core,systemctl: refuse switching root to current root properly

Fixes #28970

src/systemctl/systemctl-switch-root.c

@@ -4,6 +4,7 @@
 #include "bus-error.h"
 #include "bus-locator.h"
 #include "chase.h"
+#include "fd-util.h"
 #include "parse-util.h"
 #include "path-util.h"
 #include "proc-cmdline.h"
@@ -47,11 +48,17 @@ int verb_switch_root(int argc, char *argv[], void *userdata) {
 
         if (argc >= 2) {
                 root = argv[1];
+
                 if (!path_is_valid(root))
                         return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Invalid root path: %s", root);
+
                 if (!path_is_absolute(root))
                         return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Root path is not absolute: %s", root);
-                if (path_equal(root, "/"))
+
+                r = path_is_root(root);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to check if switch-root directory '%s' is current root: %m", root);
+                if (r > 0)
                         return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Cannot switch to current root directory: %s", root);
         } else
                 root = "/sysroot";