More Security: WxorX

From the original check-in:

This is a project to modify executables so that they do not have any
 executable regions which are writable. If a section of an executable is
 writable and executable, it is much easier for errant code to modify the
 executable's behavior.

 Two current areas in shared library environments which have this
 critical problem are the GOT (Global Offset Table) and PLT (Procedure
 Linkage Table). The PLT is required to be executable and both GOT and
 PLT are writable on most architectures. On most ELF architecture
 machines this would cause shared libraries to have data and BSS marked
 as executable.

 Padding to the linker script for programs and shared libraries/objects
 to isolate the GOT and PLT into their own load sections in the
 executables. This allows only the text(readonly) region and the PLT
 region to be marked executable with the normal data and BSS not marked
 as executable. The PLT region is still marked executable on most
 architectures because the PLT lives in the "data" or "BSS" regions
 and the dynamic loader will need to modify it. Since the GOT and PLT
 should only ever be written by the dynamic linker, it will be modified
 to mprotect those regions so that they are not writable during normal
 execution. If the dynamic linker needs to modify the regions later,
 (eg for lazy binding), it will mprotect the region, make the necessary
 changes, and mprotect it back. Since it is possible to receive a
 signal which would interrupt the program flow and perhaps cause the
 dynamic linker to modify the same (or nearby) PLT references, it is now
 necessary for signals to be blocked for the duration of the mprotect.