core: add RemoveIPC= setting

This adds the boolean RemoveIPC= setting to service, socket, mount and swap
units (i.e.  all unit types that may invoke processes). if turned on, and the
unit's user/group is not root, all IPC objects of the user/group are removed
when the service is shut down. The life-cycle of the IPC objects is hence bound
to the unit life-cycle.

This is particularly relevant for units with dynamic users, as it is essential
that no objects owned by the dynamic users survive the service exiting. In
fact, this patch adds code to imply RemoveIPC= if DynamicUser= is set.

In order to communicate the UID/GID of an executed process back to PID 1 this
adds a new "user lookup" socket pair, that is inherited into the forked
processes, and closed before the exec(). This is needed since we cannot do NSS
from PID 1 due to deadlock risks, However need to know the used UID/GID in
order to clean up IPC owned by it if the unit shuts down.

1 files changed, 16 insertions, 2 deletions

diff --git a/src/core/unit.h b/src/core/unit.h
index 513ea1614c..53875653d7 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -180,6 +180,10 @@ struct Unit {
         /* Make sure we never enter endless loops with the check unneeded logic, or the BindsTo= logic */
         RateLimit auto_stop_ratelimit;
 
+        /* Reference to a specific UID/GID */
+        uid_t ref_uid;
+        gid_t ref_gid;
+
         /* Cached unit file state and preset */
         UnitFileState unit_file_state;
         int unit_file_preset;
@@ -371,8 +375,7 @@ struct UnitVTable {
         /* Called whenever a process of this unit sends us a message */
         void (*notify_message)(Unit *u, pid_t pid, char **tags, FDSet *fds);
 
-        /* Called whenever a name this Unit registered for comes or
-         * goes away. */
+        /* Called whenever a name this Unit registered for comes or goes away. */
         void (*bus_name_owner_change)(Unit *u, const char *name, const char *old_owner, const char *new_owner);
 
         /* Called for each property that is being set */
@@ -621,6 +624,17 @@ int unit_fail_if_symlink(Unit *u, const char* where);
 
 int unit_start_limit_test(Unit *u);
 
+void unit_unref_uid(Unit *u, bool destroy_now);
+int unit_ref_uid(Unit *u, uid_t uid, bool clean_ipc);
+
+void unit_unref_gid(Unit *u, bool destroy_now);
+int unit_ref_gid(Unit *u, gid_t gid, bool clean_ipc);
+
+int unit_ref_uid_gid(Unit *u, uid_t uid, gid_t gid);
+void unit_unref_uid_gid(Unit *u, bool destroy_now);
+
+void unit_notify_user_lookup(Unit *u, uid_t uid, gid_t gid);
+
 /* Macros which append UNIT= or USER_UNIT= to the message */
 
 #define log_unit_full(unit, level, error, ...)                          \