[QUESTION] question about the errno of rename the parent dir to a subdir of a specified directory

["zhangyi (F)" <yi.zhang@huawei.com>]

Hi,

If we rename the parent-dir to a sub-dir of a specified directory, the
rename() syscall return -EINVAL because lock_rename() in lock_rename()
checks the relations of the sorece and dest dirs. But if the 'parent'
dir is a mountpoint, the rename() syscall return -EXDEV instead because
it checks the parent dir's mountpoint of the sorece and dest dirs.

For example:
Case 1: rename() return -EINVAL
# mkdir -p parent/dir
# rename parent parent/dir/subdir parent
rename: parent: rename to parent/dir/subdir failed: Invalid argument

Case 2: rename() return -EXDEV
# mkdir parent
# mount -t tmpfs test parent
# mkdir parent/dir
# rename parent parent/dir/subdir parent
rename: parent: rename to parent/dir/subdir failed: Invalid cross-device link

In case 2, although 'parent' directory is a mountpoint, it acted as a root
dir of the "test tmpfs", so it should belongs to the same mounted fs of
'dir' directoty, so I think it shall return -EINVAL.

Is it a bug or just designed as this ?

Thanks,
Yi.