virtiofsd: sshfs as submount?

[Laszlo Ersek <lersek@redhat.com>]

Hi Miklos,

the following 2019 presentation on Stefan's website:

  https://vmsplice.net/
  virtio-fs: A Shared File System for Virtual Machines at KVM Forum 2019
  https://vmsplice.net/~stefan/virtio-fs_%20A%20Shared%20File%20System%20for%20Virtual%20Machines.pdf

has a slide called "Use case: File system-as-a-service" (slide#4). It
seems to confirm my "grand" idea to expose an sshfs submount to the
guest, via virtiofsd. (The guest need not / should not know it's a
submount, just see the files.) Beyond the pure utility of this, it feels
exciting to chain FUSE to FUSE. :)

I've tried it; the FUSE_READDIRPLUS request fails.

[2020-12-20 00:32:08.64+0100] [ID: 00000006] unique: 83, opcode: READDIRPLUS (44), nodeid: 1, insize: 80, pid: 1
[2020-12-20 00:32:08.64+0100] [ID: 00000006]    unique: 83, error: -13 (Permission denied), outsize: 16

More precisely, it fails on the directory entry in the containing
directory that is the sshfs mount point, when listing the containing
directory.

I've skimmed the following thread:

  [PATCH] virtiofsd: Show submounts
  https://www.redhat.com/archives/virtio-fs/2020-April/msg00023.html

(which is now QEMU commit ace0829c0d08), and I vaguely suspect it should
work -- the MS_REC flag is present, and the MS_REC flag seems to be so
old that I think my host kernel (latest RHEL7) must support it too.

So... does the sshfs filesystem present itself as unshareable? Is it
supposed to work? Does it break for others too?

Thanks!
Laszlo