Why does Windows process all NTFS child objects when changing a parent’s ACL?

I would expect this behavior if I had checked the “Replace all child object permissions...” box, but even when that box is left unchecked, Windows will process all the children.

  • @Ben It is not done by the shell, because the cascade of inherited permissions must be replicated to all child objects with inherit enabled, so it has to be done even when calling the Windows API directly. FYI: Permissions are replicated for performance reasons. – Andreas Nov 19 at 21:43
up vote 7 down vote accepted

In Windows file permissions are not dynamically inherited. That is, when an attempt is made to open a file Windows only looks at the ACL of that file and not at the ACLs of the directories in the tree containing the file. That means when you change the ACL of a directory Windows has to immediately update the permissions of all files and subdirectories within the affected directory.

In Windows the inherit setting in an ACL does not indicate any form of dynamic inheritance. It is just a flag to indicate that when a parent directory's ACL is modified all files and subdirectories in the tree that have the inherit flag set must also be updated.

Those of us old enough to remember Novell NetWare will remember this was one of the big differences from NetWare because in NetWare inheritance of permissions is (was?) dynamic. There was much debate at the time about which approach was better, though history has rendered the issue moot. Dynamic ACLs require the OS to check the ACLs of every parent directory at the time an attempt is made to open the file, but changing ACLs is quick. In Windows opening file requires only a single ACL to be checked, but as you've found it means changing a directory ACL can be slow.

  • 1
    This makes perfect sense. ACLs are read a lot more than they're written, so it makes sense to cache the "inherited value" for faster read access, at the cost of more complexity in the rarer write use case. – Alexander Nov 19 at 20:47
  • @Alexander: What I don't understand is why this complexity has to be exposed to the end user. Why not lie and tell the end user "it's dynamic" but cache things internally? – Kevin Nov 20 at 4:34
  • @Kevin - because it can take some time to propagate permissions and in that time period you enforcement of permissions might not happen as you expect? – davidbak Nov 20 at 4:44

Any child object that is configured to inherit it’s permissions from the parent object will need to be processed. Explicitly defined permissions on the child objects are not affected.

The option, “Replace permissions on all child objects,” will not only propagate the permissions to all child objects but it will also remove and replace any explicitly defined permissions on all child objects.

By default, child folders inherit permissions from the parent folder. Assuming the default scope when you're adding/modifying permissions on the parent folder (this folder, subfolders and files) then all child folders will be updated to reflect the permissions change at the parent.

The checkbox you're referring to is a "one time" operation that will remove all explicitly defined permissions on all child folders and replace them with inheritable permissions from the parent and will re-enable permissions inheritance on the child folders.

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.