[PATCH v3 0/6] Support nested overlayfs mounts

[Alexander Larsson <alexl@redhat.com>]

There are cases where you want to use an overlayfs mount as a lowerdir for
another overlayfs mount. For example, if the system rootfs is on overlayfs due
to composefs, or to make it volatile (via tmpfs), then you cannot currently store
a lowerdir on the rootfs, becasue the inner overlayfs will eat all the whiteouts
and overlay xattrs. This means you can't e.g. store on the rootfs a prepared
container image for use with overlayfs.

This patch series adds support for nesting of overlayfs mounts by escaping the
problematic features and unescaping them when exposing to the overlayfs user.

This series is also available here:
  https://github.com/alexlarsson/linux/tree/ovl-nesting

And xfstest to test it is available here:
  https://github.com/alexlarsson/xfstests/tree/overlayfs-nesting

The overlay/083 test checks both xattr escaping, the new whiteouts as well as
actual nesting of overlayfs.

Note that this series breaks the overlay/026 test which validates that
writing overlay.* xattrs is not supported, but it now is. I'm not sure
if we should fix this test to not fail, or if we should make this an
opt-in mount feature.

Changes since v2:
 * Uses a new approach for escaping whiteouts with a regular file with an
   overlay.whiteout xattr in a lower directory with an overlay.whiteouts
   xattr.

Changes since v1:

 * Moved all xattr handling to xattr.c
 * Made creation of escaped whiteouts atomic

Alexander Larsson (6):
  ovl: Move xattr support to new xattrs.c file
  ovl: Add OVL_XATTR_TRUSTED/USER_PREFIX_LEN macros
  ovl: Support escaped overlay.* xattrs
  ovl: Add an alternative type of whiteout
  ovl: Handle escaped xwhiteouts across layers
  ovl: Add documentation on nesting of overlayfs mounts

 Documentation/filesystems/overlayfs.rst |  23 ++
 fs/overlayfs/Makefile                   |   2 +-
 fs/overlayfs/dir.c                      |   4 +-
 fs/overlayfs/inode.c                    | 124 ----------
 fs/overlayfs/namei.c                    |  15 +-
 fs/overlayfs/overlayfs.h                |  42 +++-
 fs/overlayfs/readdir.c                  |  27 +-
 fs/overlayfs/super.c                    |  67 +----
 fs/overlayfs/util.c                     |  40 +++
 fs/overlayfs/xattrs.c                   | 312 ++++++++++++++++++++++++
 10 files changed, 443 insertions(+), 213 deletions(-)
 create mode 100644 fs/overlayfs/xattrs.c

-- 
2.41.0