Re: [PATCH for-5.2] virtiofsd: Announce submounts even without statx()

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Max Reitz (mreitz@redhat.com) wrote:
> 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>

OK, that seems to have been the outcome of the discussion here:
  https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg00500.html
so


Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  tools/virtiofsd/passthrough_ll.c | 8 --------
>  1 file changed, 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,
> -- 
> 2.26.2
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK