selftests: seventh test for mounting detached mounts onto detached mounts

Add a test to verify that detached mounts behave correctly.

Link: https://lore.kernel.org/r/20250221-brauner-open_tree-v1-16-dbcfcb98c676@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/tools/testing/selftests/mount_setattr/mount_setattr_test.c b/tools/testing/selftests/mount_setattr/mount_setattr_test.c
index d2bf8b77df3fbe86097ac61f13be2e3d322e5292..1e0508cb5c2d64179718f150d37c48440831cf26 100644 (file)
--- a/tools/testing/selftests/mount_setattr/mount_setattr_test.c
+++ b/tools/testing/selftests/mount_setattr/mount_setattr_test.c
@@ -2061,4 +2061,40 @@ TEST_F(mount_setattr, two_detached_mounts_referring_to_same_anonymous_mount_name
        ASSERT_NE(move_mount(fd_tree1, "", -EBADF, "/tmp/target1", MOVE_MOUNT_F_EMPTY_PATH), 0);
 }
 
+TEST_F(mount_setattr, two_detached_subtrees_of_same_anonymous_mount_namespace)
+{
+       int fd_tree1 = -EBADF, fd_tree2 = -EBADF;
+
+       /*
+        * Copy the following mount tree:
+        *
+        * |-/mnt/A               testing tmpfs
+        *   `-/mnt/A/AA          testing tmpfs
+        *     `-/mnt/A/AA/B      testing tmpfs
+        *       `-/mnt/A/AA/B/BB testing tmpfs
+        */
+       fd_tree1 = sys_open_tree(-EBADF, "/mnt/A",
+                                AT_NO_AUTOMOUNT | AT_SYMLINK_NOFOLLOW |
+                                AT_RECURSIVE | OPEN_TREE_CLOEXEC |
+                                OPEN_TREE_CLONE);
+       ASSERT_GE(fd_tree1, 0);
+
+       /*
+        * Create an O_PATH file descriptors with a separate struct file that
+        * refers to a subtree of the same detached mount tree as @fd_tree1
+        */
+       fd_tree2 = sys_open_tree(fd_tree1, "AA",
+                                AT_NO_AUTOMOUNT | AT_SYMLINK_NOFOLLOW |
+                                AT_EMPTY_PATH | OPEN_TREE_CLOEXEC);
+       ASSERT_GE(fd_tree2, 0);
+
+       /*
+        * This must fail as it is only possible to attach the root of a
+        * detached mount tree.
+        */
+       ASSERT_NE(move_mount(fd_tree2, "", -EBADF, "/tmp/target1", MOVE_MOUNT_F_EMPTY_PATH), 0);
+
+       ASSERT_EQ(move_mount(fd_tree1, "", -EBADF, "/tmp/target1", MOVE_MOUNT_F_EMPTY_PATH), 0);
+}
+
 TEST_HARNESS_MAIN