[Virtio-fs] [RFC 2/2] virtiofsd: Set st_rdev for sub-mount points

[Dr. David Alan Gilbert]

* Max Reitz (mreitz at redhat.com) wrote:
> On 07.05.20 13:43, Miklos Szeredi wrote:
> > On Thu, May 7, 2020 at 12:56 PM Max Reitz <mreitz at redhat.com> wrote:
> >>
> >> On 06.05.20 18:04, Dr. David Alan Gilbert wrote:
> >>> * Max Reitz (mreitz at redhat.com) wrote:
> >>>> Whenever we encounter a directory with an st_dev that differs from that
> >>>> of its parent, we set st_rdev accordingly so the guest can create a
> >>>> submount for it.
> >>>>
> >>>> Make this behavior optional, so submounts are only announced to the
> >>>> guest with the announce_submounts option.  Some users may prefer the
> >>>> current behavior, so that the guest learns nothing about the host mount
> >>>> structure.
> >>>>
> >>>> Signed-off-by: Max Reitz <mreitz at redhat.com>
> >>>
> >>> Does this need to be wired to a flag in the INIT message (like say
> >>> FUSE_ASYNC_READ) to indicate that the kernel/daemon supports this, or is
> >>> it really safe just to start sending the changed rdev?
> >>
> >> I supposed it to be safe, given that rdev is never used for anything but
> >> device files, so basically it was just a reserved field for directories.
> > 
> > You assume only directories can be mounted, but that's not so.  A
> > device can also be the source or destination of a bind mount.
> 
> Well, I mostly thought this case wouldn’t be too important.  But I
> suppose it doesn’t make sense to decide now that it’s never going to be
> important.

It's quite common in kata to bind mount individual /etc files - I'm not
clear if that's this particular case.

Dave

> > We need to extend fuse_entry_out with a flags field.  We could
> > possibly reuse the upper bits of entry_valid (i.e. timeouts above say
> > INT_MAX seconds could be taken as  "infinity").  Unfortunately that's
> > not as easy as splitting it into two fields due to endianness :(
> 
> For the time being, a single bit would be sufficient to signify whether
> there should be a submount at all.  We could add a field with a submount
> ID (i.e., st_dev from the host) in the future when we decide we
> want/need to ensure that two inodes with the same st_dev on the host
> also have the same st_dev in the guest.
> 
> Max
> 



--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK