bunpen: namespace: restore the working directory (if possible) after entering the mount namespace

pkgs/additional/bunpen/restrict/namespace.ha

@@ -40,6 +40,20 @@ export fn namespace_restrict(what: *resources) void = {
   // mapped to non-ns ops by the same user, and vice-versa
   write_uid_map(uid, gid);
 
+  let pwd = strings::dup(os::getcwd());  // dup because API uses a static buffer
+  defer(free(pwd));
+  isolate_paths(what.paths);
+  // try to change to the old working directory;
+  // this can fail if it's not within the sandbox.
+  rtext::swallow_error("namespace: restore $PWD", os::chdir(pwd));
+
+  // TODO: CLONE_NEWPID (might not work without forking to also become reaper)
+};
+
+// reconfigures all the mounts so that after this call the only paths accessible
+// are those reachable from the provided `paths`.
+// N.B.: this function does NOT preserve the current working directory
+fn isolate_paths(paths: []path::buffer) void = {
   // allow new mounts to propagate from the parent namespace into the child
   // namespace, but not vice versa:
   rtext::check_error("reconfigure / as MS_SLAVE", rtext::mount("/", "/", "", rtext::MS_SLAVE | rtext::MS_REC, null));
@@ -62,7 +76,6 @@ export fn namespace_restrict(what: *resources) void = {
   rtext::check_error("mkdir new", rt::mkdir("new", 0o755));
   rtext::check_error("mount -t tmpfs tmpfs new", rtext::mount("tmpfs", "new", "tmpfs", 0, null));
 
-  {
   // bind all the user-requested paths from `old/$p` into `new/$p`.
   // use the `dirfd` abstraction so that paths meant for `old` can't crawl out
   // of that virtual fs.
@@ -73,17 +86,12 @@ export fn namespace_restrict(what: *resources) void = {
   let new_fs = os::dirfdopen(new_fd);
   defer(free(new_fs));
 
-    for (let path .. what.paths) {
+  for (let path .. paths) {
     bind_leaf(old_fs, new_fs, &path);
   };
-  };
 
   // pivot into the new rootfs
   pivot_into("new");
-
-  // TODO: `chdir` back to the original dir we were executed from
-
-  // TODO: CLONE_NEWPID (might not work without forking to also become reaper)
 };
 
 // walk from root to `p`, creating any ancestors necessary and then binding the