Re: [Virtio-fs] [PATCH] virtiofsd: Show submounts

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Apr 28, 2020 at 03:51:43PM +0100, Stefan Hajnoczi wrote:
> On Mon, Apr 27, 2020 at 06:59:02PM +0100, Dr. David Alan Gilbert wrote:
> > * Max Reitz (mreitz@redhat.com) wrote:
> > > Currently, setup_mounts() bind-mounts the shared directory without
> > > MS_REC.  This makes all submounts disappear.
> > > 
> > > Pass MS_REC so that the guest can see submounts again.
> > 
> > Thanks!
> > 
> > > Fixes: 3ca8a2b1c83eb185c232a4e87abbb65495263756
> > 
> > Should this actually be 5baa3b8e95064c2434bd9e2f312edd5e9ae275dc ?
> > 
> > > Signed-off-by: Max Reitz <mreitz@redhat.com>
> > > ---
> > >  tools/virtiofsd/passthrough_ll.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> > > index 4c35c95b25..9d7f863e66 100644
> > > --- a/tools/virtiofsd/passthrough_ll.c
> > > +++ b/tools/virtiofsd/passthrough_ll.c
> > > @@ -2643,7 +2643,7 @@ static void setup_mounts(const char *source)
> > >      int oldroot;
> > >      int newroot;
> > >  
> > > -    if (mount(source, source, NULL, MS_BIND, NULL) < 0) {
> > > +    if (mount(source, source, NULL, MS_BIND | MS_REC, NULL) < 0) {
> > >          fuse_log(FUSE_LOG_ERR, "mount(%s, %s, MS_BIND): %m\n", source, source);
> > >          exit(1);
> > >      }
> > 
> > Do we want MS_SLAVE to pick up future mounts that might happenf rom the
> > host?
> 
> There are two separate concepts:
> 
> 1. Mount namespaces.  The virtiofsd process is sandboxed and lives in
>    its own mount namespace.  Therefore it does not share the mounts that
>    the rest of the host system sees.
> 
> 2. Propagation type.  This is related to bind mounts so that mount
>    operations that happen in one bind-mounted location can also appear
>    in other bind-mounted locations.
> 
> Since virtiofsd is in a separate mount namespace, does the propagation
> type even have any effect?

Yes, propagation should work across mount namespaces if you get the mount
flags right.  You can try it out using  unshare + mount commands
to debug different scenarios.

Regards,
Daniel
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