`passwd` does not accept fully qualified usernames `(user@domain)`

[nooreldeenmansour]

As reported in canonical/authd#1487

Attempting to use passwd command for UPN formatted usernames fails

$ sudo passwd <user>@<domain>
passwd: Provided user name is not a valid name

It is failing because this function call was added is_valid_user_name(). The @ character isn't in the allowed set, causing passwd user@domain.com to fail.

[alejandro-colomar]

It seems the regression was introduced because of 326889c (2024-10-22; "Fix coverity unbound buffer issues").

This affects 4.17.0 and later.

Cc: @MarcinDigitic

[alejandro-colomar]

I think one solution would be to revert that commit, and instead remove the username from the syslog. We could log the UID instead, which is safer.