virtiofsd: Announce submounts even without statx()

Contrary to what the check (and warning) in lo_init() claims, we can
announce submounts just fine even without statx() -- the check is based
on comparing both the mount ID and st_dev of parent and child.  Without
statx(), we will not have the mount ID; but we always have st_dev.

The only problems we have (without statx() and its mount ID) are:

(1) Mounting the same device twice may lead to both trees being treated
    as exactly the same tree by virtiofsd.  But that is a problem that
    is completely independent of mirroring host submounts in the guest.
    Both submount roots will still show the FUSE_SUBMOUNT flag, because
    their st_dev still differs from their respective parent.

(2) There is only one exception to (1), and that is if you mount a
    device inside a mount of itself: Then, its st_dev will be the same
    as that of its parent, and so without a mount ID, virtiofsd will not
    be able to recognize the nested mount's root as a submount.
    However, thanks to virtiofsd then treating both trees as exactly the
    same tree, it will be caught up in a loop when the guest tries to
    examine the nested submount, so the guest will always see nothing
    but an ELOOP there.  Therefore, this case is just fully broken
    without statx(), whether we check for submounts (based on st_dev) or
    not.

All in all, checking for submounts works well even without comparing the
mount ID (i.e., without statx()).  The only concern is an edge case
that, without statx() mount IDs, is utterly broken anyway.

Thus, drop said check in lo_init().

Reported-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201103164135.169325-1-mreitz@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

1 files changed, 0 insertions, 8 deletions

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index ec1008bceb..6c64b03f1a 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -610,14 +610,6 @@ static void lo_init(void *userdata, struct fuse_conn_info *conn)
                  "does not support it\n");
         lo->announce_submounts = false;
     }
-
-#ifndef CONFIG_STATX
-    if (lo->announce_submounts) {
-        fuse_log(FUSE_LOG_WARNING, "lo_init: Cannot announce submounts, there "
-                 "is no statx()\n");
-        lo->announce_submounts = false;
-    }
-#endif
 }
 
 static void lo_getattr(fuse_req_t req, fuse_ino_t ino,