[Virtio-fs] Query about announce_submounts and ro/rw mounts

[Dr. David Alan Gilbert]

* Max Reitz (mreitz at redhat.com) wrote:
> On 03.03.21 19:20, Vivek Goyal wrote:
> > Hi Max,
> 
> Hi Vivek,
> 
> > I was playing with "announce_submounts". I have a read-only bind mounted
> > mount point in shared directory. Inside guest, when I step into that
> > directory, I see that a mount point got created but its "rw" and not "ro".
> > 
> > Is that intentional.
> 
> No, that isn’t intentional.  I just didn’t think of sharing such information
> with the guest.
> 
> > Can we send property of mount also to guest when
> > notifying guest about mount point.
> 
> I suppose we can send it (by adding a new flag alongside
> FUSE_ATTR_SUBMOUNT), and we can make the mount ro by setting the SB_RDONLY
> flag in fuse_dentry_automount().
> 
> If we implemented this for RDONLY, are there other flags that we might want
> to consider as well?  (e.g. nodev etc.)
> 
> OTOH, I just tested NFS, and it doesn’t pass through the RO flag:
> 
> [...]
> /tmp/xfs.img on ~/tmp/test-nfs/mount type xfs (ro,...)
> [...]
> 127.0.0.1:~/tmp/test-nfs on /mnt/tmp type nfs4 (rw,...)
> 127.0.0.1:~/tmp/test-nfs/mount on /mnt/tmp/mount type nfs4 (rw,...)
> 
> So is it really important or more a matter of style?

So what happens if you try and write a file in /mnt/tmp/mount ?

Dave

> > Does it make sense? I guess then next problem will be what if mount
> > changes back to "rw" and how to we propagate to guest. IIUC, we will
> > probably need monitor it and send notifications. Or notice this chagne
> > on next lookup.
> 
> Yes, I imagine that would be rather complicated.  Is there a way to monitor
> mount changes?
> 
> Max
> 
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-- 
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK