800648d8a264c

We do not want this as part of the commit message. You should place this
in the beginning of the diffstat section.

> 
> Signed-off-by: Tadeusz Struk <tadeusz.struk@...el.com>
> ---
> Changes in v2:
>  - Allow writes after only partial response is consumed to maintain
>    backwords compatibility.
> ---
>  drivers/char/tpm/tpm-dev-common.c |   38 ++++++++++++++++++++++++++++---------
>  drivers/char/tpm/tpm-dev.h        |    2 ++
>  2 files changed, 31 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm-dev-common.c b/drivers/char/tpm/tpm-dev-common.c
> index 99b5133a9d05..77e686d35384 100644
> --- a/drivers/char/tpm/tpm-dev-common.c
> +++ b/drivers/char/tpm/tpm-dev-common.c
> @@ -64,6 +64,7 @@ static void tpm_timeout_work(struct work_struct *work)
>  
>  	mutex_lock(&priv->buffer_mutex);
>  	priv->data_pending = 0;
> +	priv->partial_data = 0;

What is this variable?

Please refine the patch with better documentation.

/Jarkko