[systemd-bugs] [Bug 90769] New: user_elect_display() overwrites DISPLAY with TTY session when switching VT away from a graphical session

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=90769

            Bug ID: 90769
           Summary: user_elect_display() overwrites DISPLAY with TTY
                    session when switching VT away from a graphical
                    session
           Product: systemd
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: medium
         Component: general
          Assignee: systemd-bugs at lists.freedesktop.org
          Reporter: bugzilla at tecnocode.co.uk
        QA Contact: systemd-bugs at lists.freedesktop.org

Patch coming to fix logind’s user_elect_display() to be more stable when
switching VT from a graphical session to a TTY session, while still keeping the
graphical session open.

See the commit message for full details.

This is important because it affects the session returned by
sd_uid_get_display(), which will be used by PolicyKit to find the correct
session context to make policy decisions in. Hence this may (depending on the
particular udisks2 policy in force) break auto-mounting of USB disks if the
user starts a remote session after their X11 session, for example.

See also: bug #67728, bug #76358.

---

The incorrect behaviour can be tested by:
 1. In an X11 terminal, `cat /run/systemd/users/$uid` gives SESSIONS=c1
DISPLAY=c1.
 2. Switch VT, log in, `cat /run/systemd/users/$uid` gives SESSIONS="c2 c1"
DISPLAY=c2. Here, DISPLAY is incorrect and will result in the wrong return
value from sd_uid_get_display().
 3. Switch back to X11 and cat again; details are the same.
 4. Log out of the VT and cat again; DISPLAY=c1 SESSIONS=c1 again (correct).

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