Re: [PATCH v3] tpm: add support for partial reads

[Jarkko Sakkinen]

On Thu, Nov 15, 2018 at 12:42:22PM -0800, 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.
> 
> Signed-off-by: Tadeusz Struk <tadeusz.struk@xxxxxxxxx>
> ---
> The usecase is implemented in this TSS commit:
> https://github.com/tpm2-software/tpm2-tss/commit/ce982f67a67dc08e24683d30b05800648d8a264c
> 
> Changes in v3:
>  - Remove link to usecase implemented in TSS out of the commit message.
>  - Update the conddition in tpm_common_poll() to take into account
>    the partial_data also.
> 
> Changes in v2:
>  - Allow writes after only partial response is consumed to maintain
>    backwords compatibility.
> ---
>  drivers/char/tpm/tpm-dev-common.c |   41 ++++++++++++++++++++++++++++---------
>  drivers/char/tpm/tpm-dev.h        |    2 ++
>  2 files changed, 33 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm-dev-common.c b/drivers/char/tpm/tpm-dev-common.c
> index 99b5133a9d05..5d43b0c28565 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;
>  	memset(priv->data_buffer, 0, sizeof(priv->data_buffer));
>  	mutex_unlock(&priv->buffer_mutex);
>  	wake_up_interruptible(&priv->async_wait);
> @@ -90,22 +91,39 @@ ssize_t tpm_common_read(struct file *file, char __user *buf,
>  	ssize_t ret_size = 0;
>  	int rc;
>  
> -	del_singleshot_timer_sync(&priv->user_read_timer);
> -	flush_work(&priv->timeout_work);
>  	mutex_lock(&priv->buffer_mutex);
> +	if (priv->data_pending || priv->partial_data) {
> +		if (*off == 0)
> +			priv->partial_data = priv->data_pending;
> +
> +		ret_size = min_t(ssize_t, size, priv->partial_data);
> +		if (ret_size <= 0) {
> +			ret_size = 0;
> +			priv->data_pending = 0;
> +			priv->partial_data = 0;
> +			goto out;
> +		}
>  
> -	if (priv->data_pending) {
> -		ret_size = min_t(ssize_t, size, priv->data_pending);
> -		if (ret_size > 0) {
> -			rc = copy_to_user(buf, priv->data_buffer, ret_size);
> -			memset(priv->data_buffer, 0, priv->data_pending);
> -			if (rc)
> -				ret_size = -EFAULT;
> +		rc = copy_to_user(buf, priv->data_buffer + *off, ret_size);
> +		if (rc) {
> +			memset(priv->data_buffer, 0, TPM_BUFSIZE);
> +			priv->partial_data = 0;
> +			ret_size = -EFAULT;
> +		} else {
> +			memset(priv->data_buffer + *off, 0, ret_size);
> +			priv->partial_data -= ret_size;
> +			*off += ret_size;

You could drop these memset() calls and also one from
tpm_timeout_work(). The call could be done once in the beginning of
tpm_common_write() instead of having three different call sites.

Replacing two memsets with one could be a prepending commit.

>  		}
>  
>  		priv->data_pending = 0;
>  	}
>  
> +out:
> +	if (!priv->partial_data) {
> +		*off = 0;
> +		del_singleshot_timer_sync(&priv->user_read_timer);
> +		flush_work(&priv->timeout_work);
> +	}
>  	mutex_unlock(&priv->buffer_mutex);
>  	return ret_size;
>  }
> @@ -150,6 +168,9 @@ ssize_t tpm_common_write(struct file *file, const char __user *buf,
>  		goto out;
>  	}
>  
> +	priv->partial_data = 0;
> +	*off = 0;
> +
>  	/*
>  	 * If in nonblocking mode schedule an async job to send
>  	 * the command return the size.
> @@ -184,7 +205,7 @@ __poll_t tpm_common_poll(struct file *file, poll_table *wait)
>  
>  	poll_wait(file, &priv->async_wait, wait);
>  
> -	if (priv->data_pending)
> +	if (priv->data_pending || priv->partial_data)
>  		mask = EPOLLIN | EPOLLRDNORM;
>  	else
>  		mask = EPOLLOUT | EPOLLWRNORM;
> diff --git a/drivers/char/tpm/tpm-dev.h b/drivers/char/tpm/tpm-dev.h
> index a126b575cb8c..a2ca6a7a06f1 100644
> --- a/drivers/char/tpm/tpm-dev.h
> +++ b/drivers/char/tpm/tpm-dev.h
> @@ -11,6 +11,8 @@ struct file_priv {
>  
>  	/* Holds the amount of data passed or an error code from async op */
>  	ssize_t data_pending;
> +	/* For partial reads, holds the reminder of a response */
> +	ssize_t partial_data;

Naming becomes a mess and the comment for data_pending variable is
incorrect as it is also used for synchronous operation.

Maybe add a prepending commit to rename it as

	/* Holds the resul of the tpm_transmit() last call. */
	ssize_t transmit_result;

That is at least clear and obvious on what it contains.

The comment for partial_data is incorrect as the variable does not
contain any data.

We could use declare:

	/* Holds the count how much of the response is still unread. */
	size_t response_pending;

Observe another remark from your commit: there is no reaso to ssize_t as
the type as the value should never be a negative number.

/Jarkko