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

[Max Reitz]

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?

> 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