Suppose the user's "real" X11 display on the host is Xorg or Xwayland
listening on :42, but they also have an Xvfb server listening on :99.
If we change the X11 display number to the arbitrary value :99, and
the Flatpak sandbox shares its network namespace with the host, then
clients inside the Flatpak sandbox will prefer to connect to the
abstract socket @/tmp/.X11-unix/X99 (which is Xvfb), rather than the
filesystem-backed socket /tmp/.X11-unix/X99 in the sandbox (which is
really /tmp/.X11-unix/X42 on the host, i.e. Xorg or Xwayland).
If they're relying on Xauthority (MIT-MAGIC-COOKIE-1) for access
control (as many display managers do), then this will fail, because
we gave the sandboxed app access to the cookies for Xorg/Xwayland
(rewriting their display number from 42 to 99 as we did so), but
Xvfb does not accept those cookies.
If we're relying on `xhost +"si:localuser:$(id -nu)"` for access control
(as gdm does), then the Flatpak app will successfully (!) connect to
whatever is on :99, for example Xvfb or Xephyr, which is rarely what
anyone wants either.
Resolves: https://github.com/flatpak/flatpak/issues/3357
Signed-off-by: Simon McVittie <smcv@collabora.com>
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