For example, we have two filesystems, one is embedded into another:
/a/test # first filesystem with a directory "/a/test/b2"
/a/test/b2 # not auto mounted yet second filesystem, notice "/a/test/b2" is
# a new directory on this filesystem after this filesystem is mounted
For subpath mount "/a/test/b2", `openat("/a/test", "b2")` gets directory "b2" on the first
filesystem, then "mount -c" will use this wrong directory as source directory.
`fstatat("/a/test", "b2/")` forces triggering auto mount of second filesystem, so
`openat("/a/test", "b2")` gets correct source directory for "mount -c".
This fixes issue https://github.com/kubernetes/kubernetes/issues/110818#issuecomment-1175736550
References:
1. https://man7.org/linux/man-pages/man2/openat.2.html
If pathname refers to an automount point that has not yet
been triggered, so no other filesystem is mounted on it,
then the call returns a file descriptor referring to the
automount directory without triggering a mount.
2. https://man7.org/linux/man-pages/man2/open_by_handle_at.2.html
name_to_handle_at() does not trigger a mount when the final
component of the pathname is an automount point. When a
filesystem supports both file handles and automount points, a
name_to_handle_at() call on an automount point will return with
error EOVERFLOW without having increased handle_bytes. This can
happen since Linux 4.13 with NFS when accessing a directory which
is on a separate filesystem on the server. In this case, the
automount can be triggered by adding a "/" to the end of the
pathname.
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