gdm-{wayland,x}-session: don't overwrite user env with fallback vars

In both gdm-wayland-session and gdm-x-session, environment variables
from the systemd user manager are imported, if it is available. Those
environment variables are set up for the session-to-be-spawned, however,
environment variables that the gdm-{wayland,x}-session process itself
has are also inherited to the new session, and they are given preference
over what is imported from systemd.

That is not accidential, the gdm-{wayland,x}-session process has fresh
variables as to what the new user session will be (think $DISPLAY,
$XDG_CURRENT_DESKTOP, etc.) and those should not be overwritten by stale
data from the systemd user manager, who might have those variables still
set from a previous session.
As the gdm-{wayland,x}-session process does not inherit the environment
in which GDM itself is launched, but gets a fresh environment with only
purposefully added variables, this is in general not a problem (i.e.
there are no environment variables from the systemd system instance
overwriting those in our user session).

However, the GDM session worker sets a default fallback PATH for
gdm-{wayland,x}-session. This then gets preferred over whatever the
systemd user manager has, resulting in the session always getting the
fallback PATH.

As GDM probably needs to consider scenarios where no variables can get
imported from the systemd user manager (maybe it is not used at all),
removing the default PATH is not an option. Instead, this commit adds a
list of environment variables declared as fallbacks, for which we the
imported variables _do_ get preference over our own ones. Currently,
this is only PATH.

Closes: https://gitlab.gnome.org/GNOME/gdm/-/issues/385

2 files changed, 42 insertions, 2 deletions

diff --git a/daemon/gdm-wayland-session.c b/daemon/gdm-wayland-session.c
index 35679b19..d0404d2c 100644
--- a/daemon/gdm-wayland-session.c
+++ b/daemon/gdm-wayland-session.c
@@ -289,6 +289,17 @@ spawn_session (State        *state,
                                                     "WAYLAND_SOCKET",
                                                     "GNOME_SHELL_SESSION_MODE",
                                                     NULL };
+        /* The environment variables listed below are those we have set (or
+         * received from our own execution environment) only as a fallback to
+         * make things work, as opposed to a information directly pertaining to
+         * the session about to be started. Variables listed here will not
+         * overwrite the existing environment (possibly) imported from the
+         * systemd --user instance.
+         * As an example: We need a PATH for some of the launched subprocesses
+         * to work, but if the user (or the distributor) has customized the PATH
+         * via one of systemds user-environment-generators, that version should
+         * be preferred. */
+        static const char  *fallback_variables[] = { "PATH", NULL };
 
         g_debug ("Running wayland session");
 
@@ -320,7 +331,16 @@ spawn_session (State        *state,
                                 continue;
                         }
 
-                        g_subprocess_launcher_setenv (launcher, environment_entry[0], environment_entry[1], FALSE);
+                        /* Merge the environment block imported from systemd --user with the
+                         * environment we have set for ourselves (and thus pass on to the
+                         * launcher process). Variables we have set have precedence, as to not
+                         * import stale data from prior user sessions, with the exception of
+                         * those listed in fallback_variables. See the comment there for more
+                         * explanations. */
+                        g_subprocess_launcher_setenv (launcher,
+                                                      environment_entry[0],
+                                                      environment_entry[1],
+                                                      g_strv_contains (fallback_variables, environment_entry[0]));
                 }
 
                 /* Don't allow session specific environment variables from earlier sessions to
diff --git a/daemon/gdm-x-session.c b/daemon/gdm-x-session.c
index b1548361..5962da57 100644
--- a/daemon/gdm-x-session.c
+++ b/daemon/gdm-x-session.c
@@ -615,6 +615,17 @@ spawn_session (State        *state,
                                                      "WAYLAND_SOCKET",
                                                      "GNOME_SHELL_SESSION_MODE",
                                                      NULL };
+        /* The environment variables listed below are those we have set (or
+         * received from our own execution environment) only as a fallback to
+         * make things work, as opposed to a information directly pertaining to
+         * the session about to be started. Variables listed here will not
+         * overwrite the existing environment (possibly) imported from the
+         * systemd --user instance.
+         * As an example: We need a PATH for some of the launched subprocesses
+         * to work, but if the user (or the distributor) has customized the PATH
+         * via one of systemds user-environment-generators, that version should
+         * be preferred. */
+        static const char  *fallback_variables[] = { "PATH", NULL };
 
         g_debug ("Running X session");
 
@@ -636,7 +647,16 @@ spawn_session (State        *state,
                                 continue;
                         }
 
-                        g_subprocess_launcher_setenv (launcher, environment_entry[0], environment_entry[1], FALSE);
+                        /* Merge the environment block imported from systemd --user with the
+                         * environment we have set for ourselves (and thus pass on to the
+                         * launcher process). Variables we have set have precedence, as to not
+                         * import stale data from prior user sessions, with the exception of
+                         * those listed in fallback_variables. See the comment there for more
+                         * explanations. */
+                        g_subprocess_launcher_setenv (launcher,
+                                                      environment_entry[0],
+                                                      environment_entry[1],
+                                                      g_strv_contains (fallback_variables, environment_entry[0]));
                 }
 
                 /* Don't allow session specific environment variables from earlier sessions to