Cyringe is a code injection program, based on a description of 
Cymathoa. Cymathoa is described in an article on parasitic programs
in phrack (http://www.phrack.org) issue #68.

Cyringe uses a ptrace debugger technique to inject binary codes into
a running process' memory space. The general flow of what it does is:
* Attach to the host process as a debugger
* Write the binary input to the hosts' memory at a specified location
* Read the hosts' registers
  - Write (%RIP - offset) to the address at (%RSP + 8)
  - Write the location of your code to %RIP
* Detach from the process

We perform the first part of the third step in order to fake a
function call to our code, and allow our parasite to be friendly
(i.e. not completely kill the host). This step follows the conventions
of the X86 C ABI. The offset is a command line argument such
that you can force it to return to the instruction it was on before,
because (%RIP) technically points to the *next* instruction not the
current instruction. A good debugger can be used to figure out the
correct offset for a given host.