Re: file metadata via fs API

From: Ian Kent
Date: Wed Aug 12 2020 - 21:01:46 EST

On Wed, 2020-08-12 at 14:06 +0100, David Howells wrote:
> Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> > That presumably means the mount ID <-> mount path mapping already
> > exists, which means it's just possible to use the open(mount_path,
> > O_PATH) to obtain the base fd.
> No, you can't. A path more correspond to multiple mounts stacked on
> top of
> each other, e.g.:
> mount -t tmpfs none /mnt
> mount -t tmpfs none /mnt
> mount -t tmpfs none /mnt
> Now you have three co-located mounts and you can't use the path to
> differentiate them. I think this might be an issue in autofs, but
> Ian would
> need to comment on that.

It is a problem for autofs, direct mounts in particular, but also
for mount ordering at times when umounting a tree of mounts where
mounts are covered or at shutdown.