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

[Dr. David Alan Gilbert]

* Miklos Szeredi (mszeredi at redhat.com) wrote:
> On Tue, Apr 28, 2020 at 4:52 PM Stefan Hajnoczi <stefanha at redhat.com> wrote:
> >
> > On Mon, Apr 27, 2020 at 06:59:02PM +0100, Dr. David Alan Gilbert wrote:
> > > * Max Reitz (mreitz at 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 at 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?
> 
> It's a complicated thing.  Current setup results in propagation
> happening to the cloned namespace, but not to the bind mounted root.
> 
> Why?  Because setting mounts "slave" after unshare, results in the
> propagation being stopped at that point.  To make it propagate
> further, change it back to "shared".  Note: the result changing to
> "slave" and then to "shared" results in breaking the backward
> propagation to the original namespace, but allowing propagation
> further down the chain.

Do you mean on the "/" ?

So our current sequence is:

   (new namespace)
 1)    if (mount(NULL, "/", NULL, MS_REC | MS_SLAVE, NULL) < 0) {
 2)   if (mount("proc", "/proc", "proc",
           ....
 3)   if (mount(source, source, NULL, MS_BIND | MS_REC, NULL) < 0) {
 4)  (chdir newroot, pivot, chdir oldroot)
 5)   if (mount("", ".", "", MS_SLAVE | MS_REC, NULL) < 0) {
 6)   if (umount2(".", MNT_DETACH) < 0) {

So are you saying we need a:
       if (mount(NULL, "/", NULL, MS_REC | MS_SHARED, NULL) < 0) {

  and can this go straight after (1) ?

Dave

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