opening a file on a stacked overlayfs is broken.

[Quentin Casasnovas]

Hi,

Stacking an overlayfs on top of an overlayfs doens't work when it used to
(tested on v4.9-rc5):

  #!/bin/bash -xeu
  tmpdir=$(mktemp -d)
  pushd ${tmpdir}

  mkdir -p {upper,lower,work}
  echo 'foo' > lower/bar
  mount -t overlay level_zero upper -o lowerdir=lower,upperdir=upper,workdir=work
  cat upper/bar

  tmpdir2=$(mktemp -d)
  pushd ${tmpdir2}

  mkdir -p {upper,work}
  mount -t overlay level_one upper -o lowerdir=${tmpdir}/upper,upperdir=upper,workdir=work
  stat upper/bar  # Works fine
  cat upper/bar   # open() returns ENXIO

I _think_ (I haven't bisected it) the guilty commit is 2d902671ce1c ("vfs:
merge .d_select_inode() into .d_real()"), where vfs_open() -> uses d_real()
with a a NULL inode, which prevents any recursion from happening, when
before d_select_inode() would do the right thing and recurse.

It should be noted this has already been broken and fixed in the past year
in 1c8a47df36d7 ("ovl: fix open in stacked overlay").

Let me know if I can help,
Q

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