* Max Reitz (mre...@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 <mszer...@redhat.com>
> Signed-off-by: Max Reitz <mre...@redhat.com>
Queued
> ---
> 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 / dgilb...@redhat.com / Manchester, UK
