Re: [PATCH 3/3] tpm: make locality request return value consistent

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Mon Feb 19, 2024 at 8:29 PM UTC, Daniel P. Smith wrote:
> 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.

For all commits: your responses to my queries have much more to the
point information and buy-in than the original commit messages. So
for next version I would take them and edit a bit and then this all
makes much much more sense. Thank you.
>
> v/r,
> dps

BR, Jarkko