Re: [PATCH 1/3] virtiofsd: Find original inode ID of mount points

[Connor Kuehl <ckuehl@redhat.com>]

On 5/12/21 7:55 AM, Max Reitz wrote:
> Mount point directories represent two inodes: On one hand, they are a
> normal directory on their parent filesystem.  On the other, they are
> the
> root node of the filesystem mounted there.  Thus, they have two inode
> IDs.
> 
> Right now, we only report the latter inode ID (i.e. the inode ID of
> the
> mounted filesystem's root node).  This is fine once the guest has
> auto-mounted a submount there (so this inode ID goes with a device ID
> that is distinct from the parent filesystem), but before the
> auto-mount,
> they have the device ID of the parent and the inode ID for the
> submount.
> This is problematic because this is likely exactly the same
> st_dev/st_ino combination as the parent filesystem's root node.  This
> leads to problems for example with `find`, which will thus complain
> about a filesystem loop if it has visited the parent filesystem's root
> node before, and then refuse to descend into the submount.
> 
> There is a way to find the mount directory's original inode ID, and
> that
> is to readdir(3) the parent directory, look for the mount directory,
> and
> read the dirent.d_ino field.  Using this, we can let lookup and
> readdirplus return that original inode ID, which the guest will thus
> show until the submount is auto-mounted.  (Then, it will invoke
> getattr
> and that stat(2) call will return the inode ID for the submount.)
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---

This is a clever way of uncovering the inode ID.

Reviewed-by: Connor Kuehl <ckuehl@redhat.com>