I tried adding this line to /etc/X11/xdm/XResources:
in order to allow a user with no passwd to log in. (First I verified that the
user can log in from the console, and yes, indeed it does not ask for a
password.) Then I restarted xdm and tried to log in. The first time after starting xdm, it asks for a password, I leave the field empty, and fail to log in. The second time however, after having failed to log in, I simply type the same username again, and it does not prompt for a password, but immediately logs in, as it should have done the first time. If I log out and get back to the xdm prompt, the whole sequence can be repeated (username, empty password, fail; username, login successful).
allowNullPasswd handling is specific to each authentication backend.
If using PAM, allowNullPasswd is passed to PAM as the setting or unsetting of PAM_DISALLOW_NULL_AUTHTOK, and left to PAM to enforce.
Created attachment 47464 [details] [review]
Assign allow_null_passwd and allow_root_login variables' values before call to pam_authenticate()
Thanks for figuring this out! A modified form of this patch has been sent
on to xorg-devel for review:
Fix pushed to git master: