Segment permissions
A program to be loaded by the system must have at least one
loadable segment (although this is not required by the file
format). When the system creates loadable segments' memory
images, it gives access permissions as specified in the
p_flags member. All bits included in the
PF_MASKPROC mask are reserved for
processor-specific semantics.
Table 8-14 Segment flag bits, p_flags
--------------------------------------------------------------------- Name Value Meaning --------------------------------------------------------------------- PF_X 0x1 Execute PF_W 0x2 Write PF_R 0x4 Read PF_MASKPROC 0xf0000000 UnspecifiedIf a permission bit is 0, that type of access is denied. Actual memory permissions depend on the memory management unit, which may vary from one system to another. Although all flag combinations are valid, the system may grant more access than requested. However, a segment will never have write permission unless it is specified explicitly. The following table shows both the exact flag interpretation and the allowable flag interpretation.
Table 8-15 Segment permissions
-------------------------------------------------------------------- Flags Value Exact Allowable -------------------------------------------------------------------- none 0 All access denied All access denied PF_X 1 Execute only Read, execute PF_W 2 Write only Read, write, execute PF_W+PF_X 3 Write, execute Read, write, execute PF_R 4 Read only Read, execute PF_R+PF_X 5 Read, execute Read, execute PF_R+PF_W 6 Read, write Read, write, execute PF_R+PF_W+PF_X 7 Read, write, execute Read, write, executeFor example, typical text segments have read and execute -- but not write -- permissions. Data segments normally have read, write, and execute permissions.