Overlayfs strict feature requirements

Vivek, Miklos,

This series passes overlay/quick xfstests and I verified manually some expected mount failures with metacopy=on and override with metacopy=on,strict=off.

Still needs very carefull review and the ovl_check_rename_whiteout() helper in patch 3 is broken, so I disabled it for now.

Patches 1-3 are marked for stable apply cleanly on v4.19. Patch 4 doesn't apply to v4.19.
Patch 5 will probably apply, but not sure it is stable material.

I did not change behavior w.r.t enabling of redirect_dir, because it involves many corner cases and I don't think it matters for stable. We can always improve it later and let some mount configurations that used to fail succeed with expected user requested mount options. When we address the metacopy => redirect_dir dependency, we should also address the nfs_export => index dependency in a similar manner.

Thanks,
Amir.

Amir Goldstein (5):
ovl: return error on mount if metacopy cannot be enabled
ovl: enforce 'strict' feature requirements with metacopy=on
ovl: enforce 'strict' upper fs requirements with metacopy=on
ovl: enforce 'strict' unique uuid requirement with metacopy=on
ovl: enforce 'strict' upper fs and feature requirements with strict=on

 fs/overlayfs/Kconfig     |  23 ++++
 fs/overlayfs/ovl_entry.h |   1 +
 fs/overlayfs/super.c     | 235 ++++++++++++++++++++++++++++++---------
 3 files changed, 208 insertions(+), 51 deletions(-)

2.17.1