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

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Message-ID: <90072f09-6388-b1f7-387b-42f056f8893d@intel.com>
Date:   Sun, 18 Nov 2018 19:05:19 -0800
From:   Tadeusz Struk <tadeusz.struk@...el.com>
To:     Jarkko Sakkinen <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 v4 2/2] tpm: add support for partial reads

On 11/17/18 11:48 PM, Jarkko Sakkinen wrote:
>> +	if (priv->transmit_result || priv->partial_data) {
>> +		if (*off == 0)
>> +			priv->partial_data = priv->transmit_result;
>> +
>> +		ret_size = min_t(ssize_t, size, priv->partial_data);
>> +		if (ret_size <= 0) {
> When ret_size < 0? Shouldn't this be just "if (!ret_size)"?

What we want to check here is if ret_size is positive, which is a valid
value, or if it is negative or zero, which is an invalid value, so in
this case (!ret_size) will not work.

> 
>> 	/* Holds the resul of the last successful call to tpm_transmit() */
>>  	size_t transmit_result;
>> +	/* Holds the count how much of the response is still unread */
>> +	size_t partial_data;
> I'm otherwise happy how this look like but why call it partial_data.
> You cannot really tell from the name anything about its contents as
> data is very abstract term.

so I will rename these two to response_length and response_length_rem,
how does this sound?

> BTW, why you need the new variable anyway and not just decrease the
> variable where the length is original stored?

We need to have two variables, otherwise how do we tell if some part of
response was consumed to allow sending a new command?
The transmit_result is used for that. If it is zero then one can transmit
a new command even if the whole response is not consumed. The new variable
tracks how much of the response is still to be read. 

Thanks,
-- 
Tadeusz