README

libgerald -- library for generating a dynamic linker

On a modern Unix platform, the dynamic linker (ld.so) bootstraps the
execution of most user-space code. It's therefore a virtualisation
vector: control what the dynamic linker does, and you can transparently
extend or modify the system's behaviour. This is useful in a number of
overlapping ways:

* a way to prototype operating system extensions
* a more robust alternative to LD_PRELOAD
* a transparent way to instrument or augment whole-userland execution

This repository collects recipes and utility code that may be used to
create new dynamic linkers with new system behaviours.

In contrib/ are recipes for grabbing a build tree that can create a new
ld.so from some C library, and setting some variables that act as a shim
layer around that C library's build process. Unfortunately these are
rather grungy, being quite sensitive to which libc and version thereof
you are running, and also (because they download source code) to the OS
distribution. Currently there are rules for building variants of glibc's
ld.so on Debian-based distributions. Other recipes are welcome, as are
bug reports and the like.

In mk/ are some relatively generic rules for building a ld.so given one
of these build trees and the shim variables. Input objects can be passed
through a 'hack' recipe, and new objects can be added to the link.

In examples/ are some examples of how to use these. Currently only
examples/wrap-start-user/ is complete. This builds a new ld.so which
prints a message to stderr immediately after the bootstrap relocation.
Unfortunately it currently embeds some glibc-specific details which
really should be factored into the shim layer.