Demo: Permissions Are Per Area

Read, write and execute, against the data area and the code area. Four combinations work; three, commented out in main(), each crash.

Run

make
./memory-access

Uncomment one of the three lines at the end of main(), rebuild, run again.

Ask

• Reading code works. Why would a program be allowed to read its own instructions?
• Executing data, writing code, reading address 0: which of the three is a different kind of fault from the other two?
• Why does the kernel leave address 0 unmapped on purpose?