standalone fortify source implementation

[Dimitris Papastamos <sin@2f30.org>]

Hi everyone,

I have been working on a standalone fortify source implementation[0] that
uses GCC's #include_next to overlay over the system headers.  The current
implementation has been tested against musl libc and OpenBSD's libc.

This implementation only supports _FORTIFY_SOURCE=1.  Level 2 is the same
as level 1.  If this is to be used by default on a system it makes sense
to only catch cases where UB would be invoked (level 1) rather than trap
on suspicious but legal code (level 2).

At the moment only out of bound writes are caught.  Should the implementation
trap on out of bound reads as well?

Overlap checks have been added to memcpy() and will probably be added to
other functions as well.

A short list of functions that still need to be covered:

* wide versions of existing functions
* confstr()
* fread()
* getcwd()
* getgroups()
* gethostname()
* poll()/ppoll()
* readlink()/readlinkat()
* reentrant variants such as ttyname_r, getlogin_r() etc.

I've received very useful feedback from dalias and fabled on IRC.  I
am putting this out here so that more people can have a look and share their
comments.

Thanks,
Dimitris

[0] http://git.2f30.org/fortify/