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[Serge Hallyn <1531747@xxxxxxxxxxxxxxxxxx>]

Does it require the workdir to be empty?

I.e. is there a way (symlink, bind mount, something else) that a user
could use a dir they own which has a child which they don't own?

It looks like no, since

root@w1:/tmp# mount -t overlay -o lowerdir=lower,upperdir=upper,workdir=workdir overlay /mnt
root@w1:/tmp# ls /mnt
cisco
root@w1:/tmp# rmdir /mnt/cisco
rmdir: failed to remove ‘/mnt/cisco’: Read-only file system
root@w1:/tmp# mv /mnt/cisco /mnt/c2
mv: cannot move ‘/mnt/cisco’ to ‘/mnt/c2’: Read-only file system

(here w1 is a unpriv container with /hostopt a bind mount of /opt on the
host;  cisco a directory both in host's /opt and in /tmp/lowerdir)

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https://bugs.launchpad.net/bugs/1531747

Title:
  overlay: mkdir fails if directory exists in lowerdir in a user
  namespace

Status in linux package in Ubuntu:
  Triaged
Status in linux source package in Wily:
  Triaged
Status in linux source package in Xenial:
  Triaged

Bug description:
  If a directory exists in the lowerdir but not in the mounted
  overlay, then mkdir of the directory in the target dir results
  in a mysterious -EPERM.  I've seen this both in wily kernel
  (4.2.0-22-generic #27-Ubuntu) and in a hand-built xenial
  master-next (with unrelated patches added).

  =====================================================
  #!/bin/sh -ex
  dir=`mktemp -d`
  cleanup() {
   umount -l $dir/t
   rm -rf $dir
  }

  trap cleanup EXIT

  echo "dir is $dir"
  mkdir -p $dir/l $dir/u $dir/w $dir/t
  mkdir $dir/l/dev
  mount -t overlay -o lowerdir=$dir/l,upperdir=$dir/u,workdir=$dir/w o $dir/t
  stat $dir/t/dev
  rmdir $dir/t/dev
  mkdir $dir/t/dev
  echo $?
  echo "mkdir should have succeeded"
  =====================================================

  The above will work on the host, but fail in a user namespace, i.e
  in a regular lxd container.

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