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, 1 insertions, 0 deletions

diff --git a/src/core/execute.h b/src/core/execute.h
index 106154f81a..6082c42aba 100644
--- a/src/core/execute.h
+++ b/src/core/execute.h
@@ -178,6 +178,7 @@ struct ExecContext {
         bool no_new_privileges;
 
         bool dynamic_user;
+        bool remove_ipc;
 
         /* This is not exposed to the user but available
          * internally. We need it to make sure that whenever we spawn