i386 PROT_EXEC best-effort


i386 lacks per-page X bit

Only significant relevant hardware feature:
code segment limit
3.2/3.3: points below bottom of stack -> no-exec stack

Link each shared object to have 1GB gap between code & data

Map all text segments low, all data segments 1GB higher

Set code segment limit register to point up to highest PROT_EXEC page
(floating CS limit)
Normally somewhere below 1GB

Will be in 3.4: W^X on i386

Changing CS limit is a bit expensive: slight overhead