[PATCH 0/3] virtiofsd: Find original inode ID of mount points

[Max Reitz <mreitz@redhat.com>]

Hi,

Not announcing submounts to the guest may lead to duplicated
st_dev/st_ino combinations, because inodes on different filesystems
(different st_dev) may share the same inode ID (st_ino).  If the guest
only sees a single st_dev, those inodes will appear to have the same
st_dev/st_ino combination.

Announcing submounts is supposed to solve this problem by making the
guest report different st_dev IDs for different filesystems.

However, there is one loop hole: Submounts are implemented as
auto-mounts, meaning when the virtio-fs filesystem is mounted, the
submounts will not be mounted immediately.  In the guest, it is possible
to stat() these mount points with AT_NO_AUTOMOUNT to receive information
without mounting them, so they will then show whatever st_ino virtiofsd
passes, and the st_dev of the parent filesystem.

Unfortunately, as far as we understood, the only st_ino that virtiofsd
could inquire was the one on the submounted filesystem, i.e. the st_ino
of that FS’s root node.

Thus, we again got a collision: In said case, the guest would see st_dev
of the parent FS, but st_ino of the submounted FS.  This is very likely
to be the same st_dev/st_ino combination as the root node of the parent
FS.

For nested mount structures, this can be reproduced with `find`:
Mounting several filesystems under each other, passing everything into a
guest, mounting the root virtio-fs FS there, and then invoking `find`
would likely result in it complaining about filesystem loops and
refusing to visit the submount points.  (These loops are reported
because it takes notes of st_dev/st_ino combinations, and then reports a
loop when some combination reappears.)  This can only be fixed by
forcing all submounts to be (auto-)mounted.

What we’d need to do is report the inode ID of the mount point directory
that it has on the parent filesystem to the guest, until the submount is
auto-mounted there.  It’s just that we thought there was no way to
inquire this parent FS st_ino.

It turns out that our understand was wrong, though: There is a way,
namely readdir().  The dirent.d_ino field represents (in practice) the
st_ino of some directory entry on the directory’s filesystem, and so is
exactly the value we want to give to the guest.

Let’s do that.


Max Reitz (3):
  virtiofsd: Find original inode ID of mount points
  virtiofs_submounts.py: Do not generate ssh key
  virtiofs_submounts.py: Check `find`

 tools/virtiofsd/passthrough_ll.c              | 104 +++++++++++++++++-
 tests/acceptance/virtiofs_submounts.py        |  10 +-
 .../virtiofs_submounts.py.data/guest.sh       |  56 ++++------
 3 files changed, 122 insertions(+), 48 deletions(-)

-- 
2.31.1