From the Shon Harris book:
“So, partially ordered means the system has to apply the most restrictive access controls to this set, and “least upper bound” means the system looks at one access control’s statement (Kathy can read the file) and the other access control’s statement (Kathy cannot write to the file) and takes the least upper bound value. Since no write is more restrictive than read, Kathy’s least upper bound access to this file is read, and her greatest lower bound is no write.”
Arrrrgh. No. No. No.
Partially ordered means a comparison (call it dominates) can return less than, greater than, equal, or incomparable. Least Upper Bound means there is a value in the set that is greater than or equal to every value in the set, and it is the lowest such value. Greatest Lower Bound means there is a value in the set that is less than or equal to every value in the set, and is the greatest such value.
This is why this chapter frustrates me so.
