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Date: Mon, 19 Feb 2024 15:29:30 -0500
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Subject: Re: [PATCH 3/3] tpm: make locality request return value consistent
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To: Jarkko Sakkinen <jarkko@kernel.org>, Jason Gunthorpe <jgg@ziepe.ca>,
 linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org
Cc: Ross Philipson <ross.philipson@oracle.com>,
 Peter Huewe <peterhuewe@gmx.de>
References: <20240131170824.6183-1-dpsmith@apertussolutions.com>
 <20240131170824.6183-4-dpsmith@apertussolutions.com>
 <CYU3XUGOX6QT.1GL070ONNPBWQ@suppilovahvero>
From: "Daniel P. Smith" <dpsmith@apertussolutions.com>
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On 2/1/24 17:49, Jarkko Sakkinen wrote:
> On Wed Jan 31, 2024 at 7:08 PM EET, Daniel P. Smith wrote:
>> The function tpm_tis_request_locality() is expected to return the locality
>> value that was requested, or a negative error code upon failure. If it is called
>> while locality_count of struct tis_data is non-zero, no actual locality request
>> will be sent. Because the ret variable is initially set to 0, the
>> locality_count will still get increased, and the function will return 0. For a
>> caller, this would indicate that locality 0 was successfully requested and not
>> the state changes just mentioned.
>>
>> Additionally, the function __tpm_tis_request_locality() provides inconsistent
>> error codes. It will provide either a failed IO write or a -1 should it have
>> timed out waiting for locality request to succeed.
>>
>> This commit changes __tpm_tis_request_locality() to return valid negative error
>> codes to reflect the reason it fails. It then adjusts the return value check in
>> tpm_tis_request_locality() to check for a non-negative return value before
>> incrementing locality_cout. In addition, the initial value of the ret value is
>> set to a negative error to ensure the check does not pass if
>> __tpm_tis_request_locality() is not called.
> 
> This is way way too abtract explanation and since I don't honestly
> understand what I'm reading, the code changes look bunch of arbitrary
> changes with no sound logic as a whole.

In more simpler terms, the interface is inconsistent with its return 
values. To be specific, here are the sources for the possible values 
tpm_tis_request_locality() will return:
1. 0 - 4: _tpm_tis_request_locality() was able to set the locality
2. 0: a locality already open, no locality request made
3. -1: if timeout happens in __tpm_tis_request_locality()
4. -EINVAL: unlikely, return by IO write for incorrect sized write

As can easily be seen, tpm_tis_request_locality() will return 0 for both 
a successful(1) and non-successful request(2). And to be explicit for 
(2), if tpm_tis_request_locality is called for a non-zero locality and 
the locality counter is not zero, it will return 0. Thus, making the 
value 0 reflect as success when locality 0 is successfully requested and 
as failure when a locality is requested with a locality already open.

As for failures, correct me if I am wrong, but if a function is 
returning negative error codes, it should not be using a hard coded -1 
as a generic error code. As I note, it is unlikely for the -EINVAL to be 
delivered, but the code path is still available should something in the 
future change the backing call logic.

After this change, the possible return values for 
tpm_tis_request_locality() become:
1. 0 - 4: the locality that was successfully requested
2. -EBUSY: tpm busy, unable to request locality
3. -EINVAL: invalid parameter

With this more consistent interface, I updated the return value checks 
at the call sites to check for negative error as the means to catch 
failures.

v/r,
dps