5.9 Call Gates

5.9 Call Gates

1. 2 Main functions

   • To define an entry point for a procedure
     • Used to txfer to a procedure in a new segment
   • To specify the privilege level required to enter a procedure
     • Provides a way for less privileged code to access more privileged segments in a controlled way
       • Known as controlled invocation
       • When using a gate the CPL changes to the level of the code to which the gate is pointing
         • after using the gate the CPL is restored

2. Call gates guarantee that all control transfers to other sgments go to a valid entry point rather than to the middle of a procedure

3. Call gates allow execute only access to a procedure in an inner ring or higher privilege level

4. for a procedure to use a gate the gate must be in the same ring as the procedure

5. Access Control checks the following levels before transfer of execution through a gate

   • The CPL
   • RPL of the selector used to specify the gate
   • DPL of the gate descriptor
   • DPL of the segment descriptor of the destination code segment
   • The following must both be satisfied otherwise an exception is generated
     • DPL of the gate must be greater than the CPL and the RPL
     • The DPL of the code segment is less than or equal to the CPL
     • Hence the DPL of the code segment is less than or equal to the DPL of the gate
       • The DPL of the gate descriptor determines the privilege level from which the gate must be used