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
