Hands-On System Programming with Linux

Memory-region protection

Perhaps the most important benefit of VM is this: the ability to define protections on virtual memory and have them honored by the OS.

Unix and friends (including Linux), allow four protection or permission values on memory pages:

Protection or permission type	Meaning
None	No permission to do anything on the page
Read	Page can be read from
Write	Page can be written to
Execute	Page (code) can be executed

Table 5: Protection or permission values on memory pages

Let's consider a small example: we allocate four pages of memory in our process (numbered 0 to 3). By default, the default permission or protections on the pages is RW (Read-Write), which means the pages can be both read from and written to.