[fprint] fprintd single sign on limitation

[Bastien Nocera]

On Fri, 2018-12-07 at 14:46 +0200, Igor Filatov wrote:
> I think Dave means that the scanner would capture the print when the
> device is powered on. Not as instructed by a userspace driver, but as
> a "hardware feature" of the button/sensor itself.

Which might need to be enabled in the hardware. I have a older Lenovo
X1 Carbon with that feature, though the fingerprint reader and power
button are separate, and the hardware would change colour on boot if it
was expecting a fingerprint read.

>  It would store a single image until asked by libfrint to perform a
> scan, at which point it would just submit the image it has.

That's probably not something we want to "hide" completely inside the
driver. I would much rather this was a separate call if the feature is
enabled at all because there are likely cases where admins might want
to deactivate this feature.

>  libfprint doesn't need to know. It only needs to match the print not
> aginst the library of a chosen user, but of all users and return a
> pair of (ok, user_id).

It depends if the enrollment fingerprint is stored in the device, or in
user-space in the OS (or if it's possible to enroll multiple users, or
multiple fingerprints...)

Anyway, there are too many unknowns to discuss this more without some
sort of access to the specifications.