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17. Access Control Lists > The ACL_MASK Entry and the ACL Group Class

The ACL_MASK Entry and the ACL Group Class

If an ACL contains ACL_USER or ACL_GROUP entries, then it must contain an ACL_MASK entry. If the ACL doesn’t contain any ACL_USER or ACL_GROUP entries, then the ACL_MASK entry is optional.

The ACL_MASK entry acts as an upper limit on the permissions granted by ACL entries in the so-called group class. The group class is the set of all ACL_USER, ACL_GROUP, and ACL_GROUP_OBJ entries in the ACL.

The purpose of the ACL_MASK entry is to provide consistent behavior when running ACL-unaware applications. As an example of why the mask entry is needed, suppose that the ACL on a file includes the following entries:


  

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