switch-root: don't require /mnt/ when switching root into host OS

So far, we invoked pivot_root() specifying /mnt/ as second argument,
which then unmounted right-after. We'd create /mnt/ if needed. This
sucks, because it means /mnt/ must strictly be pre-created on immutable
images.

Remove this limitation, by using pivot_root() with "." as source and
target, which will result in two stacked mounts afterwards: the new one
underneath, the old one ontop. We can then simply unmount the top one,
and have what we want without needing any extra /mnt/ dir.

Since we don't need /mnt/ anymore we can get rid of the extra
unmount_old_root parameter and simply specify it as NULL if we don't
want the old mount to stick around.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/core/main.c b/src/core/main.c
index 932ea64e45..af88fe34bb 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -1797,7 +1797,7 @@ static int do_reexecute(
                 broadcast_signal(SIGTERM, false, true, arg_default_timeout_stop_usec);
 
                 /* And switch root with MS_MOVE, because we remove the old directory afterwards and detach it. */
-                r = switch_root(switch_root_dir, "/mnt", true, MS_MOVE);
+                r = switch_root(switch_root_dir, /* old_root_after= */ NULL, MS_MOVE);
                 if (r < 0)
                         log_error_errno(r, "Failed to switch root, trying to continue: %m");
         }