Comment 0 for bug 1983640

[Impact]

Starting with 5.13 we've incorrectly dropped the following sauce patch:

    UBUNTU: SAUCE: overlayfs: fix incorrect mnt_id of files opened from map_files

This patch is required to use overlayfs on top of shiftfs and without this patch we may break containers that rely on shiftfs (using zfs/ceph as storage pool w/ shiftfs enabled).

However, we made this patch dependent on AUFS, starting with Jammy we're not enabling AUFS anymore, so this fix becomes a no-op.

So we need to re-introduce this fix with a bit of refactoring to not depend on AUFS.

[Test case]

The following script can be used to trigger the issue:

  #!/bin/bash

  cat > test.py << EOF
  import sys

  f = open("/proc/self/maps")

  for l in f.readlines():
    if "python" not in l:
      continue
    print(l)
    s = l.split()
    start, end = s[0].split("-")
    fname = s[-1]
    print(start, end, fname)
    break
  else:
    sys.exit(1)

  test_file1 = open(fname)
  test_file2 = open("/proc/self/map_files/%s-%s" % (start, end))

  fdinfo1 = open("/proc/self/fdinfo/%d" % test_file1.fileno()).read()
  fdinfo2 = open("/proc/self/fdinfo/%d" % test_file2.fileno()).read()

  if fdinfo1 != fdinfo2:
    print("FAIL")
    print(test_file1)
    print(fdinfo1)
    print(test_file2)
    print(fdinfo2)
    sys.exit(1)
  print("PASS")
  EOF
  sudo docker run -it --privileged --rm -v `pwd`:/mnt python python /mnt/test.py

[Fix]

Import the right pieces from AUFS to properly support the fix and get rid of the AUFS dependency across all our kernels.

[Regression potential]

This patch is touching overlayfs, so we may see potential regressions in overlayfs, especially when containers are used.