mount: warn about symlinks pointing outside of the mountpoint, see #9254

src/borg/archiver/mount_cmds.py

@@ -79,6 +79,11 @@ class MountMixIn:
         archives and the directory structure below these will be loaded on-demand from
         the repository when entering these directories, so expect some delay.
 
+        Care should be taken, as Borg backs up symlinks as-is. When an archive 
+        or repository is mounted, it is possible to “jump” outside the mount point 
+        by following a symlink. If this happens, files or directories (or versions of them)
+        that are not part of the archive or repository may appear to be within the mount point.
+
         Unless the ``--foreground`` option is given, the command will run in the
         background until the filesystem is ``unmounted``.
 

src/borg/fuse.py

@@ -565,6 +565,11 @@ class FuseOperations(llfuse.Operations, FuseBackend):
         )
         self._create_filesystem()
         llfuse.init(self, mountpoint, options)
+        logger.warning(
+            "Warning: The mounted archive is capable of containing symlinks that point outside the archive tree. "
+            "When following such symlinks you may see files and directories within the mountpoint "
+            "that do not reflect the archive content."
+        )
         if not foreground:
             if isinstance(self.repository_uncached, RemoteRepository):
                 daemonize()